genetically modified animal models are an important part of medical research . by inducing conditions similar to human diseases in animals , pathophysiologic hypotheses and therapeutic strategies can be tested and evaluated . the endothelin ( et ) system plays a complex role in physiologic and pathophysiologic functions of the body . in order to analyse the role of single components of the endothelin system in specific organs and tissues the et system consists of the three peptides et-1 , et-2 and et-3 , their g - protein - coupled receptors endothelin receptor a and b ( etra and etrb ) and the two endothelin - converting enzymes ( ece-1 and ece-2 ) which convert the precursor big - ets to the biologically active forms . its different components are often counteracting in an autocrine and paracrine way . because of its strong vasoconstrictive effect the et system plays a role in the development of hypertension and atherosclerosis . et has also been shown to be involved in states of portal hypertension and acute and chronic renal failure . in the following , this article outlines the genetically modified animal models , which have been established in order to obtain a better understanding of the et system . a model in which the human et-1 gene under the control of its natural promoter was transferred into the germline of mice was established independently by two research groups . both groups observed similar results : the over expression of the et-1 gene creates a phenotype with characteristic , age - dependent pathologic alterations of the kidney whilst blood pressure remains normal . the most evident effects of the elevated plasma and tissue concentrations of et-1 are the development of renal interstitial fibrosis , renal cysts and glomerulosclerosis , leading to a progressive decrease in glomerular filtration rate in aged mice . in these transgenic models the profibrotic effect of et-1 is most ostentatious whereas hypertension can only be induced by salt - loading . this was unexpected since previous studies showed et-1 to be an potent vasoconstrictor when administered intravenously [ 4 - 7 ] . a bolus injection of the nitric oxide synthesis inhibitor l - name induces an exaggerated hypertensive response in et-1 transgenic mice as well as in et-2 transgenic rats . this suggests that the nitric oxide system as a vasodilator system is involved . in an animal model of crossbreds of et-1 transgenic mice ( et+/+ ) and mice in which the gene for the endothelial nitric oxide synthase ( enos ) was knocked out   this assumption was supported . compared to the normotensive wild type and et+/+ , the crossbred et+/+ enos ko have a significantly elevated blood pressure . this is also seen in et+/+ inducible nos ko crossbreds   which corroborates the hypothesis that the no system counterregulates the et overexpression in the et-1 transgenic model leading to a normotensive phenotype . the et-1 transgenic model also brought out some interesting findings about the role of endothelin in the etiology of pulmonary diseases . the development of pulmonary fibrosis and chronic lymphocytic inflammation in the absence of pulmonary hypertension was observed in the transgenic animals , suggesting et-1 to be involved in the pathogenesis of lung fibrosis and bronchiolitis . endothelium - restricted overexpression of human et-1 does not result in elevated blood pressure , however altered vascular structure and function in resistance vessels can be observed . this is a sign of blood pressure independent effects of endothelium - generated et-1 on vasculature . conditional cardiac overexpression of endothelin-1 induces inflammation and dilated cardiomyopathy in mice indicating that et-1 acts as a proinflammatory molecule in the heart . in rats overexpressing the human et-2 gene under control of its natural promoter a blood pressure - independent fibrotic remodelling of the kidney this is most probably due to the preferential expression of the transgene within the glomeruli in et-2 transgenic animals , whereas the transgene is ubiquitously expressed within the entire kidney of et-1 transgenic mice . a tetracyclin - responsive melanocyte - specific lineage transgenic system was created and used to conditionally regulate the overexpression of et-3 in different stages of melanocyte development . the activation of the transgene in mice leads to a hyperpigmented phenotype both in embryonic development and in mature mice . homozygous et-1 ko mice die within 15 - 30 minutes after birth due to craniofacial abnormalities resulting in an inability to breath normally . these affected tissues and organs are predominantly formed by cranial neural crest - derived ectomesenchymal cells . no abnormalities in other organs such as the lung , kidney and central nervous system can be observed . similar craniofacial and cardiovascular malformations are seen in mice with disruption of the etar gene . the important role of et-1 in skeletal development during intrauterine life was recently confirmed by using prepro - et-1-lacz - transgenic mice . the examination of expression patterns of etars and et-1 suggests that et-1/etar interaction is essential in cranial bone and connective tissue formation as well as in the development of the heart and its outflow tract . heterozygous et-1 ko mice which have lower serum levels of et-1 than wild type mice paradoxically develop elevated blood pressure . enhanced basal and hypercapnia - induced renal sympathetic