g protein  coupled receptors ( gpcrs ) are nodal regulators of mammalian cell physiology because they transduce cell signals from diverse ligands such as neurohormones , sensory stimuli , and ions through heterotrimeric g proteins . in the heart , they represent the major modulators of both function and morphology with -adrenergic receptors ( ars ) representing  the heads of the line  , and for this reason they are considered the most important molecular targets in the cardiovascular system . currently , 3 ar subtypes ( 1ar , 2ar , and 3ar ) have been identified in the myocardium , with 1 and 2ars the most expressed and studied . however , since its discovery in 1989 , it soon seemed clear that 3ars , the isoform with minor expression , can influence cardiovascular physiology . in particular , 3ars seem to have multiple roles that go from regulation of metabolism , vasodilation , and relaxation to cardiac contractility . thus , this receptor is of high interest especially for new potential therapeutic approaches for heart disease . in this review , we will discuss what is known about the cardiac role of 3ars and how not only their activation but also the blockade could be beneficial or not in cardiac physiology and in disease . the mammalian 3ar sequence consists of about 400408 amino acids in a protein that has the typical structure of all gpcrs . the 3ar has 7 transmembrane domains ( 7-tmds ) with an extracellular n - terminal that is glycosylated , and an intracellular c - terminal domain . further , the cys361 residue in the fourth intracellular domain is palmitoylated , a feature that has been shown to be associated with g protein - coupling and adenylyl cyclase stimulation following agonist stimulation of the receptor ( fig . as shown in figure 2 , the protein sequence alignment between different mammalian species demonstrates that most of the homology between the 3ar amino acid sequences is concentrated in the 7-tmds and in the membrane - proximal regions of the intracellular loops . interestingly , when the 3ar protein sequence is compared with other ar ( 1 and 2ar ) isoforms , it is still possible to observe a high level of homology in the 7-tmd sequence , but a significant divergence is present both in the third intracellular loop and in the c - terminal domain ( fig . this difference probably represents the major factor affecting the pharmacologic regulation of the receptors and their response to a ligand . in this regard , the c - terminus of both 1 and 2ars is rich in serine and threonine residues , and is subjected to gpcr kinase ( grk)-mediated regulation through phosphorylation . further , these receptors also harbor a consensus sequence for protein kinase a ( pka ) . of note , the 3ar lacks all of these sites , and is more resistant to agonist - induced desensitization / downregulation . finally , these sequence divergences also support differential and intracellular signaling ( including g protein - coupling ) between the 3 ar isoforms , which may determine their relative roles in physiology and in the disease . the receptor is a gpcr with 7-tmds , an extracellular n - terminal domain ( exd1 ) , and an intracellular c - terminal domain ( ind4 ) . the receptor presents also 6 loops , 3 are intracellular ( ind1 , ind2 , and ind3 ) , and 3 are extracellular ( exd2 , exd3 , and exd4 ) . indicated with arrows are the asparagine ( n ) residues , in the exd1 , that are sites of n - glycosylation ; tryptophan ( w ) in position 64 that is the location of 3ar - polymorphism ( trp64arg ) and the cysteine ( c ) in position 361 that is a site subjected to palmitoylation . protein sequence alignment of mouse , dog , pig , human , and macaca mulatta 3ar . the tmd is highlighted in yellow ; the tryptophan ( w ) in position 64 is in red ; the cysteine ( c ) in position 361 is in green . the human 3ar was cloned in 1989 , and the studies demonstrated that this new ar isoform was mainly implicated in lipolysis and thermogenesis regulation in adipose tissues . however , over the last 2 decades , different reports have also clearly shown that 3ars are present in the cardiovascular system , mainly in myocardium and endothelium , where they have a prominent role in modulating cardiac function and angiogenesis , respectively . in this context , determining the specific pathways associated with these effects represents a tough challenge and remains a largely unresolved question regarding 3ar function . this is due , at least in part , to the fact that the role of the cardiac 3ar has not been studied with the same intensity as the 1- and 2ars . moreover , all the studies concerning 3ar function have not been focused on similar cell types , and the agonist and the doses used are significantly different between most studies . in fact , it is known that in the mouse , the gene encoding for the 3ar undergoes alternative splicing and gives rise to 3aar and 3bar variants . these 2 3ar isoforms are coupled to adenylyl cyclase stimulatory g ( gs ) or to the inhibitory g ( gi ) protein subunits ( 3bar ) , or exclusively to the gs protein ( 3aar ) . by contrast , in humans , although some reports have proposed that 3ar can activate gs signaling in cho / k1 cells and in adipocytes , it is a general assumption that , at least in ventricular myocardium , 3ars are mainly coupled with gi proteins . for these reasons , the 3ar leads to effects that are either comparable or opposite to those elicited by 1- and 2ar stimulation . in fact , stimulation of 3ar , through gs activation , increases the generation of cyclic amp ( camp ) and the activation of the pka , similar to 1- and 2ars . in the myocardium , after catecholamines stimulation of ars , pka phosphorylates many ca handling proteins and some myofilament components leading to positive inotropic , lusitropic , and chronotropic effects ( fig . 