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BioMed Research International
Volume 2013 (2013), Article ID 730789, 14 pages
Tight Junction Proteins and Oxidative Stress in Heavy Metals-Induced Nephrotoxicity
Physiology, Biophysics and Neurosciences Department, Center for Research and Advanced Studies, National Polytechnic Institute, Avenida Instituto Politécnico Nacional 2508, Colonia San Pedro Zacatenco, 07360 Mexico, DF, Mexico
Received 20 December 2012; Accepted 27 March 2013
Academic Editor: Cheng-An J. Lin
Copyright © 2013 José L. Reyes et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Kidney is a target organ for heavy metals. They accumulate in several segments of the nephron and cause profound alterations in morphology and function. Acute intoxication frequently causes acute renal failure. The effects of chronic exposure have not been fully disclosed. In recent years increasing awareness of the consequences of their presence in the kidney has evolved. In this review we focus on the alterations induced by heavy metals on the intercellular junctions of the kidney. We describe that in addition to the proximal tubule, which has been recognized as the main site of accumulation and injury, other segments of the nephron, such as glomeruli, vessels, and distal nephron, show also deleterious effects. We also emphasize the participation of oxidative stress as a relevant component of the renal damage induced by heavy metals and the beneficial effect that some antioxidant drugs, such as vitamin A (all-trans-retinoic acid) and vitamin E (α-tocopherol), depict on the morphological and functional alterations induced by heavy metals.
1. Introduction
Kidney plays the most important role in elimination of xenobiotics, including drugs and toxic environmental agents. Due to its secretory mechanisms, tubular proximal cells are often exposed to higher concentrations of toxic substances than those occurring in plasma or extracellular fluids. In addition to these circumstances, kidney depicts several mechanisms to absorb and excrete xenobiotics. They use molecular protein components located at the cell membrane, that are involved in the transport through the epithelial cells (transcellular route) and those located between cells (paracellular route). In this review we focus on the alterations induced by heavy metals on the proteins that participate in the paracellular transport (junctional adhesion molecules, claudins, occludin, and zonula occludens). Awareness of the damage to these proteins and the relevant consequences, it has on renal function and morphology, has been recently developed.
Heavy metals are accumulated in the kidney and liver, for this reason it is more accurate to estimate degree and duration of exposition to them, by measuring their concentrations in renal and hepatic tissues than in blood. Other options to measure them are in nails and hair. Kidney is the target of heavy metals and the proximal tubule has been recognized for a long time, as the main site of accumulation and damage; however, in this review we will describe that other segments of the nephron are also damaged after exposition to them, and that this damage leads to severe alterations in renal function, that may be permanent or reversible.
The toxic effect of heavy metals on human body has been recognized. In the past century due to the development of industrialized world, large amounts of these elements were produced and most of them were not biodegradable, remaining in the environment for long periods of time. The problem of disposal of toxic products is more extended in emerging economies. In Latin America despite of strict legal regulations, high levels of these elements are found in soil and sediments, leading to the risk of chronic exposure in the general population. In addition, in those countries with volcanic areas, such as Mexico, heavy metals are present in fossil waters that are extracted for human and animal use [1, 2].
Some heavy metals are necessary for vital functions of the human body, such as iron (Fe), cobalt (Co), copper (Cu), manganese (Mn), molybdenum (Mb), and zinc (Zn). Although it is unknown whether metals such as lead (Pb), cadmium (Cd), and arsenic (As) have a physiological role, they adversely affect several organs, including the liver, lungs, and kidney, being the latest particularly sensitive to their toxic effects even at low levels, due to its ability to reabsorb and accumulate these divalent metals [3]. The extent of renal damage depends on the nature, the dose, route, and duration of exposition displaying diverse degree of severity ranging from mild tubular dysfunction to severe renal failure.
2. Tight Junction (TJ) Proteins in Renal Physiology
2.1. Renal Transport through Transcellular and Paracellular Pathways
Reabsorption of filtered solutes and water is one of the vital functions performed by the kidney. It occurs in several segments of the nephron, the functional unit of the kidney. Reabsorption occurs through two pathways, transcellular and paracellular. The transcellular route is dependent on specialized molecules, such as transporters, ionic channels, and water channels. The paracellular reabsorption is mediated by the proteins that form the TJ: junctional adhesion molecules (JAMs), occludin, claudins, tricellulin, and scaffolding proteins zonula occludens (ZO-1, ZO2, and ZO-3), for reviews see [4, 5]. These proteins have differential distribution along the nephron and this distribution determines the paracellular permeability of the respective segments. The magnitude of this transport is variable, according to the specific ion. For instance 90% of the absorption of calcium (Ca2+) in proximal tubule occurs through the paracellular route, while in the distal tubule 100% of Ca2+ reabsorption occurs through the transcellular route. In Henle's loop 50% is transcellular and 50% is paracellular. In contrast, renal glucose reabsorption is transcellular and it is achieved at the proximal tubule through specific transporters, sodium-glucose cotransporters (SGLT 1 and 2) located at the apical brush border and glucose basolateral transporters (GLUT 1 and 2) [6, 7].
From 60 to 70% of the absorption of the glomerular filtered load of ions, organic compounds, and water occurs in proximal tubules, through both, transcellular and paracellular routes [8].
2.2. Distribution of TJ Proteins along the Nephron
Distribution of TJ proteins along the nephron has been shown to be similar in several mammal species, such as mouse, rat, and rabbit, and has been related to the permeability characteristics of the segments that constitute the nephron [9] (Figure 1). This permeability can be estimated by measuring the transepithelial electrical resistance (TER). TER is high in tight epithelia with low paracellular permeability, while in leaky epithelia TER is low. In the nephron this resistance increases from the glomerulus, where no TER has been measured, to the proximal tubule (5 to 6 Ω·cm2), and to the collecting tubule that depicts the highest electrical resistance (200 to 600 Ω·cm2) [10].
Glomerulus. Claudin 5, an endothelial TJ protein, has been identified in glomeruli and vessels [11] (Figure 2). Also claudin 3 is located at glomerular capillaries. In Bowman's capsule, an epithelial structure, claudin 2 has been observed (Figure 3(a)).
Proximal Tubule. Claudin 2 is the most abundant of the claudin's family (formed by more than 20 isoforms) in the proximal tubule (Figure 3(b)). This claudin has been recognized as a cation and water channel [12, 13]. It has been located in other highly permeable epithelia that depict low TER [14]. The location of claudin 2 is in agreement with the characteristics of the paracellular pathway in the proximal tubule, which is the most permeable tubular segment of the nephron [15]. Claudins 1, 9, 11, and 17 are expressed in this tubular segment. Occludin, ZO-1, and ZO-2 are also located along the nephron, with increasing amounts from proximal tubule to collecting duct [11] (Figure 1).
Thin Descending and Ascending Loops of Henle. In descending loop of Henle claudins 7 and 8 are located at the aldosterone-sensitive distal nephron and at descending and ascending thin segments [16, 17]. In contrast to proximal tubule, claudin 2 is not present at these segments.
Thick Ascending Loop of Henle. In this segment the role of claudin 16 has been identified as a critical factor for the paracellular absorption of Ca2+ and magnesium (Mg2+) [18]. Claudins 11 and 14 are also in this segment [19].
Distal Tubule. In this segment the presence of claudin 3, 4, and 8 has been reported. These three proteins have a role as barriers for cations absorption [15].
