Aryl propanolamines: comparison of activity at human beta3 receptors, rat beta3 receptors and rat atrial receptors mediating tachycardia

Adrenoceptors: Receptors, Ligands and Their Clinical Uses, Molecular Pharmacology and Assays

The nine G protein-coupled adrenoceptor subtypes are where the endogenous catecholamines adrenaline and noradrenaline interact with cells. Since they are important therapeutic targets, over a century of effort has been put into developing drugs that modify their activity. This chapter provides an outline of how we have arrived at current knowledge of the receptors, their physiological roles and the methods used to develop ligands. Initial studies in vivo and in vitro with isolated organs and tissues progressed to cell-based techniques and the use of cloned adrenoceptor subtypes together with high-throughput assays that allow close examination of receptors and their signalling pathways. The crystal structures of many of the adrenoceptor subtypes have now been determined opening up new possibilities for drug development.

Cardiac β3 -adrenoceptors-A role in human pathophysiology?

As β₃ -adrenoceptors were first demonstrated to be expressed in adipose tissue they have received much attention for their metabolic effects in obesity and diabetes. After the existence of this subtype had been suggested to be present in the heart, studies focused on its role in cardiac function. While the presence and functional role of β₃ -adrenoceptors in the heart has not uniformly been detected, there is a broad consensus that they become up-regulated in pathological conditions associated with increased sympathetic activity such as heart failure and diabetes. When detected, the β₃ -adrenceptor has been demonstrated to mediate negative inotropic effects in an inhibitory G protein-dependent manner through the NO-cGMP-PKG signalling pathway. Whether these negative inotropic effects provide protection from the adverse effects induced by overstimulation of β₁ /β₂ -adrenoceptors or in themselves are potentially harmful is controversial, but ongoing clinical studies in patients with congestive heart failure are testing the hypothesis that β₃ -adrenceptor agonism has a beneficial effect. LINKED ARTICLES: This article is part of a themed section on Adrenoceptors-New Roles for Old Players. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.14/issuetoc.

Application of pH-zone-refining countercurrent chromatography in the chiral separation of two β-adrenergic blocking agents

Two β-adrenergic blocking agents, 1-[(1-methylethyl)amino]-3-phenoxy-2-propanol (1) and 1-[(1-methylethyl)amino]-3-(3-methylphenoxy)-2-propanol (2; Toliprolol), were enantioseparated by pH-zone-refining countercurrent chromatography. A two-phase solvent system composed of chloroform containing 0.10 mol/L of di-n-hexyl l-tartrate/0.10 mol/L of boric acid aqueous solution (1:1, v/v) was selected, in which 20 mmol/L triethylamine was added in the organic phase as a retainer and 2 mmol/L HCl was added in the aqueous phase as an eluter. Fifty milligrams of each racemate was completely enantioseparated by pH-zone-refining countercurrent chromatography to yield each enantiomer with a purity of more than 98%, and the recovery of each separated enantiomer reached around 76-82%.

β3 -AR and the vertebrate heart: a comparative view

Recent cardiovascular research showed that, together with β1- and β2-adrenergic receptors (ARs), β3-ARs contribute to the catecholamine (CA)-dependent control of the heart. β3-ARs structure, function and ligands were investigated in mammals because of their applicative potential in human cardiovascular diseases. Only recently, the concept of a β3-AR-dependent cardiac modulation was extended to non-mammalian vertebrates, although information is still scarce and fragmentary. β3-ARs were structurally described in fish, showing a closer relationship to mammalian β1-AR than β2-AR. Functional β3-ARs are present in the cardiac tissue of teleosts and amphibians. As in mammals, activation of these receptors elicits a negative modulation of the inotropic performance through the involvement of the endothelium endocardium (EE), Gi/0 proteins and the nitric oxide (NO) signalling. This review aims to comparatively analyse data from literature on β3-ARs in mammals, with those on teleosts and amphibians. The purpose is to highlight aspects of uniformity and diversity of β3-ARs structure, ligands activity, function and signalling cascades throughout vertebrates. This may provide new perspectives aimed to clarify the biological relevance of β3-ARs in the context of the nervous and humoral control of the heart and its functional plasticity.