4 ) . however , because 3ars are also coupled with gi , they can act as a brake to prevent 1 and 2ars overactivation , and this has been proposed as a mechanism in the heart ( fig . moreover , in the heart , the stimulation of 3ars leads to increased endothelial nitric oxide ( no ) synthase ( enos ) or neuronal ( nnos ) activation , giving rise to no generation and activation of soluble guanylate cyclase to produce cgmp and cgmp - dependent protein kinase g ( pkg ) activation ( fig . pkg is a serine / threonine kinase that mediates many of the biological effects of no / cgmp . in particular , pkg downstream of 3ars can enhance myocytes relaxation but cause negative inotropy , possibly through the phosphorylation of troponin i and l - type ca channel ( fig . importantly , the 3ar / no - cgmp / pkg signaling axis seems to be a robust cardioprotective mechanism that can be beneficial in failing myocardium . in fact , pkg activation downstream of cgmp has been proven to reduce ca oscillations which can cause ventricular arrhythmias , hypercontracture and sarcolemmal rupture as well as mitochondrial permeability transition pore that is a cause of cell death . of note , both these phenomena occur during reperfusion of ischemic myocardium and can lead to progression toward heart failure ( hf ) . schematic representation of 3ar signaling activation in cardiomyocytes . 3ars are coupled to both stimulatory g proteins ( gs ) and inhibitory g proteins ( gi ) . although the gs pathway induces the generation of camp and cgmp which , in turn , activates the pka and pkg , respectively , the activation of gi signaling pathway is able to stimulate only the generation of cgmp and the activation of pkg . pka has multiple roles in cardiomyocytes and is able to induce the phosphorylation of several key factors involved in the regulation of contractility , such as cardiac troponin i ( ctni ) and phospholamban ( pln ) . furthermore , pka can phosphorylate the l - type ca channel ( ltcc ) increasing the influx of extracellular ca . importantly , pka can also activate protein kinase b ( akt ) with the subsequent activation of the endothelial no synthase ( enos ) . of note , enos activation increases the generation of no which , in turn , activates the soluble guanylate cyclase to produce cgmp and pkg activation . however , pkg can induce the inactivation of ltcc , thus reducing the extracellular ca influx . moreover , cgmp can stimulate phosphodiesterase 2 ( pde2 ) , reducing the camp levels and the activation of pka . of note , after the gi signaling pathway activation , the 3ar is able to give rise to no through both enos and neuronal nos ( nnos ) located on the sarcoplasmic reticulum ( sr ) . in fact , there is a nonsynonymous polymorphism at amino acid 64 where a tryptophan can exist instead of arginine ( figs . 1 , 2 ) . this mutation makes the 3ar less responsive to catecholamine stimulation , which has been associated with some pathophysiological conditions ( ie , obesity ) ( box 1 ) . this important difference suggests that nonhuman 3ar and its signaling may not fully mirror the human system . obesity , susceptibility todiabetesinsulin resistancehyperuricemia obesity , susceptibility to accumulating evidence has revealed that the 3ar , present in endothelium and myocardium , may have specific beneficial effects in the cardiovascular system including cardioprotection . this becomes critically crucial in cardiac diseases such as hf , a syndrome characterized by decreased cardiac output , caused by deficits in contractility and/or relaxation . importantly , after an injury such as ischemia , to preserve cardiac output , there is an increase in sympathetic activity and in catecholamine release to stimulate ar - mediated inotropic capacity . however , chronic exposure of the heart to high levels of catecholamines can lead to further pathologic changes in the heart that can induce a progressive deterioration of cardiac function and structure . of note , catecholamines directly stimulate ars , and the sustained activation of these receptors correlate with left ventricular ( lv ) dysfunction and mortality . in this regard , grk2 , the principal grk involved in ar regulation within the cardiomyocytes , phosphorylates the receptors attenuating their increased responsiveness . this process , called desensitization , at early stage represents a protective mechanism , but in chronic stage can cause 1 and 2ars dysregulation and signaling abnormalities ( eg , downregulation and overdesensitization ) and promote the progression of the disease . importantly , as discussed above , 3ars lack grk recognition sites and are not subject to desensitization and downregulation , and in fact , their levels within human failing myocardium remain