Collecting Duct. This is the segment that depicts the highest electrical resistance in the nephron. Claudins present in collecting tubules share the property of conferring low ionic paracellular permeability, especially to cations. It is noteworthy that expression of claudin 8, present in this segment, has been reported to be augmented by aldosterone. Claudins 4, 7, and 18 are also present in collecting tubules, as well as occludin, ZO-1, and ZO-2 [9] (Figure 1).
In general terms, distribution of claudins along the nephron follows the pattern of TER present in this structure. It increases from the proximal tubule to collecting duct and in opposite direction, paracellular permeability decreases from proximal to collecting duct [11, 20, 21].
3. Nephrotoxicity of Heavy Metals on TJs
3.1. Absorption and Metabolism of Divalent Metals
The absorption of heavy metals, such as Cd and Pb, is carried out in the small intestine by a divalent metal transporter characterized as DMT-1. This transporter is expressed in the duodenum, red blood cells, liver, and in the proximal convoluted tubular cells of the kidney. This protein transports Fe and displays a high affinity for other divalent metals such as Cd, Nickel (Ni), Pb, Co, Mn, Zn, and Cu [22].
Once absorbed, heavy metals are accumulated in liver where they bind to metallothioneins (MTs). These proteins are widely expressed through the body and have a particular feature; they contain a large quantity of cysteine, which confers both a high affinity and a storage capability to heavy metals, such as Zn, Cd, Mercury (Hg), Cu, Pb, Ni, Co, and Fe. The main role of MT is to transfer heavy metals to metalloproteins, transcription factors, and enzymes [23].
3.2. Cadmium
Cadmium is one the most important toxic elements to which general population is exposed. The most common exposition circumstance occurs by contact with tobacco smoke, contaminated water, and foodstuffs such as vegetables, grains, and molluscs. Cadmium progressively accumulates in the body compartments, notably in liver and kidney, given rise to its long half-life, which is more than 20 years in human [24].
Once absorbed and dissociated from MT by the action of gastric environment, Cd binds to albumin and then it is transported to liver, where it binds to glutathione (GSH) and MT-1. This complex has a low molecular weight (<7 kDa), and then it easily filtrated by the glomerulus being almost totally uptaken in the S1 segment of proximal tubule, in a process mediated by megalin and cubilin [25]. Approximately 10% is reabsorbed in distal tubule [26].
Cadmium induces cellular damage through several mechanisms, one of the best characterized is that it accumulates in mitochondria, where it blocks the respiratory chain at complex III resulting in an increased production of free radicals that enhances caspase activity leading to apoptosis [27]. Free Cd also binds to chemical functional groups in several proteins. It has been shown that Cd affects the distribution of paracellular TJ proteins decreasing the TER [28] (Figure 4).
The best characterized clinical findings of Cd renal toxicity are low molecular weight proteinuria, aminoaciduria, bicarbonaturia, glycosuria, and phosphaturia [29]. Also, it has been observed pulmonary obstructive disorders and different types of neoplasias, such as bladder and lung carcinomas [29–31].
Nephrotoxic effects are severe and include the development of a Fanconi-like syndrome, with glucosuria, aminoaciduria, and phosphaturia. Hazen-Martin et al. [32] reported in cultures of human proximal cells exposed to Cd, alterations in TER and in the structure of apical cell membrane junctions, latest observed by freeze-fracture microscopy. Adult rats exposed to Cd showed alterations in claudin 5 pattern in glomeruli, indicating that in addition to proximal tubule, this metal also damages glomerular endothelial cells. As expected, claudin 2 showed severe disruption of the "chicken fence" pattern in proximal tubules of animals intoxicated with Cd. Zinc administration partially protected against deleterious effects of Cd [33]. Prozialeck et al. [34] reported damage induced by this metal, to N-cadherin, E-cadherin, and β-catenin. It is interesting that Zimmerhackl et al. [35] reported that Cd is more toxic to LLC-PK1 cells (from porcine origin) than in MDCK cells (from canine origin), acting on the cadherin-catenin complex. These findings suggest that Cd toxicity may elicit differences related to species. Chronic exposure to Cd leads to hypertension and renal failure. Jacquillet et al. [28] described severe alteration in the pattern of claudin 5 in glomeruli from adult offsprings exposed in uterus to Cd. After birth those animals did not receive additional doses of this metal and their blood pressure and renal function were normal. However, they developed hypertension and renal damage in adult life. These animals also depicted alteration of the pattern and distribution of claudin 2 in proximal tubules. These findings showed the risk of in utero exposition to Cd on the development of arterial hypertension and renal damage in adult life. Taking into consideration that tobacco smoke is one of the main sources of contamination by Cd and the long lasting half-life of this metal in humans (10 to 30 years), exposure during gestation might be a critical hazard for hypertension and renal damage in offsprings [28].
3.3. Chromium
This metal is accumulated in the kidney, as other heavy metals. The toxic effects of this accumulation have been recognized mostly in acute intoxication episodes. Much less is known for chronic exposure. In a model of acute renal failure induced by potassium dichromate, Perez et al. [36] reported that abundant death renal cells were found in urinary sediment. Even more relevant was the finding of alive cells in urine. This finding suggested that the mechanisms, responsible for maintenance of cells attached to basal membrane and to neighbor cells (tight junctions, desmosomes, and gap junctions), were altered by this metal. Chromium (Cr) induces oxidative stress and it has been reported that oxidative stress damages TJs function. In the same study it was observed that administration of all-trans-retinoic acid, the active form of vitamin A, reduced the elimination of living cells [36]. Arreola-Mendoza et al. [37] reported that in addition to damage to proximal tubules demonstrated by increased excretion of glucose and decreased capacity of the secretory pathway of organic anions, distal functions of the nephron, as water regulation, were altered by this metal. Arreola-Mendoza et al. [38] confirmed in the same model that Cr severely altered expression and distribution of occludin and claudin 2; α-tocopherol, active form of vitamin E, protected renal cells from damage induced by this metal, through the participation of extracellular signal-regulated kinase 1/2 (ERK1/2).
Exposure to hexavalent chromium (Cr6+) causes mutagenic, carcinogenic, and toxic effects, some of which have been associated with its oxidative capacity. In the kidney, TJs are especially sensitive to oxidative stress. In this sense, Basuroy et al. [39] demonstrated that changes in the tubular oxide-reductive environment change the TJ proteins distribution affecting its permeability properties. Subsequently, Arreola-Mendoza et al. [38] and Perez et al. [36] showed, in rats exposed to Cr6+, proximal and distal tubule dysfunction, decreased glomerular filtration, as well as increased oxidative damage.
Chromium induces variations in serum creatinine, creatinine clearance (Ccr), and fractional excretion of sodium (FeNa), and the latest can be mediated by the mislocation of claudin 2 and occludin induced by Cr [36] (Figure 5). These data implicate that oxidative stress induced by heavy metals affects the distribution of TJ proteins in proximal and distal tubules altering its permeability properties.
3.4. Lead
Lead nephrotoxicity has been recognized for more than a century; however, cell mechanisms involved have not been fully disclosed. This is more evident in the case of effect on intercellular junctions. Navarro-Moreno et al. [40] showed alterations in the intercellular apical region of proximal tubules from rats intoxicated during seven months with Pb. Glycosuria, aminoaciduria, hematuria, oxidative stress, and loss of apical microvilli were also observed. Lead shares with Cd the toxic effect of inducing arterial hypertension and vascular alterations, including glomerulonephritis [41, 42].