Human atrial β(1L)-adrenoceptor but not β₃-adrenoceptor activation increases force and Ca(2+) current at physiological temperature

BACKGROUND AND PURPOSE

It has been proposed that BRL37344, SR58611 and CGP12177 activate β₃-adrenoceptors in human atrium to increase contractility and L-type Ca(2+) current (I(Ca-L)). β₃-adrenoceptor agonists are potentially beneficial for the treatment of a variety of diseases but concomitant cardiostimulation would be potentially harmful. It has also been proposed that (-)-CGP12177 activates the low affinity binding site of the β₁-adrenoceptor in human atrium. We therefore used BRL37344, SR58611 and (-)-CGP12177 with selective β-adrenoceptor subtype antagonists to clarify cardiostimulant β-adrenoceptor subtypes in human atrium.

EXPERIMENTAL APPROACH

Human right atrium was obtained from patients without heart failure undergoing coronary artery bypass or valve surgery. Cardiomyocytes were prepared to test BRL37344, SR58611 and CGP12177 effects on I(Ca-L). Contractile effects were determined on right atrial trabeculae.

KEY RESULTS

BRL37344 increased force which was antagonized by blockade of β₁- and β₂-adrenoceptors but not by blockade of β₃-adrenoceptors with β₃-adrenoceptor-selective L-748,337 (1 µM). The β₃-adrenoceptor agonist SR58611 (1 nM-10 µM) did not affect atrial force. BRL37344 and SR58611 did not increase I(Ca-L) at 37°C, but did at 24°C which was prevented by L-748,337. (-)-CGP12177 increased force and I(Ca-L) at both 24°C and 37°C which was prevented by (-)-bupranolol (1-10 µM), but not L-748,337.

CONCLUSIONS AND IMPLICATIONS

We conclude that the inotropic responses to BRL37344 are mediated through β₁- and β₂-adrenoceptors. The inotropic and I(Ca-L) responses to (-)-CGP12177 are mediated through the low affinity site β(1L)-adrenoceptor of the β₁-adrenoceptor. β₃-adrenoceptor-mediated increases in I(Ca-L) are restricted to low temperatures. Human atrial β₃-adrenoceptors do not change contractility and I(Ca-L) at physiological temperature.

Exercício físico, receptores β-adrenérgicos e resposta vascular

O exercício aeróbio promove efeitos benéficos na prevenção e tratamento de doenças como hipertensão arterial, aterosclerose, insuficiência venosa e doença arterial periférica. Os receptores β-adrenérgicos estão presentes em várias células. No sistema cardiovascular, promovem inotropismo e cronotropismo positivo cardíaco e relaxamento vascular. Embora os efeitos do exercício tenham sido investigados em receptores cardíacos, estudos focados nos vasos são escassos e controversos. Esta revisão abordará os efeitos do exercício físico sobre os receptores β-adrenérgicos vasculares em modelos animais e humanos e os mecanismos celulares envolvidos na resposta relaxante. Em geral, os estudos mostram resultantes conflitantes, onde observam diminuição, aumento ou nenhum efeito do exercício físico sobre a resposta relaxante. Assim, os efeitos do exercício na sensibilidade β-adrenérgica vascular merecem maior atenção, e os resultados mostram que a área de fisiopatologia vascular é um campo aberto para a descoberta de novos compostos e avanços na prática clínica.

Beta3-adrenergic receptors in cardiac and vascular tissues emerging concepts and therapeutic perspectives

Catecholamines released by the orthosympathetic system play a major role in the short- and long-term regulation of cardiovascular function. Beta1- and beta2-adrenoreceptors (ARs) have classically been considered as mediating most of their effects on cardiac contraction. After their initial cloning and pharmacologic characterization in the late 1980s, beta3-ARs have been mostly thought of as receptors mediating metabolic effects (e.g., lipolysis) in adipocytes. However, definitive evidence for their expression and functional coupling in cardiovascular tissues (including in humans) has recently initiated a re-examination of their implication in the pathophysiology of cardiovascular diseases. Distinctive pharmacodynamic properties of beta3-AR, e.g., their upregulation in disease and resistance to desensitization, suggest that they may be attractive targets for therapeutic intervention. They may substitute efficient vasodilating pathways when beta1/2-ARs are inoperative. In the heart, their contractile effects, which are functionally antipathetic to those of beta1/2-AR, may protect the myocardium against adverse effects of excessive catecholamine stimulation and perhaps mediate additional ancillary effects on key aspects of electrophysiology or remodeling. Longitudinal studies in animals and patients with different stages of heart failure are now needed to identify the optimal therapeutic scheme using specific combinations of agonists or antagonists at all three beta-ARs.