unchanged or become upregulated . enhancement of 3ars could represent either a protective mechanism against the detrimental effects of chronic ar stimulation or a detrimental mechanism that may lead to further deterioration of hf . in this regard , the role of this receptor in the heart has been debated for years , and some reports have suggested that due to its cardiodepressant effect , sustained activation of 3ars in hf could contribute to impaired cardiac function . consistently , the antagonism of this receptor has been proposed as a potential strategy against hf development . however , by contrast with this hypothesis , it has been demonstrated that in the failing myocardium , 3ars are able to inhibit , through activation of na - k pump , the deleterious accumulation of na in cardiac myocytes thus blocking camp generation , and consequently , reducing the activation of the camp - downstream oxidative pathways . mechanistically , this effect decreases the glutathionylation of the 1 na - k pump subunit and enhances the na - k pump activity in presence of 3ar agonists . in line with the notion that 3ar activation in failing myocardium is not detrimental , studies strongly support the idea that overexpression or persistent activation of 3ar is cardioprotective and can attenuate pathological lv hypertrophy induced by continuous infusion of isoproterenol and angiotensin ii , or by transaortic constriction , in mice . importantly , as shown in these studies , the activation of nos and subsequent no generation represent the main mechanism responsible for 3ar - induced cardioprotection . in agreement with this mechanism of action , another study , using small ( mice ) and large ( pigs ) animal models of ischemia / reperfusion ( i / r ) injury , demonstrated that administration of selective 3ar agonist brl 37344 positively affected infarct size ( acutely ) and lv function ( chronically ) . further , this study showed that 3ar / no signaling decreased opening of the mitochondrial permeability transition pore , thus conferring protection to the cardiac cells against cell death . finally , it is well established that the role of exercise in cardioprotection , and 3ars , has been also shown to be a mediator of this effect , especially in a setting of cardiac i / r injury . of note , during exercise , it seems that gs is the key mediator of 3ar - induced protection that leads to the pka / akt / enos activation , thus suggesting that , in some conditions , the 3ar , similarly to the cardioprotective 2ar , is coupled with both gs and gi in cardiomyocytes ( fig . regard , this signaling pathway activation has a multiple protective role in the injured myocardium like the promotion of revascularization of the ischemic tissue . in fact , activation of akt and enos , and the consequent secretion of no , has been proven to directly stimulate endothelial cell function and promote the neoangiogenesis . further , as discussed above , the enos - mediated generation of no is also responsible for cgmp and pkg activation , thus directly conferring beneficial effects on the myocardium . as described above , 3ars have emerged as novel potential targets for the treatment of certain cardiovascular diseases including hf . the clinical relevance of this is further supported by the successful effects obtained with -blocker treatment in patients with hf as they can block the noxious effects of catecholamines and prevent further 1 and 2ars downregulation . importantly , as proposed by us and others , the use of -blockers may influence the expression / activity of 3ars . however , because there are some specific differences in -blockers ( 1ar selective or nonselective ) , the full extent of whether any molecular changes in 3ars are significant contributors to the therapeutic mechanisms of ar antagonism in hf still remains to be elucidated . accordingly , answering these mechanistic questions is important and may lead to novel therapeutic advances in hf . in 2007 in particular , in an hf rat model induced by transaortic constriction , it has been shown that , although metoprolol ( a selective 1-blocker ) treatment did not affect the expression levels of 3ar which was increased after hf , the use of carvedilol ( a nonselective -blocker ) resulted in a robust 3ar downregulation . nevertheless , soon after this report , a number of studies proposed that 1-blockers and their subsequent beneficial effects in hf could induce enhancement of the 3ar expression and activity . for example , sharma et al showed that metoprolol treatment was able to improve cardiac function in diabetic rats mainly through 3ar upregulation and no generation . analogous , in a canine model of mitral regurgitation , we recently found that metoprolol can promote 3ar upregulation and enhance its protective signaling ( ie , nnos / no / cgmp ) . moreover , correlative data have been recently shown for the potential of 3ar agonistic activity of nebivolol , a highly selective 1-blocker . in particular , in a model of i / r injury , nebivolol administration activated cardiac 3ars leading to a significant reduction of infarct size . similarly , zhang et al demonstrated , in a setting of hf in mice , induced by left anterior descending artery ligation , that nebivolol was able to reduce cardiac fibrosis and apoptosis and