Lead is one of the most frequent divalent metals that induces nephrotoxicity. It has been demonstrated that even at low doses, Pb increases cardiovascular morbility [43].
Although, the maximum nontoxic dose of Pb blood levels has not been established, there is increasing evidence that even previously considered nontoxic Pb levels increase morbility and mortality rates in general population [43].
Once absorbed by intestine, lung and to a lesser extent through the skin, Pb binds to erythrocyte proteins at above 90%, and then it is distributed to soft tissues and bone. The latest is the main reservoir for Pb in the body. Its bloodstream levels increase during augmented bone turnover, notably during adolescence and pregnancy [44].
Lead binds to low molecular weight proteins in a proportion lower than 1%. For this reason it is freely filtered at the glomerulus and it is reabsorbed in the proximal tubular cells by a mechanism mediated by endocytosis. Once into the cell, Pb induces mitochondrial damage, uncoupling of respiratory chain, intracellular depletion of GSH, oxidative stress, and apoptosis [45].
It has been demonstrated that Pb enhances proinflammatory processes through activation of nuclear factor kappa B (NFκB), which in turn activates intrarenal renin-angiotensin system and thus activates macrophages, generating an interstitial inflammatory process into the kidney [46]. In endothelial cells, the increased production of free radicals elicited by Pb inactivates vasorelaxant mediators such as nitric oxide (NO). These effects may partially explain the hypertension found in Pb intoxication [47].
The clinical findings of Pb toxicity can be divided in two categories depending on duration of exposure; acute toxicity manifested as aminoaciduria, glycosuria, hyperphosphatemia, haemolytic anemia, gout, and encephalopathy [48]; and chronic toxicity depicted as tubule-interstitial nephritis and progressive decrease of renal function. The toxic effects of Pb on proximal tubular epithelial cells decrease the urate excretion resulting in increased urate levels in the bloodstream [40, 49].
3.5. Mercury
Effects of Hg on proximal tubule were recognized long time ago leading to its use as diuretic. Recognition of its toxic effects precluded this therapeutic application. To our knowledge to date there is no available information on the effects of Hg exposure on intercellular renal TJs. As in the case of Cr toxicity, in Hg-intoxicated rats vitamin E protected against the renal damage and development of interstitial renal fibrosis [50].
Mercuric chloride (HgCl2) causes acute oxidant renal failure that affects mainly proximal tubules. Proteinuria induced by chronic exposure to Hg and the relationship between urinary Hg and renal damage were explored in rats. The results showed that the primary site of damage was the proximal renal tubule and that the glomerulus was eventually involved, due to the capability of Hg to be freely filtered at the glomerulus. The tubular ultrastructural analysis revealed that the lysosome was the most sensitive organelle to Hg, and there was a close relationship between the excretion of urinary Hg and the Hg detoxication mechanisms of the kidney. There is evidence that Hg was accumulated also in endothelia and mesangia of the glomeruli [51].
The mechanisms involved in renal damage (membranous nephropathy) induced by Hg are in relationship with the deposit of immune complexes directed both against laminin β1 and several complement factors. It has been previously reported that this mechanism can be mediated by the release of interferon-gamma (IFN-γ) and interleukin-4 (IL-4) from infiltrating glomerular macrophages which exerted toxic effects, resulting in a rapid decrement of TER of confluent monolayers. In addition, it has been demonstrated that podocytes exposed to IFN-γ and IL-4 also altered vimentin and laminin expression; indeed both of them interfered with monolayer integrity when added to the basolateral side of podocytes, indicating a selective damage [52]. Indeed, experimental models of HgCl2-induced renal glomerular injury showed increased deposits of fibronectin and lipids and enhanced cellularity in glomeruli [53].
Experimental models of HgCl2-induced renal glomerular injury showed alterations in urine osmolality, volume, and protein levels were seen within 24 h in response to 1 mg/kg of HgCl2 [54].
3.6. Bismuth
Toxicity of this metal on renal attachment mechanisms has been only scarcely reported. Acute renal failure induced by bismuth (Bi) has been reported [55]. This issue is relevant taking into consideration the wide use of Bi compounds to peptic ulcers treatment and to eradicate Helicobacter pylori. Loss of renal epithelial cell adhesion by selective N-cadherin displacement was reported by Leussink et al. [56]. They found that this effect was present both, in vivo and in vitro models. After 1 h treatment, N-cadherin had disappeared from adherent junctions of vital proximal tubular cells, whereas ZO-1, a TJ protein, remained present. Similar results were obtained in two cell lines derived from proximal tubules (NRK-52E and LLC-PK1) [57]. Interestingly, Bi salts have been found to be protective against cisplatin nephrotoxicity [57].
3.7. Nickel
Nickel compounds are associated with several human diseases mainly in lung and kidney cancers [58]. Vyskocil et al. [59] showed that even at low oral doses, soluble Ni induced changes in glomerular permeability in female laboratory animals, or enhanced the normal age-related glomerular nephritic lesions. Compared with male, female rats seem to be more sensitive to the nephrotoxic effect of Ni [59].
Subsequently, Horak and Sunderman [60] demonstrated that rats exposed to inhalation of Ni carbonyl display an increased urinary protein excretion. Among the acidic amino acids and amides, only the excretion of glutamic acid was increased. Urinary excretion of ammonia was greatly increased after exposure to Ni carbonyl. The alterations of urinary excretions of amino acids were apparently mediated by nephrotoxicity rather than by mobilization of amino acids from tissues, since plasma concentrations of amino acids were not significantly affected by exposure to Ni carbonyl. The pronounced diminution of glutamine excretion and the marked increase of ammonia excretion were consistent with enhanced renal production of ammonia from glutamine by action of glutaminase [60].
Accordingly, administration of Ni (6 mg per kg, i.p., three days) significantly enhanced the urinary excretion of alkaline phosphatase (ALP), lactate dehydrogenase (LDH), glutamate oxaloacetate transaminase (GOT), amino acids, and proteins. In addition, it increased the activity of serum ALP, GOT, and glutamate pyruvate transaminase (GPT) [61].
4. Mechanisms of Heavy Metals-Induced Nephrotoxicity
Renal toxicity induced by heavy metals has been recognized for over a century; however, the intracellular mechanisms of this nephrotoxicity remain unclear. Recent studies performed in vivo and in vitro (renal cell lines and isolated mitochondria) have indicated that oxidative stress, apoptosis, and necrosis are common phenomena in the course of nephrotoxicity elicited by these metals [62]. In recent years, the pathogenetic mechanisms of acute kidney injury (AKI) have been associated with endothelial and epithelial cell injury. Endothelial damage in glomeruli leads to proteinuria. Tubular cells undergo alteration of cell polarity, mislocalization of TJ proteins, and membrane transporters and finally develop necrosis and apoptosis. A common event in the action of all heavy metals in the proximal tubular cells is generation of oxidative stress that is manifested by (a) depletion of intracellular GSH and free radical scavenger levels, (b) inhibition of the activity of several antioxidant enzymes that participate in the detoxication of free radicals, and (c) increased reactive oxygen species (ROS) production.