Potent oxindole based human β3 adrenergic receptor agonists

The synthesis and biological evaluation of a series of oxindole beta(3) adrenergic receptor agonists is described. A modulation of rat atrial tachycardia was observed with substitution at the 3-position of the oxindole moiety.

Demonstration of functional neuronal beta3-adrenoceptors within the enteric nervous system

BACKGROUND & AIMS

Although the beta(3)-adrenoceptor (AR) has been suggested to be involved in regulation of gut motility and visceral algesia, the precise mechanisms have been unknown. beta(3)-AR has been postulated to have a nonneuronal expression, being initially characterized in adipocytes and subsequently in the smooth muscle. We aimed to investigate the expression of beta(3)-AR in human enteric nervous system and its role in motility and visceral algesia.

METHODS

The expression of beta(3)-AR in human colon myenteric and submucosal plexus was investigated using immunohistochemistry. The effects of a beta(3)-AR agonist on nerve-evoked and carbachol-induced contractions as well as somatostatin release were investigated in strips of human colon. The effect of an agonist on diarrhea and visceral pain was investigated in vivo in rat models.

RESULTS

beta(3)-AR is expressed in cholinergic neurons in the myenteric plexus and submucosal plexus of human colon. Activation of beta(3)-AR causes the release of somatostatin from human isolated colon. In a rat model of visceral pain, beta(3)-AR agonist elicits somatostatin-dependent visceral analgesia. beta(3)-AR agonists inhibit cholinergically mediated muscle contraction of the human colon, as well as chemically induced diarrhea in vivo in a rat model.

CONCLUSIONS

This is the first demonstration of expression of beta(3)-AR in the enteric nervous system. Activation of these receptors results in inhibition of cholinergic contractions and enhanced release of somatostatin, which may lead to visceral analgesia and inhibition of diarrhea. Therefore, beta(3)-AR could be a novel therapeutic target for functional gastrointestinal disorders.

Tools to study beta3-adrenoceptors

Beta(3)-adrenoceptors mediate some of the effects of catecholamines on tissues such as blood vessels or the urinary bladder and are putative targets for the treatment of diseases such as the overactive bladder syndrome. Progress in the understanding of the presence, function, and regulation of beta(3)-adrenoceptors has been hampered by a lack of highly specific tools. "Classical" beta(3)-adrenoceptor agonists such as BRL 37,344 [(R*, R*)-(+/-)-4[2-[(3-chlorophenyl)-2-hydroxyethyl) amino] propyl] phenoxyacetic acid] and CGP 12,177 [(+/-)-4-(3-t-butylamino-2-hydroxypropoxy)benzimidazol-2-one] are only partial agonists in many settings, have limited selectivity over other beta-adrenoceptor subtypes, and may additionally act on receptors other than beta-adrenoceptors. More efficacious and more selective agonists have been reported and, in some cases, are in clinical development but are not widely available for experimental studies. The widely used antagonist SR 59,230 [3-(2-ethylphenoxy)-1-[(1,S)-1,2,3,4-tetrahydronapth-1-ylamino]-2S-2-propanoloxalate] is not selective for beta(3)-adrenoceptors, at least in humans, and may actually be a partial agonist. Radioligands, which are suitable either for the selective labeling of beta(3)-adrenoceptors or for the nonselective labeling of all beta-adrenoceptor subtypes, are also missing. beta(3)- and beta(1)/beta(2) double knockout mice have been reported, but their usefulness for extrapolations in humans is questionable based upon major differences between humans and rodents with regard to the ligand recognition and expression profiles of beta(3)-adrenoceptors. While the common availability of more selective agonists and antagonists at the beta(3)-adrenoceptor is urgently awaited, the limitations of the currently available tools need to be considered in studies of beta(3)-adrenoceptor for the time being.

Potent benzimidazolone based human β3-adrenergic receptor agonists

The synthesis and biological evaluation of a series of benzimidazolone beta(3) adrenergic receptor agonists are described. A trend toward the reduction of rat atrial tachycardia upon increasing steric bulk at the 3-position of the benzimidazolone moiety was observed.