improved cardiac function . interestingly , this report showed also that after 4 weeks postmyocardial infarction , there was a significant reduction of cardiac 3ar levels , and that nebivolol was able to restore the expression of this receptor . importantly , further to the direct effect in cardiomyocytes , 1-blockers , such as nebivolol have been also reported to act on endothelium . of note , enhancement of neoangiogenesis in the failing heart is considered one of the mechanisms of protection activated by this class of drugs . in this context , nebivolol through 3ar activation increases the generation of no , a key mediator of endothelial function , enhancing endothelial proliferation and increasing vasodilation . as described above , the 3ar represents an emerging attractive target for pharmacological modulation in the injured heart that , for its compensatory effects , prevents the effects of excessive catecholamines stimulation on the heart . in fact , selective 3ar agonism has been proven to confer protection in the failing heart through a specific cgmp / no signaling pathway . a particular role for no has been associated with the enhancement of endothelial cell proliferation , and within failing myocardium it can lead to a beneficial effect on cardiac function and remodeling . as described above , 3ars are expressed not only in cardiomyocytes but also in endothelial cells , thus supporting the idea that this receptor can also promote proangiogenic mechanisms . moreover , in adipocytes , the 3ar is implicated in metabolic regulation [ fatty acid ( fa ) oxidation , lipolysis , and thermogenesis ] , that can also be a crucial mechanism to rescue the heart from failure . importantly , this mechanism is particularly important in several pathological conditions that affect cardiac function , such as ischemia and pressure overload . in fact , in these conditions , the heart , in the presence of limited oxygen supply , suppresses glucose and fa oxidation with a shift in cardiac substrate metabolism from fa oxidation to glycolysis . glycolysis without a concomitant increase in glucose oxidation results in an accumulation of different harmful catabolites such as lactate and protons that are the cause of intracellular acidosis and na and ca overload . these effects strongly reduce the capacity of the heart to provide sufficient energy for contractile work because more energy is spent to restore ion homeostasis and lead to an increase in lipid accumulation within the cardiac cells with consequent lipotoxicity . in this context , as recently demonstrated in adipocytes , nebivolol , through 3ar activation , is able to improve adipocyte metabolism . therefore , it is plausible that long - term 3ar stimulation , through selective agonists and/or -blockers ( ie , nebivolol ) , can be used as a novel therapeutical approach also to improve metabolism within the failing myocardium . currently , 2 clinical trials , the beta 3 agonist treatment in heart failure ( beat - hf ; clinicaltrials.gov : nct01876433 ) , or in the prevention of hf development , the assessment of efficacy of mirabegron , a new beta 3-adrenergic receptor in the prevention of heart failure ( beta3_lvh ; clinicaltrials.gov : nct02599480 ) , are trying to evaluate the effects of 3ar agonism on hf progression and development . in this context , patients will be treated with the selective 3ar agonist , mirabegron ( also known as ym-178 ) , a drug already approved in the united states , japan , and europe , for the treatment of overactive bladder . this drug , through the activation of 3ar , exerts a myorelaxant effect in the detrusor muscle thus improving the bladder filling . anyway , in the meantime that these trials will give us the proof - of - concept of the beneficial role of 3ars in human hf , the most important clinical evidence that we currently have regarding the role of this receptor remains the proved efficacy of -blockers . the cardioprotective effect of ar - blockade has been largely attributed , for decades , to antagonism of cardiac 1- and 2ars and a resulting heart rate reduction . however , as discussed above in this review , there is emerging evidence that this class of drugs may also act through 3ar signaling pathway activation which is not blocked by the drugs currently used in clinical hf - therapy ( box 2 ) . however , the main question that still remains unanswered is why not all patients respond favorably to these agents . for these reasons , further elucidation on specific mechanisms of action of these drugs is extremely interesting . in particular , it will be important to evaluate how different -blockers can impact , in a positive or a negative manner , 3ar activity , and if this effect is a significant contributor of therapeutic mechanisms in hf . the mammalian 3ar sequence consists of about 400408 amino acids in a protein that has the typical structure of all gpcrs . the 3ar has 7 transmembrane domains ( 7-tmds ) with an extracellular n - terminal that is glycosylated , and an intracellular c - terminal domain . further , the cys361 residue in the fourth intracellular domain is palmitoylated , a feature that has been shown to be associated with g protein - coupling and adenylyl cyclase stimulation following agonist stimulation of the receptor ( fig . as