Other phenomena such as the loss of the function of transporters, ATPases and ion channels, deranged metabolism, cytoskeleton, and cell polarity destabilization with loss of cell membrane integrity are present. Increased synthesis of MT proteins, upregulation of heat shock proteins (Hsp), increase in cytoplasmic concentration of Ca2+, impaired endocytosis, enhancement of ion conductances, and structural and functional damage in mitochondria have been described in the nephrotoxicity induced by heavy metals such as Cd, Hg, and Cr.
In this review we focus on the deleterious effects induced by heavy metals on the renal TJ structure and function and the participation of oxidative stress and renal cells response to these toxicants. It is known that TJ structure is compromised under oxidative stress conditions [71]. Nevertheless, there are few reports on the effects of heavy metals on the expression of renal claudins, occludin and ZO proteins, and the functional consequences. In vitro studies have shown that oxidative stress induced by H2O2 disrupts the TJs of cultured MDCK cells through the activation of G protein α12 [72].
Multiple pathways participate in Cd-induced nephrotoxicity [73]. Cd exposure causes Ca2+ release from the endoplasmic reticulum, and activation of ERK and depolarization of the mitochondrial membrane potential, which induces autophagy and apoptosis, leading to the death of mesangial cells. Thus, the calcium-mitochondria-caspase signaling pathway contributes significantly to the Cd-induced death of mesangial cells [73]. Another mechanism involved in Cd toxicity is the production of ROS, which in turn activates glycogen synthase kinase (GSK-3β) leading to autophagy and cell death. ROS production plays a pivotal role in the pathogenesis of several kidney diseases, and Cd might act on the mitochondria to induce the release of ROS.
Cadmium can selectively damage the TJ in LLCPK1 cultures [34, 35], and in toad epithelial (A6) cells [74], with decrements of TER without affecting cell viability. Jacquillet et al. [33] reported that Cd intoxication alters the renal expression and localization of claudin proteins in rats. The same group reported that exposure of rats to Cd during gestation disrupts claudins 2 and 5 in the kidney of the adult offspring [28]. In conclusion, in utero exposure of Cd leads to toxic renal effects in adult offspring. Cd-induced contraction of both mesangial cells and isolated rat glomeruli occurs through activation of p38 mitogen-activated protein kinase (p38 MAPK) [75]. It has been described that exposure of MDCK cells to CdCl2 caused a robust increase in cellular levels of Hsp70 and eliminated vectorial active transport by proximal tubular cell lines. However, Cd exposure did not induce alterations in occludin expression [76].
Cr6+ compounds are oxidizing agents that directly induce tissue damage. The kidney is the main target for Cr accumulation, which might result in acute tubular necrosis in humans after oral or dermal absorption [77]. ROS production, DNA damage, and apoptotic cell death play a pivotal role in the nephrotoxicity induced by Cr6+ [63]. In a model of acute kidney injury induced by Cr6+, it was shown that oxidative stress disrupts distribution of occludin and claudin 2, and that the treatment with the antioxidant α-tocopherol prevents these changes, through ERK 1/2 pathway [38]. There are reports on the toxicity of Cr on the TJ structure in other organs; Murthy et al. [78] reported that daily administration of Cr6+ in adult rats for 15 days produced significant increases in the blood and testicular Cr levels, and lanthanum perfusion in treated rats revealed leakage at Sertoli-cell TJs.
The effects of Hg are highly dependent on the different chemical forms of this metal. Dental amalgam is a major source of Hg ingestion [79]. In humans and other mammals, kidneys are the primary targets for accumulation of mercuric ions after exposure to elemental or inorganic forms of Hg [80]. In vitro studies have shown that Hg exposure decreased TER in confluent monolayers of podocytes in which IFN-γ and IL-4 expression are increased, which was closely associated with altered immune-reactivity of the TJ protein ZO-1 [52]. Kawedia et al. [81] reported that Hg-treated salivary epithelial cells showed a decreased TER, Hg2+ triggered phosphorylation of occludin via a protein kinase A (PKA) dependent mechanism decreasing TJ expression of occludin and increasing the paracellular permeability. The actions of two Hg compounds, the inorganic HgCl2, and the organic methyl mercury chloride (MeHg), on ion transport across the rat colon, were studied by Bohme et al. [82]; they found that Hg compounds, administered to the luminal side, induced a large, concentration-dependent increase of tissue conductance. The transepithelial movement of the extracellular marker, mannitol, was enhanced in the presence of the Hg compounds, indicating that they cause an increase in the paracellular permeability of the epithelium.
Renal toxicity by Pb was recognized in 19th century; however, the cellular mechanisms involved in the renal damage induced by chronic exposure to this metal remain largely unknown. Navarro-Moreno et al. [40] described alteration of intercellular junction at the proximal tubules of rats intoxicated with Pb for seven months. In addition, they described oxidative damage, glucosuria, proteinuria, and morphological changes in mitochondria, nuclei, lysosomes, and loss of apical microvilli. Environmental Pb intoxication causes irreversible neurological disturbances by mechanisms remaining to be identified. It has been reported that Pb induced a reduction in the expression of occludin in the TJ of the blood-brain barrier of young rats altering the paracellular permeability of this structure [83]. Another study performed by Shi and Zheng [84] showed that early exposure to Pb (prior to the formation of tight barrier) significantly reduced the tightness of blood-cerebrospinal fluid barrier (BCB), as evidenced by reduction in TER and increase in the paracellular permeability of [14]C-sucrose. Exposure to Pb after the formation of tight barrier, however, did not cause any detectable barrier dysfunction. In the same study, Pb exposure decreased both the mRNA and protein levels of claudin 1. Pb exposure, however, had no significant effect on ZO-1 and occludin expression. These data suggest that Pb exposure selectively alters the cellular level of claudin 1, which, in turn, reduces the tightness and augments the permeability of tight BCB.
5. Oxidative and Nitrosative Stress in Kidney
Oxidative stress is a state in which the cell undergoes altered intracellular redox homeostasis; that is, the balance between oxidants and antioxidants is lost. This imbalance is caused by excessive production of ROS and/or deficiency in antioxidant mechanisms. ROS and reactive nitrogen species (RNS) often act together to create a state of oxidative stress. The three molecular targets more susceptible to be damaged by ROS are DNA, lipids, and proteins [85]. Protection against oxygen free radicals involves enzyme activity: catalase (CAT), superoxide dismutases (SOD), glutathione peroxidase (GPx), glutathione reductase (GR), and nonenzymatic (vitamin E, vitamin C, GSH) systems of protection [86–89]. Balanced combination of the actions of antioxidant enzymes such as SOD, GPx, and CAT is very important to achieve a better protection against oxidative stress damage [90].
Oxidative stress has been identified as one of the factors that trigger certain pathologies such as asthma, cardiopulmonary disorders, hypertension, Parkinson, and Alzheimer and plays an important role in the development of kidney diseases [91–94]. A significant imbalance in the activities in pro-oxidants and antioxidants in patients with renal dysfunction has been reported [95]. In patients with acute renal failure, generation of ROS is increased due to a deficiency of the major antioxidant systems. In these patients increased oxidation of proteins occurs in plasma, mainly albumin (carbonylation and disulfide bond formation). Other complications, such as tissue damage, favour severe irreversible kidney damage [96–98].