Downregulation of propranolol-sensitive beta-adrenoceptor signaling after inhibition of nitric oxide synthesis

The beta-adrenoceptor agonist, isoprenaline, elicits vasodilation and tachycardia in anesthetized rats via activation of propranolol-sensitive beta1- and beta2-adrenoceptors and also by propranolol-insensitive beta1- and beta3-adrenoceptors. The aim of this study was to determine whether the relative contribution of propranolol-sensitive and -insensitive beta-adrenoceptors to the changes in heart rate (HR) and vascular resistances elicited by isoprenaline is altered after blockade of nitric oxide (NO) synthase, in pentobarbital-anesthetized rats. The hemodynamic responses elicited by isoprenaline (0.1 and 0.5 microg kg(-1), i.v.) were determined before and after injection of saline or the NO synthase inhibitor, N(G)-nitro-L-arginine methylester (L-NAME, 50 micromol kg(-1), i.v.), and again after injection of the beta1- and beta2-adrenoceptor antagonist, propranolol (1 mg kg(-1), i.v.). The responses elicited by the above doses of isoprenaline were also determined before and during infusion of the alpha1-adrenoceptor agonist, phenylephrine (3 microg kg(-1) min(-1), i.v.), and again 15-20 min after injection of propranolol (1.0 mg kg(-1), i.v.). Both doses of isoprenaline elicited tachycardia and reductions in vascular resistances. Propranolol eliminated the responses elicited by the lower dose of isoprenaline and substantially diminished the responses elicited by the higher dose of the beta1-, beta2- and beta3-adrenoceptor agonist. The maximal vasodilator responses elicited by both doses of isoprenaline were not diminished whereas the maximal increases in HR were higher after injection of L-NAME. The ability of propranolol to diminish the hemodynamic actions of isoprenaline was substantially diminished in L-NAME-treated rats, whereas propranolol retained its potency in rats that received an equi-pressor infusion of the alpha1-adrenoceptor agonist, phenylephrine. The finding that the maximal vasodilator responses elicited by isoprenaline were not diminished by L-NAME suggests that the vasodilation elicited by this drug was due to direct activation of beta-adrenoceptors on vascular smooth muscle and that the full compliment of isoprenaline-sensitive receptors was not changed after inhibition of NO synthesis. However, these results suggest that the activities of propranolol-sensitive beta-adrenoceptors are downregulated, whereas propranolol-insensitive beta-adrenoceptors are upregulated upon the loss of exposure to endothelial nitrosyl factors.

Differential effects of nitroblue tetrazolium on the hemodynamic responses elicited by activation of alpha1-adrenoceptors and 5-HT2 receptors in conscious rats

This study determined the effects of the lipophobic electron acceptor, nitroblue tetrazolium (5 micromol/kg, i.v.) on the vasoconstrictor responses elicited by the 5-hydroxytryptamine2 (5-HT2) receptor agonist, alpha-methyl-5-HT (5-50 microg/kg, i.v.) and the alpha1-adrenoceptor agonist, phenylephrine (2.5-20 microg/kg, i.v.) in conscious Sprague-Dawley rats. The systemic injection of nitroblue tetrazolium elicited pronounced hemodynamic responses that subsided by 10-15 min. Prior to the administration of nitroblue tetrazolium, the injections of alpha-methyl-5-HT and phenylephrine elicited dose-dependent increases in mean arterial blood pressure and mesenteric, renal and hindquarter vascular resistances. After administration of nitroblue tetrazolium, the vasoconstrictor responses elicited by alpha-methyl-5-HT were augmented whereas those elicited by phenylephrine were diminished. These results are consistent with the possibility that nitroblue tetrazolium interacts with the extracellular ligand-binding domains of 5-HT2 receptors and alpha1-adrenoceptor and that this interaction has opposite effects on activities of these G protein coupled receptors.

Potential role of nitration and oxidation reactions in the effects of peroxynitrite on the function of β-adrenoceptor sub-types in the rat

This study examined the hemodynamic responses elicited by the beta-adrenoceptor agonist, isoproterenol (1 and 10 microg/kg, i.v.) before and after administration of (i) peroxynitrite (10 x 10 micromol/kg, i.v.), (ii) the thiol chelator, para-hydroxymercurobenzoic acid (pHMBA, 75 micromol/kg, i.v.), and (iii) the electron acceptor, nitroblue tetrazolium (NBT, 10 micromol/kg, i.v.) in pentobarbital-anesthetized rats. The tachycardia elicited by the lower dose of isoproterenol was diminished whereas the tachycardia elicited by the higher dose was not attenuated after administration of peroxynitrite. The falls in hindquarter and renal vascular resistances elicited by both doses of isoproterenol were substantially diminished whereas the isoproterenol-induced falls in mesenteric vascular resistance were not changed after administration of peroxynitrite. All of the isoproterenol-induced responses were markedly attenuated after administration of pHMBA or NBT. These findings suggest that the oxidation and/or nitration of beta-adrenoceptors impair the ability of isoproterenol to bind to and/or activate these G protein-coupled receptors. beta1-, beta2- and beta3-adrenoceptors contain extracellular cysteine residues susceptible to oxidation (i.e., disulfide-bridge formation) whereas only the beta1- and beta2-adrenoceptors contain extracellular tyrosine residues susceptible to nitration. These findings also suggest that sustained impairment of beta1- and beta2-adrenoceptor function by peroxynitrite is due to nitration of extracellular tyrosine residues in these receptors. By analogy, beta3-adrenoceptors may not be permanently affected by peroxynitrite because these receptors are devoid of extracellular tyrosine residues.