shown in figure 2 , the protein sequence alignment between different mammalian species demonstrates that most of the homology between the 3ar amino acid sequences is concentrated in the 7-tmds and in the membrane - proximal regions of the intracellular loops . interestingly , when the 3ar protein sequence is compared with other ar ( 1 and 2ar ) isoforms , it is still possible to observe a high level of homology in the 7-tmd sequence , but a significant divergence is present both in the third intracellular loop and in the c - terminal domain ( fig . this difference probably represents the major factor affecting the pharmacologic regulation of the receptors and their response to a ligand . in this regard , the c - terminus of both 1 and 2ars is rich in serine and threonine residues , and is subjected to gpcr kinase ( grk)-mediated regulation through phosphorylation . further , these receptors also harbor a consensus sequence for protein kinase a ( pka ) . of note , the 3ar lacks all of these sites , and is more resistant to agonist - induced desensitization / downregulation . finally , these sequence divergences also support differential and intracellular signaling ( including g protein - coupling ) between the 3 ar isoforms , which may determine their relative roles in physiology and in the disease . human 3ar structure . the receptor is a gpcr with 7-tmds , an extracellular n - terminal domain ( exd1 ) , and an intracellular c - terminal domain ( ind4 ) . the receptor presents also 6 loops , 3 are intracellular ( ind1 , ind2 , and ind3 ) , and 3 are extracellular ( exd2 , exd3 , and exd4 ) . indicated with arrows are the asparagine ( n ) residues , in the exd1 , that are sites of n - glycosylation ; tryptophan ( w ) in position 64 that is the location of 3ar - polymorphism ( trp64arg ) and the cysteine ( c ) in position 361 that is a site subjected to palmitoylation . protein sequence alignment of mouse , dog , pig , human , and macaca mulatta 3ar . the tmd is highlighted in yellow ; the tryptophan ( w ) in position 64 is in red ; the cysteine ( c ) in position 361 is in green . the human 3ar was cloned in 1989 , and the studies demonstrated that this new ar isoform was mainly implicated in lipolysis and thermogenesis regulation in adipose tissues . however , over the last 2 decades , different reports have also clearly shown that 3ars are present in the cardiovascular system , mainly in myocardium and endothelium , where they have a prominent role in modulating cardiac function and angiogenesis , respectively . in this context , determining the specific pathways associated with these effects represents a tough challenge and remains a largely unresolved question regarding 3ar function . this is due , at least in part , to the fact that the role of the cardiac 3ar has not been studied with the same intensity as the 1- and 2ars . moreover , all the studies concerning 3ar function have not been focused on similar cell types , and the agonist and the doses used are significantly different between most studies . in fact , it is known that in the mouse , the gene encoding for the 3ar undergoes alternative splicing and gives rise to 3aar and 3bar variants . these 2 3ar isoforms are coupled to adenylyl cyclase stimulatory g ( gs ) or to the inhibitory g ( gi ) protein subunits ( 3bar ) , or exclusively to the gs protein ( 3aar ) . by contrast , in humans , although some reports have proposed that 3ar can activate gs signaling in cho / k1 cells and in adipocytes , it is a general assumption that , at least in ventricular myocardium , 3ars are mainly coupled with gi proteins . for these reasons , the 3ar leads to effects that are either comparable or opposite to those elicited by 1- and 2ar stimulation . in fact , stimulation of 3ar , through gs activation , increases the generation of cyclic amp ( camp ) and the activation of the pka , similar to 1- and 2ars . in the myocardium , after catecholamines stimulation of ars , pka phosphorylates many ca handling proteins and some myofilament components leading to positive inotropic , lusitropic , and chronotropic effects ( fig . 4 ) . however , because 3ars are also coupled with gi , they can act as a brake to prevent 1 and 2ars overactivation , and this has been proposed as a mechanism in the heart ( fig . moreover , in the heart , the stimulation of 3ars leads to increased endothelial nitric oxide ( no ) synthase ( enos ) or neuronal ( nnos ) activation , giving rise to no generation and activation of soluble guanylate cyclase to produce cgmp and cgmp - dependent protein kinase g ( pkg ) activation ( fig . pkg is a serine / threonine kinase that mediates many of the biological effects of no / cgmp . in particular , pkg downstream of 3ars can enhance myocytes relaxation but cause negative inotropy , possibly through the phosphorylation of troponin i and l - type ca channel ( fig . importantly , the 3ar / no - cgmp / pkg signaling axis seems to be a robust cardioprotective mechanism that can be beneficial in failing myocardium . in fact , pkg activation downstream of cgmp has been proven to reduce ca oscillations which can cause ventricular arrhythmias , hypercontracture and sarcolemmal rupture as well as mitochondrial permeability transition pore that is a cause of cell death . of note , both these phenomena