5.1. Metal-Induced Toxicity and Carcinogenicity by the Generation of ROS and RNS
Metal-mediated formation of free radicals causes various modifications to DNA bases, augmented lipid peroxidation, and altered Ca2+ and sulfhydryl homeostasis. Lipid peroxides, formed by the attack of radicals on polyunsaturated fatty acid residues of phospholipids, can further react with redox metals, finally producing mutagenic and carcinogenic malondialdehyde, 4-hydroxynonenal, and other exocyclic DNA adducts (etheno and/or propano adducts). Whilst Fe, Cu, Cr, Vanadium (V), and Co undergo redox-cycling reactions, for a second group of metals, Hg, Cd and Ni, the primary route for their toxicity is depletion of glutathione and binding to sulfhydryl groups of proteins. Arsenic (As) is thought to bind directly to critical thiols; however, other mechanisms, involving formation of hydrogen peroxide under physiological conditions, have been proposed. The unifying factor in determining toxicity and carcinogenicity for all these metals is the generation of ROS and RNS. Common mechanisms involving the Fenton reaction, generation of the superoxide radical (), and the hydroxyl radical (OH•) appear to be involved for Fe, Cu, Cr, Vanadium, and Co, primarily associated with mitochondria, microsomes, and peroxisomes damage. Nitric oxide seems to be involved in arsenite-induced DNA damage and pyrimidine excision inhibition.
Lipids in the kidney suffer damage due to oxidative stress generated by exposure to certain metals such as Cr (Table 1). Lipoperoxidation is produced in different regions of kidney and it is attenuated by the α-tocopherol [37, 38].
Renal tissue has high requirements for metabolic energy and relies heavily on aerobic metabolism for the production of ATP through oxidative phosphorylation. The reduction of molecular O2 along the electron transport chain within mitochondria is vital for renal cellular function, yet with a potentially devastating long-term effect [99]. Oxidative and nitrosative stress, by reducing the activity of complexes III, IV, and V of the respiratory chain and decreasing ATP levels, might contribute to mitochondrial dysfunction [100]. Mitochondrial injury leads to mitochondria dysfunction and perpetuated oxidative stress. The loss of mitochondrial membrane potential causes release of cytochrome-C and activation of caspase pathways that causes apoptotic detachment of renal cells. Removal of dysfunctional mitochondria through autophagy leads to "autophagic cell death" or apoptosis [101].
Chromium also causes damage to the kidney through oxidative stress. Potassium dichromate administered intraperitoneally to Sprague-Dawley rats for 5 days at doses of 2.5, 5.0, 7.5, and 10 mg/kg body weight per day caused a significant increase of ROS production in both liver and kidney, with increments in superoxide dismutase and catalase activities and DNA damage [63].
Potassium dichromate (K2Cr2O7) is a chemical compound that is widely used in metallurgy, chrome plating, chemical industry, textile manufacture, wood preservation, photography, photoengraving, stainless steel industry, and cooling systems. Its nephrotoxicity is associated with oxidative and nitrosative stress [102].
Cd is a toxic trace metal that is absorbed through lung, gastrointestinal tract, and skin. Following absorption, Cd is taken up by the hepatocytes and also circulates in blood, bound to MT. The Cd-MT complex, due to its small molecular size, is easily filtered through the glomerular membrane and taken up by renal tubular cells. In the cell this complex induces oxidative stress by interaction of Cd with mitochondrial structures and causes renal damage [103]. The intoxication of Cd leads to enhanced lipid peroxidation and protein carbonylation and to decrease of SOD activity and GSH levels [70] (Table 1).
Hg and its compounds are environmental and industrial agents that induce severe nephrotoxicity, both in man and animals. HgCl2 administration is a classic model for the study of the pathogenesis of inorganic Hg toxicity, both in vitro and in vivo systems, as reviewed by Zalups [80]. HgCl2 affects cell oxidative function due to its high affinity for cellular cysteine thiols [104]. Hg nephrotoxicity is characterized by altered antioxidant enzymes [105], lipid peroxidation [106], reduced ATP content [107] and it leads to tubular epithelium necrosis, even after a single exposure [108]. HgCl2-induced damage is strictly dependent on the route of administration, time, and dose [109]. The nephrotoxic dose of inorganic Hg is significantly different in various species. In humans it is fatal in the range of 10–42 mg/kg [110], while in mice the oral lethal dose 50 (LD50) is higher, between 25.9 and 77.7 mg/kg [111]. McDowell et al. [112] reported progressive morphological and functional alterations from 6 h on in the straight portion of rat proximal tubules after a dose of 4 mg/kg HgCl2. After administration to rats of 1.5 mg/kg HgCl2, Girardi and Elias [113] reported an increase in glutathione reductase and peroxidase and reduced urinary volume. Severe tubular necrosis was detected after 16 h in rat proximal tubules and after 48 h in the entire cortical area.
Exposure to Pb is ubiquitous, with the highest levels found in some environmental exposures that have been associated with a number of serious systemic adverse effects involving the nervous system, blood-forming organs, lungs, and kidneys. Pb is also one of the most prevalent and nephrotoxic metals to human. It induces oxidative stress conditions in specific organ targets not only through generation of ROS but also by decrement of cellular antioxidant mechanisms [114] (Table 1).
A proposed mechanism through which heavy metals disrupt the structure and function of the TJ and cause renal dysfunction is shown in Figure 6.
6. Conclusion
Renal toxicity induced by heavy metals is a circumstance that may occur at any age and disrespect of gender. Acute exposition may cause renal failure, but chronic exposure results in a great variety of alterations, depending on the dose and duration of exposure. Occupational and environmental sources are of relevance. Damage induced by heavy metals on the proteins that constitute TJs is under current investigation, as well as the severe physiological consequences. It should be emphasized that most heavy metals depict oxidative stress in the kidney and this is one of the mechanisms involved in disruption of TJs. Therefore, to disclose the role of antioxidants in the prevention of oxidative stress might prove helpful in the prevention of the disassembly of the TJ structure.
This review was partially supported by Consejo Nacional de Ciencia y Tecnologia (CONACyT), Mexico (Grant 0179870). Eduardo Molina-Jijón, Yazmin Debray-García, and Pablo Bautista-García have postgraduate fellowships from CONACyT.
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text:
Nero CaroZiv
A Child Dream - Poem by Nero CaroZiv
In my childhood dream I rush over a hill and over a dale,
Thorough thicket of a bush, thorough a tangle of a brier,
Over spring blooming park, over a mild slope pale,
Thorough winter flood channel streets, thorough dry fields summer fire!
I am pert and nimble in my dream, I do wander everywhere,
Swifter than the moon orbiting sphere; or the fire glare
And I visit the ghostly silhouettes in the secret forest garden
To dew the wild rose orbs without asking the night wide staring awl's pardon
The cowslips in the garden have the night flies courtship
In their gold glaring lights you see;
And those ruby colored sprouts up hill reign like lordship
In their spotted freckles live their savors to be;
And then at the end of my dream I seek some dewdrops here,
And hang a pearl in every empty cowslip's ear.
Copy rights 2010
All rights reserved
Topic(s) of this poem: dream
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subset: commoncrawl


text:
DC Heroes get the spud treatment this year
The annual Toy Fair is happening this week in New York, and with it comes all sorts of announcements and pictures of various toys you'll be able to buy later this year, from Avengers Legos to Back to the Future hoverboards.
And joining Spider-Man, Elvis, Optimus Prime, R2D2 and a whole bunch of other pop culture icons in becoming spuds are Batman, Superman and Wonder Woman, who get the Mr. Potato Head treatment this year. According to USA Today, the trio of tators is just the tip of the iceberg, as more DC heroes and villains are set to become Potato Heads next year.