nerve activity in the heterozygous et-1 ko mice seem to be involved in elevation of blood pressure . some other causes that might contribute to blood pressure elevation in heterozygous et-1 ko mice cd et-1 ko mice are hypertensive but show no differences in body weight , urine volume , creatinine clearance , sodium and potassium excretion , urine and plasma osmolality , plasma aldosteron concentration and renin activity . cd et-1 ko mice are salt - sensitive and exhibit reduced sodium excretion during the first three days of high salt diet . amiloride and furosemide are able to prevent sodium retention and exacerbation of hypertension . however , they do not reduce blood pressure in cd et-1 ko mice on a normal sodium diet . plasma vasopressin ( avp ) concentrations are substantially reduced in cd et-1 ko mice , despite all other aspects of water metabolism being similar . the response to continuous infusion of 1-deamin-8-darginine vasopressin is stronger in cd et-1 ko mice . there seems to be an increased renal sensitivity to the effects of avp suggesting that et-1 acts as a physiological autocrine regulator of avp action in the collecting duct . thus , cd - derived et-1 seems to be an important physiological regulator of renal salt and water handling and systemic blood pressure . cd - specific ko of the etar has no effect on blood pressure or urinary sodium excretion in mice , independently of salt intake . those mice have increased plasma avp levels but do not show differences of renal water excretion under baseline conditions . however they have a more rapid decrease in urine osmolality following an acute water loading . during exogenous avp infusion cd etar ko mice increase urine osmolality similar to wt mice but have a more rapid subsequent fall in urine osmolality during sustained avp administration . the lower avp responsiveness in cd etar ko mice is contradictory to the results seen in cd et-1 ko mice . these animals have elevated avp sensitivity , and impaired ability to excrete an acute water load . this implies that et-1 must exert its influence on vasopressin through cd etbr and/or through paracrine effects . at 6 month of age mice with disruption of the et-1 gene in cardiomyocytes ( cm et-1 ko ) have a significantly lower fractional shortening and develop a dilated left ventricle ( lv ) . during the next month the left ventricle systolic function of cm et-1 ko mice declines and ultimately , they die at much younger ages ( median life expectancy of cm et-1 ko : 11 months , wt : 2 years ) . histological characterization of the cm et-1 ko hearts reveal dilated heart chambers with heterogeneity of myocyte size , increased fibrosis and raised apoptosis . at the age of 2 months , aortic banding leads to dilated cardiomyopathy in cm et-1 ko mice but to lv hypertrophy in wt mice . transcriptional and western blot analyses suggest that increased apoptosis in cm et-1 ko is mediated by enhanced activity of tumor necrosis factor ( tnf ) . cm et-1 ko hearts also have diminished nf-b activity , amounting to diminution of downstream inhibitors of tnf signaling . cm et-1 ko mice are resistant to the hypertrophic stress of treatment with thyroid hormone . while mice with intact et-1 gene develop a pronounced lv hypertrophy , cm et-1 ko mice show only a marginal increase in lv mass . these findings indicate that the interaction of locally produced et-1 with thyroid hormone , is essential in the development of thyroid hormone induced myocardial hypertrophy . et-3 ko mice exhibit aganglionic megacolon and pigmentary disorders of the skin and choroidal layer of the retina . these findings indicate that et-3 is essential in the development of two neural crest - derived cell lineages , epidermal and choroidal melanozytes and enteric ganglion neurons . blood pressure and heart rate of infant et-3 ko mice are not different from those in age matched wt mice . these results suggest that et-3 is not involved in cardiovascular regulation at least in early life before they die of complications of the megacolon . phenotypes of animals with natural mutations or with a targeted disruption of the etbr gene are similar to that of et-3 ko animals . this underlines the importance of et-3/etbr - mediated signaling in the development of melanocytes and enteric neurons . a naturally occurring rodent model of the described phenotype is the spotting lethal ( sl / sl ) rat which carries a deletion of the etbr gene . to establish a viable animal model it was necessary to support a normal enteric nervous system development . etbr deficient sl / sl rats were rescued by breeding them with rats harboring a dopamine - betahydroxylase ( dbh)-etbr transgene . the resulting transgenic rats ( dbh - etbr ; etbrsl / sl ) are healthy but present with total absence of etbr in all non - adrenergic tissues including the kidneys . those etbr - deficient rats are normotensive on a sodium - deficient diet , mildly hypertensive on a standard diet and exhibit severe hypertension on a high - sodium diet . their endothelial function is impaired but it was shown that it is independent of the salt - enriched diet and therefore not responsible for the hypertension . in young sl / sl rats fractional sodium excretion is markedly reduced . when treated with the specific epithelial sodium channel blocker amiloride this seems to be due to a lack of the inhibitory property of the etbr on the epithelial sodium channel activity . this data indicates