occur during reperfusion of ischemic myocardium and can lead to progression toward heart failure ( hf ) 3ars are coupled to both stimulatory g proteins ( gs ) and inhibitory g proteins ( gi ) . although the gs pathway induces the generation of camp and cgmp which , in turn , activates the pka and pkg , respectively , the activation of gi signaling pathway is able to stimulate only the generation of cgmp and the activation of pkg . pka has multiple roles in cardiomyocytes and is able to induce the phosphorylation of several key factors involved in the regulation of contractility , such as cardiac troponin i ( ctni ) and phospholamban ( pln ) . furthermore , pka can phosphorylate the l - type ca channel ( ltcc ) increasing the influx of extracellular ca . importantly , pka can also activate protein kinase b ( akt ) with the subsequent activation of the endothelial no synthase ( enos ) . of note , enos activation increases the generation of no which , in turn , activates the soluble guanylate cyclase to produce cgmp and pkg activation . however , pkg can induce the inactivation of ltcc , thus reducing the extracellular ca influx . moreover , cgmp can stimulate phosphodiesterase 2 ( pde2 ) , reducing the camp levels and the activation of pka . of note , after the gi signaling pathway activation , the 3ar is able to give rise to no through both enos and neuronal nos ( nnos ) located on the sarcoplasmic reticulum ( sr ) . in fact , there is a nonsynonymous polymorphism at amino acid 64 where a tryptophan can exist instead of arginine ( figs . 1 , 2 ) . this mutation makes the 3ar less responsive to catecholamine stimulation , which has been associated with some pathophysiological conditions ( ie , obesity ) ( box 1 ) . this important difference suggests that nonhuman 3ar and its signaling may not fully mirror the human system . accumulating evidence has revealed that the 3ar , present in endothelium and myocardium , may have specific beneficial effects in the cardiovascular system including cardioprotection . this becomes critically crucial in cardiac diseases such as hf , a syndrome characterized by decreased cardiac output , caused by deficits in contractility and/or relaxation . importantly , after an injury such as ischemia , to preserve cardiac output , there is an increase in sympathetic activity and in catecholamine release to stimulate ar - mediated inotropic capacity . however , chronic exposure of the heart to high levels of catecholamines can lead to further pathologic changes in the heart that can induce a progressive deterioration of cardiac function and structure . of note , catecholamines directly stimulate ars , and the sustained activation of these receptors correlate with left ventricular ( lv ) dysfunction and mortality . in this regard , grk2 , the principal grk involved in ar regulation within the cardiomyocytes , phosphorylates the receptors attenuating their increased responsiveness . this process , called desensitization , at early stage represents a protective mechanism , but in chronic stage can cause 1 and 2ars dysregulation and signaling abnormalities ( eg , downregulation and overdesensitization ) and promote the progression of the disease . importantly , as discussed above , 3ars lack grk recognition sites and are not subject to desensitization and downregulation , and in fact , their levels within human failing myocardium remain unchanged or become upregulated . enhancement of 3ars could represent either a protective mechanism against the detrimental effects of chronic ar stimulation or a detrimental mechanism that may lead to further deterioration of hf . in this regard , the role of this receptor in the heart has been debated for years , and some reports have suggested that due to its cardiodepressant effect , sustained activation of 3ars in hf could contribute to impaired cardiac function . consistently , the antagonism of this receptor has been proposed as a potential strategy against hf development . however , by contrast with this hypothesis , it has been demonstrated that in the failing myocardium , 3ars are able to inhibit , through activation of na - k pump , the deleterious accumulation of na in cardiac myocytes thus blocking camp generation , and consequently , reducing the activation of the camp - downstream oxidative pathways . mechanistically , this effect decreases the glutathionylation of the 1 na - k pump subunit and enhances the na - k pump activity in presence of 3ar agonists . in line with the notion that 3ar activation in failing myocardium is not detrimental , studies strongly support the idea that overexpression or persistent activation of 3ar is cardioprotective and can attenuate pathological lv hypertrophy induced by continuous infusion of isoproterenol and angiotensin ii , or by transaortic constriction , in mice . importantly , as shown in these studies , the activation of nos and subsequent no generation represent the main mechanism responsible for 3ar - induced cardioprotection . in agreement with this mechanism of action , another study , using small ( mice ) and large ( pigs ) animal models of ischemia / reperfusion ( i / r ) injury , demonstrated that administration of selective 3ar agonist brl 37344 positively affected infarct size ( acutely ) and lv function ( chronically ) . further , this study showed that 3ar / no signaling decreased opening of the mitochondrial permeability transition pore , thus conferring protection to the cardiac cells against cell death . finally , it is well established that the role of exercise in cardioprotection , and 3ars , has been also shown to be a mediator of this effect , especially in a setting of cardiac i / r injury . of note , during exercise , it seems that gs is the key mediator of 3ar - induced protection that leads to the pka / akt / enos activation , thus suggesting that , in some conditions , the 3ar , similarly to the cardioprotective 2ar , is coupled with both gs and gi in cardiomyocytes ( fig . regard , this signaling pathway activation has a multiple protective role in the injured myocardium like the promotion of revascularization of the ischemic tissue . in fact , activation of akt and enos , and the consequent secretion of no , has been proven to directly stimulate endothelial cell function and promote the neoangiogenesis . further , as discussed above , the enos - mediated generation of no is also responsible for cgmp and pkg activation , thus directly conferring beneficial effects on the myocardium . as described above , 3ars have emerged as novel potential targets for the treatment of certain cardiovascular diseases including hf . the clinical relevance of this is further supported by the successful effects obtained with -blocker treatment in patients with hf as they can block the noxious effects of catecholamines and prevent further 1 and 2ars downregulation . importantly , as proposed by us and others , the use of -blockers may influence the expression / activity of 3ars . however , because there are some specific differences in -blockers ( 1ar selective or nonselective ) , the full extent of whether any molecular changes in 3ars are significant contributors to the therapeutic mechanisms of ar antagonism in hf still remains to be elucidated . accordingly , answering these mechanistic questions is important and may lead to novel therapeutic advances in hf . in 2007 , it was first reported that a relationship between -blockers and 3ar may exist . in particular , in an hf rat model induced by transaortic constriction , it has been shown that , although metoprolol ( a selective 1-blocker ) treatment did not affect the expression levels of 3ar which was increased after hf , the use of carvedilol ( a nonselective -blocker ) resulted in a robust 3ar downregulation . nevertheless , soon after this report , a number of studies proposed that 1-blockers and their subsequent beneficial effects in hf could induce enhancement of the 3ar expression and activity . for example , sharma et al showed that metoprolol treatment was able to improve cardiac function in diabetic rats mainly through 3ar upregulation and no generation . analogous , in a canine model of mitral regurgitation , we recently found that metoprolol can promote 3ar upregulation and enhance its protective signaling ( ie , nnos / no / cgmp ) . moreover , correlative data have been recently shown for the potential of 3ar agonistic activity of nebivolol , a highly selective 1-blocker . in particular , in a model of i / r injury , nebivolol administration activated cardiac 3ars leading to a significant reduction of infarct size . similarly , zhang et al demonstrated , in a setting of hf in mice , induced by left anterior descending artery ligation , that nebivolol was able to reduce cardiac fibrosis and apoptosis and improved cardiac function . interestingly , this report showed also that after 4 weeks postmyocardial infarction , there was a significant reduction of cardiac 3ar levels , and that nebivolol was able to restore the expression of this receptor . importantly , further to the direct effect in cardiomyocytes , 1-blockers , such as nebivolol have been also reported to act on endothelium . of note , enhancement of neoangiogenesis in the failing heart is considered one of the mechanisms of protection activated by this class of drugs . in this context , nebivolol through 3ar activation increases the generation of no , a key mediator of endothelial function , enhancing endothelial proliferation and increasing vasodilation . as described above , the 3ar represents an emerging attractive target for pharmacological modulation in the injured heart that , for its compensatory effects , prevents the effects of excessive catecholamines stimulation on the heart . in fact , selective 3ar agonism has been proven to confer protection in the failing heart through a specific cgmp / no signaling pathway . a particular role for no has been associated with the enhancement of endothelial cell proliferation , and within failing myocardium it can lead to a beneficial effect on cardiac function and remodeling . as described above , 3ars are expressed not only in cardiomyocytes but also in endothelial cells , thus supporting the idea that this receptor can also promote proangiogenic mechanisms . moreover , in adipocytes , the 3ar is implicated in metabolic regulation [ fatty acid ( fa ) oxidation , lipolysis , and thermogenesis ] , that can also be a crucial mechanism to rescue the heart from failure . importantly , this mechanism is particularly important in several pathological conditions that affect cardiac function , such as