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Senior Member
Thanks Meter: 3
I tried to add some apps to a clean official 2.1 generic rom!
I added all to data/app,but tried system/app,too and then signed
the rom again!
But if I flashed the rom not all apps shown at phone!
What is the problem,why some apps works and other not?
Rootexplorer,Titanium Backup,Autokiller and Linpack works,but
Tapatalk Pro and Zeam Launcher or other Launcher don`t works!
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Halter Tie-Dyed Backelss One-Piece Swimwear - Brown - S
Swimwear Type: One Piece
In the sea or on the beach, the time when a girl gets to show of her figure the most is when she is wearing a sexy one piece swimsuits. Created for the purpose of swimming, cute one piece bathing suits are now more of a fashion accessory, and their main purpose being to look good and attract attention.
How To Choose
Since we have over 1000 different kinds of one piece swimsuits, it is not an easy way of not being dazzled. The most common type is black one piece swimsuit. However, even just the most common type of black one piece, we have a lot embellishments for it. Trust us: even the most basic among these styles will speak volumes. Thanks to alluring upgrades such as unlined halter, cutout frilled and push up details, these cheap one piece swimsuits in Rosegal are designed to flatter every body type combining specific personal style. For those who opt for girly types, cute one piece bathing suits including crochet one piece swimsuit and flounce one piece swimsuit should be in your wardrobe. As for bold ladies, we have sexy one piece bathing suits like low back one piece swimsuits or backless one piece swimsuits. Of course, deep V collar or monokini swimsuits are our best sellers as well. If you are worried about our sizes, we can proud to tell that our plus size one piece swimsuits have launched! And no matter you want the push up monokinis or wireless one piece cutout swimsuit, we provide the most comfortable and the chicest style you'll need.
Take a plunge and perfect your poolside look with Rosegal's one piece bikini collection!


text:
Ideally potatoes chosen for processing
should have a high specific gravity/ percent solids as this
will minimize the volume of water that needs evaporating,
but the reality is that these potatoes are frequently selected
for the production of chips and French fries leaving a mixed
feed stock for the dried potato manufacturers to work with.
Dryers are typically set up to ensure that the wettest of
product is dried sufficiently for shipment, but this results
in over drying the bulk of the product costing the processor
twice; in terms of unnecessary energy usage and in lost sales
Dehydrating Process
Potatoes are cleaned, skinned, sliced, blanched, cooled, washed
and steam cooked for 15 minutes to an hour depending on the
% solids, prior to drum drying(dehydration).
Measurement Location & Gauge
The MCT NIR gauge is typically located above the collection
conveyor for the potato flakes that peel off the drum. NIR
is a surface biased measurement. In order to obtain representative
gauge readings, the gauge should be viewing a minimum bed
depth of 0.5" of well mixed product.
Owing to the large variance in the % solids of the incoming
potatoes, a moisture swing of +/- 3% in product exiting the
dryers is not uncommon if the dryers aren't automated.
With Process Sensors NIR meters, moisture can be continually
measured and the information used in a feedback loop to control
the drum speed of the dryer. The uncontrolled +/- 3% swing
can be reduced to +/- 0.5% . With moisture variation minimized,
the moisture set point can be raised resulting in increased
production volume and reduced energy costs.
460: Used in a wide variety
of industrial processes, the MCT 460 Multi-Constituent NIR
Transmitter, a True Stand-Alone Transmitter, provides off-line
accuracy under on-line operating conditions.
more about the MCT 460 moisture sensors...
460-T: The MCT 460-T measures
moisture... the most critical variable in tobacco processing...
providing the world's most accurate, reliable and stable moisture
measurement for the tobacco industry.
more about the MCT 460-T moisture measurement tool...
460-WP: The MCT 460-WP uses
a well proven algorithm which works on a wide range of wood
varieties and forms. You choose the mode of operation, stand-alone
or networked.
more about the MCT 460-WP moisture measurement tool...
466HD: The MCT466HD features
a heavy duty stainless steel enclosure, is resistant to high
temperatures, is sealed for wash down environments, and provides
reliable, continuous moisture measurement.
more about the MCT466HD moisture measurement tool...
466-SF: The MCT 466-SF features
a food grade optical window, a robust stainless steel enclosure,
and reliably operates in high temperature locations, measuring
moisture, oil & product temperatures.
more about the MCT 466-SF moisture measurement tool...
466 QuikCheck: Used for quality
control in a wide variety of industrial processes, the MCT
466 QuikCheck bench top NIR Tester provides fast accurate
results every time.
more about the MCT 466 QuikCheck moisture analyzer...
The Guardian Scanning Frame System is comprised of three components:
MCT-460 NIR Transmitter, the automated Scanning Frame and
a PC with Automated Guardian Profile Software.
more about the GUARDIAN Scanning Frame System...


text:
Bakugan Battle BrawlersIt is time to fight against Mascarade and Hal-G with Dan and his bakugan, Drago who are fighting for freedom. In Bakugan Battle Brawlers, you will play Dan and his bakugan, Dragonoid more well-know as Drago. By the way, Drago is in this game at the first step of evolution. Who is fighting Dan? Mascarade or somebody else who is able to send much bakugans against Drago. It is you to discover it in 10 levels.On Bakugan Battle Brawlers , you play Drago with your keyboard.
You like Bakugan Battle Brawlers? Share it with your friends !!
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text:
In international test results released this week, 15-year-old students in Florida scored worse than their peers in the United States overall and much worse than their counterparts in the world's best-performing schools in Asia. In some categories, Florida students trailed students from countries like Poland and Vietnam. Is Florida's sub-par performance a sign that public education in the state is failing? Or is it a mistake to draw conclusions from this one test -- especially since a greater share of disadvantaged students take it in Florida? Do you think Florida schools are on the right track to improve? Talk about it!
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text:
Development of New High-Strength Carbide-Free Bainitic Steels
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An attempt was made to optimize the mechanical properties by tailoring the process parameters for two newly developed high-strength carbide-free bainitic steels with the nominal compositions of 0.47 pct C, 1.22 pct Si, 1.07 pct Mn, 0.7 pct Cr (S1), and 0.30 pct C, 1.76 pct Si, 1.57 pct Mn, and 0.144 pct Cr (S2) (wt pct), respectively. Heat treatment was carried out via two different routes: (1) isothermal transformation and (2) quenching followed by isothermal tempering. The results for the two different processes were compared. The bainitic steels developed by isothermal heat treatment were found to show better mechanical properties than those of the quenched and subsequently tempered ones. The effect of the fraction of the phases, influence of the transformation temperatures, the holding time, and the stability of retained austenite on the mechanical properties of these two steels was critically analyzed with the help of X-ray diffraction, optical metallography, scanning electron microscopy, and atomic force microscopy. Finally, a remarkable combination of yield strength of the level of 1557 MPa with a total elongation of 15.5 pct was obtained.