that a reduced renal etbr activity might contribute to a salt - sensitive hypertension . the effect of the etbr on the progression of diabetic nephropathy was analyzed by rendering the transgenic animals diabetic with streptozotocin . these animals develop severe hypertension and show an enhanced functional renal impairment compared to diabetic wt animals . further analyses suggested the elevated plasma levels of et-1 to be responsible for the hypertension and not the renin - angiotensin system or the no system . in a model of endothelial cell - specific etbr ko , the animals showed endothelial dysfunction but no hypertension in response to a high salt diet . this points out that the salt - induced hypertension seen in the model of rescued etbr - deficient rats is not mediated by endothelial cells . taken together with the results from the cd - specific et-1 ko , these findings point out an et-1/etbr axis which has influence on systemic blood pressure at least partially mediated through autocrine inhibition of cd sodium reabsorption . yet it is not only the renal etbr which is crucial for the hypertension since etbr deficient mice with transplanted normal kidneys are hypertensive as well . the highly specific et converting - enzymes ( eces ) are membrane - bound metalloproteases which catalyse the cleavage of a trp21 bond in the precursor big - endothelins resulting in active endothelins . two isoenzymes ece-1 and ece-2 have been identified   and their role in the et system was investigated by creating ko models . most of the ece-1 ko animals die in utero because of an embryonic lethal phenotype of cardiac abnormalities . those severe defects of patterning of the great vessels and formation of the outflow tract are not found to this extent in other ko models of the endothelin system . the surviving mice have craniofacial abnormalities strikingly similar to those of the et-1 ko and etar ko animals . the animals also present with an absence of epidermal melanocytes and enteric neurons of the distal gut , the phenotype known from the et-3 ko and etbr ko mice . this combined phenotype corroborates the assumption that ece-1 by activating both et-1 and et-3 in vivo has an influence on the development of distinct subsets of neural crest cell lineages . the tissue levels of et-1 , et-2 and et-3 were measured as whole embryo extracts and compared to those of wt animals . et-3 is markedly reduced whereas the levels of et-1 and et-2 are still about 50% of the wt mice . however , biologically active et seems not to be sufficiently produced at the sites crucial for normal embryonic development . unlike the ece-1 ko mice , animals with a homozygous ko of ece-2 show no embryonic developmental disorders . a cross breeding of the two knockout lines brought out animals with a lack of both ece genes . these ece1 -/- ; ece2 -/-animals show a similar but more profound embryonic ece-1 ko phenotype with the same amount of et-1 and et-2 as in the ece-1 ko suggesting yet another protease to be also responsible for the activation of ets . taken together , results from the different transgenic and knockout models disclose that the et system plays a major role in embryonic development . two et system - dependent neural crest - driven developmental pathways become obvious : one of them being an et-1/etar axis , responsible for normal cardiac and cranial development ; the other seems to be a et-3/etbr mediated signalling pathway . these findings led to the discovery of similar findings in humans with hirschsprung disease . in adult life the et system is most important in the renal and cardiovascular system as shown by a chronically activated et system which results in a blood pressure - independent fibrosis of kidneys and lungs . especially collecting duct - derived et-1 seems to have influence on the physiologic regulation of renal salt and water handling and systemic blood pressure via etbr . the creation and investigation of genetically modified animals has contributed significantly to the understanding of et function in vivo .
shortly after discovery of et-1 in 1988 , the entire endothelin system was characterized . 
 the endothelin system consists of the three peptides et-1 , et-2 and et-3 , their g - protein - coupled receptors endothelin receptor a and b ( etra and etrb ) and the two endothelin - converting enzymes ( ece-1 and ece-2 ) . 
 genetically modified animal models are an important tool in biomedical research . here 
 we describe the key findings obtained from genetically modified animal models either over - expressing compounds of the et system or lacking these compounds ( knockout mice ) . 
 results from the different transgenic and knockout models disclose that the et system plays a major role in embryonic development . 
 two et system - dependent neural crest - driven developmental pathways become obvious : one of them being an et-1/etar axis , responsible for cardio - renal function and development as well as cranial development ; the other seems to be an et-3/etbr mediated signalling pathway . 
 mutations within this axis are associated with disruptions in epidermal melanocytes and enteric neurons . 
 these findings led to the discovery of similar findings in humans with hirschsprung disease . in adult life 
 the et system is most important in the cardiovascular system and plays a role in fibrotic remodelling of the heart , lung and kidney as well as in the regulation of water and salt excretion .