ischemia and pressure overload . in fact , in these conditions , the heart , in the presence of limited oxygen supply , suppresses glucose and fa oxidation with a shift in cardiac substrate metabolism from fa oxidation to glycolysis . glycolysis without a concomitant increase in glucose oxidation results in an accumulation of different harmful catabolites such as lactate and protons that are the cause of intracellular acidosis and na and ca overload . these effects strongly reduce the capacity of the heart to provide sufficient energy for contractile work because more energy is spent to restore ion homeostasis and lead to an increase in lipid accumulation within the cardiac cells with consequent lipotoxicity . in this context , as recently demonstrated in adipocytes , nebivolol , through 3ar activation , is able to improve adipocyte metabolism . therefore , it is plausible that long - term 3ar stimulation , through selective agonists and/or -blockers ( ie , nebivolol ) , can be used as a novel therapeutical approach also to improve metabolism within the failing myocardium . currently , 2 clinical trials , the beta 3 agonist treatment in heart failure ( beat - hf ; clinicaltrials.gov : nct01876433 ) , or in the prevention of hf development , the assessment of efficacy of mirabegron , a new beta 3-adrenergic receptor in the prevention of heart failure ( beta3_lvh ; clinicaltrials.gov : nct02599480 ) , are trying to evaluate the effects of 3ar agonism on hf progression and development . in this context , patients will be treated with the selective 3ar agonist , mirabegron ( also known as ym-178 ) , a drug already approved in the united states , japan , and europe , for the treatment of overactive bladder . this drug , through the activation of 3ar , exerts a myorelaxant effect in the detrusor muscle thus improving the bladder filling . anyway , in the meantime that these trials will give us the proof - of - concept of the beneficial role of 3ars in human hf , the most important clinical evidence that we currently have regarding the role of this receptor remains the proved efficacy of -blockers . the cardioprotective effect of ar - blockade has been largely attributed , for decades , to antagonism of cardiac 1- and 2ars and a resulting heart rate reduction . however , as discussed above in this review , there is emerging evidence that this class of drugs may also act through 3ar signaling pathway activation which is not blocked by the drugs currently used in clinical hf - therapy ( box 2 ) . however , the main question that still remains unanswered is why not all patients respond favorably to these agents . for these reasons , further elucidation on specific mechanisms of action of these drugs is extremely interesting . in particular , it will be important to evaluate how different -blockers can impact , in a positive or a negative manner , 3ar activity , and if this effect is a significant contributor of therapeutic mechanisms in hf . the 3ar is a novel and intriguing receptor , with multiple functions within the cardiovascular system . in this review , we have discussed how many disparities have been generated around the signaling and the function of this receptor . however , the emerging concept in the literature is that 3ar is mostly protective for the cardiovascular system and its agonism with selective ligands or activation during 1- and 2ar blockade could represent a future therapeutic strategy to prevent development of hf . anyway , as discussed above , the 3ar function in the heart is still poorly investigated , and for this reason , further investigations are required to clarify the causal mechanistic relationship between 3ar expression and cardiac dysfunction and protection . more importantly , since presently , only 1ar - blockers have been associated with improvement of the 3ar signaling pathway , it will be crucial to define the specific signaling pathways associated with -blocker  dependent activation / inhibition of 3ars because it can help in personalizing anti - hf therapy .
abstract : cardiac diseases , such as heart failure , remain leading causes of morbidity and mortality worldwide , with myocardial infarction as the most common etiology . 
 hf is characterized by -adrenergic receptor ( ar ) dysregulation that is primarily due to the upregulation of g protein  coupled receptor kinases that leads to overdesensitization of 1 and 2ars , and this clinically manifests as a loss of inotropic reserve . 
 interestingly , the  minor  ar isoform , the 3ar , found in the heart , lacks g protein  coupled receptor kinases recognition sites , and is not subject to desensitization , and as a consequence of this , in human failing myocardium , the levels of this receptor remain unchanged or are even increased . in different preclinical studies , it has been shown that 3ars can activate different signaling pathways that can protect the heart . 
 the clinical relevance of this is also supported by the effects of -blockers which are well known for their proangiogenic and cardioprotective effects , and data are emerging showing that these are mediated , at least in part , by enhancement of 3ar activity . in this 
 regard , targeting of 3ars could represent a novel potential strategy to improve cardiac metabolism , function , and remodeling .