1 Introduction
Carbide-free bainitic steels are emerging as a very promising class of steels, because they exhibit a remarkable combination of high strength with good ductility and can be produced at low costs.[1] The mechanical properties of carbide-free bainitic steels were found to supersede those of quenched and tempered alloys of the same hardness because of their interesting microstructure consisting of thin plates of bainite in a matrix of retained austenite. This microstructure can be generated by quenching to a certain temperature followed by a simple isothermal holding treatment route. The strength of these steels is due to the small length scale of the microstructure (relative fineness of bainite lathes) and high dislocation densities, whereas ductility was attributed to the film of retained austenite present in between the bainitic lathes.[2, 3, 4] The detrimental effect of carbides is overcome by suppressing their precipitation with the addition of a sufficient amount of silicon,[5,6] and the thin film of austenite acts as a cushion for any crack propagation.[7,8] A carbon-enriched austenite results in increased stability and thereby helps in minimizing the fraction of retained austenite transforming to martensite.[9] A series of bainitic steels with strengths more than 1200 MPa along with an elongation of 13 pct can be used as high performance alloys possessing good toughness and tribological properties.[10] The design criterion for the development of very strong bainitic steels lies in the engineering of metastable temperature (To) where the free energies of austenite and ferrite are the same. Coarse blocks of retained austenite can be avoided by shifting the To curve to higher carbon concentrations in austenite.[10, 11, 12] One of the best combinations of strength and toughness was achieved in a bainitic steel exhibiting 1600 MPa and 130 \({\text{MPa}} \; \sqrt {\text{m}}\), respectively, by meeting this criterion.[7,8] Chakraborty et al.[13] recently showed improved mechanical properties of 52100 steel (1.1C, 1.46Cr, 0.27Si, 0.33Mn, 0.14V, 0.04Ni, 0.02P, balance Fe, all in wt pct) with a dual-phase structure made of bainite and martensite over quenched and tempered structure by adopting the austempering route. Some prior cold deformation was given to refine the thickness and size of the bainitic sheaves. A maximum strength of 2250 MPa with an impact strength of 72 J was achieved. Brown and Baxter[14] found the isothermal transformation characteristics of a high carbon and high silicon steel, which exhibited a tensile yield stress of 1673 MPa with an elongation of 8 pct. They designated them as super bainitic steels, which are virtually free of any carbides, and the sheaf thickness is very fine due to the low transformation temperatures of 473 K to 523 K (200 °C to 250 °C).
A recent trend in the field of bainitic steels is to develop carbide-free novel bainitic structures.[15,16] The cementite particles in bainitic steels are detrimental to the properties as they are the source of crack and void nucleation.[10] In the novel bainitic structures, these cementite precipitates are avoided by adding inhibitors such as Si and Al. It was shown[17] that an addition of more than 1.5 wt pct Si results in carbide-free bainitic steels. Caballero and Bhadeshia[5] have worked on designing novel high-strength steels and have successfully produced a mixed microstructure consisting of fine plates of upper bainitic ferrite separated by a thin film of retained austenite. They were able to achieve strength of 2500 MPa and a toughness of 30 to 40 \({\text{MPa}} \; \sqrt {\text{m}}\) at a transformation temperature of 398 K (125 °C). With careful design, Caballero et al.[16] were able to achieve an impressive combination of a toughness of 130 \({\text{MPa}} \; \sqrt {\text{m}}\) with a strength of 1600 to 1700 MPa. However, the full benefit of carbide-free bainitic steels is yet to be realized. The alloy design and its incomplete reaction phenomena can further be explored to obtain more impressive properties. Present work investigates two steels with different compositions. They were treated isothermally at different temperatures as a function of time to form bainitic microstructure with different morphologies. The steels were also quenched to obtain martensite and subsequently tempered to obtain hardnesses similar to those of bainitic steels formed by isothermal treatment, and their mechanical properties were compared. The objective of the present work is to develop a structure-property correlation of carbide-free bainitic steels. It is shown that high strength in combination with high ductility can be obtained by suitable heat treatments.
2 Experimental Procedure
Chemical Composition of the Investigated Steels S1 and S2
Element (Wt Pct)
Bs [K (°C)]
Ms [K (°C)]
833 (560)
563 (290)
765 (492)
579 (306)
Different Heat Treatment Cycle Followed for Both the Steels S1 and S2
Isothermal Transformation in Salt Bath
Soaking Temperature and Time
Salt Bath Temperature [K (°C)]
Holding Time (Min)
1173 K (900 °C) for 20 min
573 (300)
623 (350)
1173 K (900 °C) for 20 min
598 (325)
623 (350)
648 (375)
673 (400)
Water quenching followed by isothermal tempering
Soaking Temperature and Time
Tempering Temperature and Time
1173 K (900 °C) for 20 min
673 K (400 °C) for 60 min
1173 K (900 °C) for 20 min
623 K (350 °C) for 90 min
The amount of retained austenite in the heat-treated samples was determined by X-ray diffraction using Cr Kα radiation (wavelength = 2.2909 Å) in a Siefert ISO-DEBYEFLEX 1001 diffractometer (RICH SEIFERT & Co. GmbH & Co. KG, Ahrensburg, Germany) operated at 40 KV and 30 mA fitted with a graphite monochromator. The diffracting angle (2θ) was scanned from 60 to 130 to include the strongest reflection from ferrite. The scan rate was 3 deg per minute with a step size of 0.05. The optical metallography of the steel samples was carried out in a Zeiss optical microscope. Scanning electron microscopy (SEM) was carried out using an FEI Quanta scanning electron microscope (FEI Co., Hillsboro, OR) operated at 20 kV using the secondary electron mode. In the case where martensite co-existed with bainite, quantitative analysis was done using image analyzer software. Atomic force microscopy (AFM) was also carried out to clearly reveal the ultrafine morphology of the heat-treated samples. There was also a need to see any other phases or carbides present in the microstructure. The AFM was done in the contact mode with a Solver Pro NT-MDT atomic force microscope (NT-MDT Co., Moscow, Russia) at a frequency of 1 KHz. The samples for metallography as well for AFM analysis were mechanically ground using emery paper down to mesh number 800, followed by cloth polishing with 5-μm and then 1-μm alumina paste. They were etched with 2 pct nital to reveal the microstructure in optical microscopy and SEM. For AFM, these samples were further polished with 0.1-μm diamond paste and etched with 2 pct nital. Hardness measurements were done in a Rockwell hardness tester in C scale using 150 kg load and a Brale indenter (Wilson, American Chain & Cable Co., New York). Tensile tests on the steel samples were carried out in a LR 10 K Lloyds Tensile Testing Machine (Lloyd Instrument Ltd., West Sussex, UK). The crosshead speed was kept at 0.2 mm/min to maintain a strain rate of 2.66 × 10−4 s1. The samples were made according to the ASTM E8M standards for subsize round specimen.
3 Results and Discussion
- (1)
With the increase in the transformation temperature, the sheaf thickness increases for both steels S1 and S2. Holding duration does not produce any significant variation.
- (2)
The volume fraction of bainite increases with the increase in holding duration.
Summary of the Best Mechanical Properties Attained with Their Processing Conditions
YS (MPa)
UTS (MPa)
Elongation (Pct)
Hardness (HRC)
Steel S1
Air cooled
Q&T (673 K (400 °C), 60 min)
Isothermal transformation at 623 K (350 °C) for 30 min
Steel S2
Air cooled
Q&T (623 K (350 °C), 90 min)
Isothermal transformation at 623 K (350 °C) for 30 min
The optical micrographs for S1 at 623 K (350 °C) for different holding durations in Figures 2(a) through (d) show the variation in the amount of the phases present. As concluded earlier, with the progress of the isothermal transformation, the volume percent of bainite is found to increase, as shown in Figure 8(a). In steel S2, the bainite volume fraction is greater compared to that of steel S1, and this is expected due to its faster kinetics. As the weight percent of carbon increases in the retained austenite, it becomes enriched in carbon, which is more stable to martensite transformation. The hardness values for S1 in Figure 11(a) decrease with an increase in holding duration due to the decrease in the volume fraction of martensite, which is formed from the untransformed austenite during quenching (Figure 8(a)) and formation of upper bainite. The hardness trend in Figure 11(b) for steel S2 shows a variation according to the bainite volume fraction and the scale of the microstructure, as indicated by the relative size of bainite lathes with respect to the micron bar (Figures 5, 6, and 10). With increasing length scale of microstructure because of increasing temperature, the hardness was observed to decrease. It is evident from Figure 13(a) that the YS decreases with the increase in the isothermal holding temperature. Also, it can be seen that there is a decreasing trend with increasing holding duration after the initial increase in YS and UTS. It can be seen that remarkable values of YS (1800 MPa) in the case of the sample of S1 heat treated at 573 K (300 °C) for 30 minutes were achieved but with a compromise in the total elongation (~6 pct). The high strength level can be attributed to the formation of a large amount of martensite due to slow transformation rates at the lower isothermal holding temperature. The improvement in ductility over the quenched and tempered sample (Table III) is due to the presence of the softer bainitic phase.
Large elongation in combination with relatively lower YS and UTS of the samples of the steel S2 heat treated at higher isothermal holding temperatures [648 K and 673 K (375 °C and 400 °C)] is attributed to the presence of a large amount of retained austenite, as indicated in Figure 8(b). A YS of 1550 with a total elongation of 15.5 pct was achieved on the sample of steel S2 heat treated at the isothermal temperature of 623 K (350 °C) with a holding time of 30 minutes. This marks the optimization of strength and ductility. This result can be attributed to the presence of the optimum amount of retained austenite present in the microstruture in the form of film, which governs the ductility obtained in the steel.[23, 24, 25] Retained austenite plays a role in blunting propagating cracks and reducing their effective fracture size; thus, it contributes to ductility and toughness.[12,15] The strength is a function of the scale of the microstructure, as indicated earlier. In both steels, it is evident that at higher transformation temperatures, the strength values decreased. This can be attributed to two factors. Primarily, it would be a function of the sheaf thickness. At higher transformation temperature, the sheaf thickness can be seen to be greater for both steels in Figures 3 and 5; hence, the strength values decreased. Second, the increasing volume fraction of bainite at higher transformation temperatures results in a decrease in the strength.
The preceding observation can be schematically shown in Figure 15, where the relative positions of the isothermal heat treatments were marked on the time-temperature-transformation (TTT) diagram. Bs and Ms temperatures for both steels are indicated on the diagram. It is very clear that the bainite that forms in steel S1 at the higher isothermal holding temperatures is coarser and the fraction of martensite that is formed from the untransformed austenite is reduced (Figure 3) compared to the sample heat treated at 573 K (300 °C). This leads to the lowering of the hardness and strength of the steel, as observed in Figure 11(a). It can even be seen that the microstructure of steel S1 due to isothermal heat treatment at a constant temperature contains a progressively lesser amount of martensite because of prolonged holding (Figure 2). This relates to the schematic too. Prolonged holding at the same isothermal temperature leads to lowering of the fraction of untransformed austenite, which transforms to martensite on quenching at the end of holding time. It is intersting to note that the fraction of retained austenite is maximum in the sample isothermally treated at 673 K (400 °C) (Figures 5 and 6) for steel S2. However, the fraction of retained austenite in the samples isothermally treated at 648 K and 623 K (375 °C and 350 °C) are almost on the same level, as seen from Figures 5(b) and (c) and Figures 6(b) and (c), whereas the fraction of retained austenite in the sample isothermally treated at 598 K (325 °C) again increased. This observation in microstructure is clearly associated with the position of the holding temperatures, as indicated in Figure 15. The optimization in the microstructure on the basis of fraction of bainite, retained austenite, and length scale of microstructure is achieved when the isothermal treatment temperature is 623 K (350 °C) for a holding period of 30 minutes. This is also reflected with the appearance of peak in the YS and UTS in Figures 14(b) and (c) at the same temperature and time. It is interesting to note that the fraction of untransformed austenite at the end of the isothermal treatment transforms to martensite in the case of steel S1, whereas it remains as retained austenite in steel S2. This is also clearly evident in Figures 9 and 10, where the AFM images show the presence of martensite in steel S1 and retained austenite in steel S2 in addition to the major fraction of bainite. This could be related to the fraction of untransformed austenite as well as the composition of two different steels, but it is not understood fully. However, carbide was not observed during any of the treatments given to both steels. This must be related to the higher content of silicon in both cases. The same observation was also noted earlier, where Si acts as inhibitors for the precipitation of carbide.[18] It is well understood that bainite rejects carbon to the remaining austenite during its formation.[5,26] The carbon percentage in the untransformed austenite at the end of isothermal transformation in the case of S2 is suppressed higher than that in the samples of S1. This is also evident in Figure 15, where it was observed that the formation of bainite is greater in the case of steel S2 compared to steel S1 at all the isothermal temperatures (Figure 8(a)).
The quenching and subsequent tempering treatment showed a best result of 1166 MPa as YS and 1313 MPa as UTS with a total elongation of 11.5 pct for steel S1 (Table III). However, these values are significantly lower than those obtained by isothermal transformation to bainite. The best result obtained for this steel from the isothermal transformation route is YS of 1330 MPa, UTS of 1517 MPa, with a total elongation of 12 pct. For steel S2, the quenching and tempering treatment shows a best result of YS of 1440 MPa and UTS of 1560 MPa with an elongation of 12 pct (Table III), while the isothermal transformation route resulted in a YS of 1557 MPa and UTS of 1656 MPa with a total elongation of 15.5 pct. Exceptional mechanical properties obtained in the case of steel S2 are better than some of the steels developed by other investigators.[7,8,14] The toughness for steel S2 was calculated empirically to be 236 \({\text{MPa}} \; \sqrt {\text{m}}\).[27] The combination of strength of 1557 MPa and toughness 236 \({\text{MPa}} \; \sqrt {\text{m}}\) is more impressive than a combination of the strength of 1600 MPa and toughness of 130 \({\text{MPa}} \; \sqrt {\text{m}}\) reported in the literature.[16,22] The quenching and subsequent tempering heat treatment cycles for both steels are shown by the dotted line in Figure 15. Such a remarkable combination of mechanical properties is attributed to the bainite morphology.
4 Conclusions
Structure-property correlations were developed for the two steels (S1 and S2) heat treated at different temperatures and time. The mechanical properties are a strong function of bainite, retained austenite, and martensite content in the steels. The volume fraction of bainite was found to increase with time at an isothermal holding temperature, and the volume fraction of the same is also closely associated with the position of isothermal temperature with respect to the TTT diagram. Finally, the mechanical properties of both the steels are also a strong function of the constituents (bainite, retailed austenite, and martensite) and length scale of the microstructures or finer bainite lathes. The addition of silicon resulted in carbide-free bainitic steels. Impressive results (the combination of strength of 1557 MPa and toughness 236 \({\text{MPa}} \; \sqrt {\text{m}}\)) were obtained for intermediate temperature of transformation due to the optimization in the fraction of bainite and retained austenite. Finally, it can be concluded that the isothermal quenching temperature of 623 K (350 °C) with a holding duration of 30 minutes was found to be the best processing condition for high silicon steel (S2).
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