A search for presynaptic beta3-adrenoceptors in the rat

Presynaptically localized adrenoceptors occur on a variety of neurones. In particular, alpha2-adrenoceptors, occurring on neurones of the peripheral and central nervous system, inhibit the release of the respective transmitters whereas beta2-adrenoceptors on some types of postganglionic sympathetic neurones facilitate noradrenaline release. Since only little information is available whether there are also presynaptic beta3-adrenoceptors, we examined the effect of beta3-adrenoceptor agonists on noradrenaline release from the resistance vessels and the hippocampus of the rat and on serotonin and acetylcholine release from the rat hippocampus. In rat hippocampal slices preincubated with (H-noradrenaline, 3H-serotonin and 3H-choline and superfused in the presence of an inhibitor of the neuronal transporter of the respective neurone, the beta3-adrenoceptor agonist CL 316243 did not affect the electrically evoked tritium overflow. The latter was, however, inhibited by at least 50% by agonists of the respective autoreceptors. CL 316243 and another three beta3-adrenoceptor agonists (BRL 37344, ZD 2079 and CGP 12177) failed to affect the electrically evoked tritium overflow also in slices preincubated with 3H-noradrenaline and superfused in the presence of the alpha2-adrenoceptor antagonist rauwolscine whereas prostaglandin E2 caused a marked inhibition. In pithed and vagotomized rats, the increase in diastolic blood pressure induced by electrical stimulation of the sympathetic outflow was also not affected by CL 316243 but markedly inhibited by the cannabinoid receptor agonist WIN 55212-2. CL 316243 and WIN 55212-2 were devoid of an effect on the rise in diastolic blood pressure induced by exogenous noradrenaline. In conclusion, our data suggest that the noradrenergic neurones innervating the resistance vessels of the rat and the noradrenergic, serotoninergic and cholinergic neurones of the rat hippocampus are not endowed with presynaptic beta3-adrenoceptors.

Atypical beta-adrenoceptors, different from beta 3-adrenoceptors and probably from the low-affinity state of beta 1-adrenoceptors, relax the rat isolated mesenteric artery

(1) We examined whether beta3- and/or atypical beta-adrenoceptors relax the rat isolated mesenteric artery. (2) Mesenteric arteries precontracted with phenylephrine were relaxed by beta-agonists with the following potencies (pD2): nonselective agonist isoprenaline (6.00)>nonconventional partial agonist cyanopindolol (5.45)>beta2-agonist fenoterol (4.98)>nonconventional partial agonist CGP 12177 (4.19)>beta3-agonist ZD 2079 (3.72). The beta3-agonist CL 316243 1 mm relaxed the vessel only marginally. (3) The concentration-response curves (CRCs) for cyanopindolol, CGP 12177 and ZD 2079 were not affected by the nonselective beta-antagonist propranolol 0.3 microm, the beta2-antagonist ICI 118551 1 microm and by CL 316243 60 microm, but shifted to the right by bupranolol (pA2 5.3-5.7), CGP 20712 (5.4) and SR 59230A (6.5-6.7) (the latter three drugs block atypical and/or beta3-adrenoceptors at high concentrations). (4) The CRC for isoprenaline was shifted to the right by propranolol (pA2 7.0) but, in the presence of propranolol 0.3 microm, not affected by SR 59230A 1 microm. The CRC for fenoterol was shifted to the right by propranolol (pA2 6.9) and ICI 118551 (6.8). (5) Removal of endothelium diminished the vasorelaxant effects of cyanopindolol, CGP 12177 and ZD 2079. (6) Fenoterol and cyanopindolol also relaxed (endothelium-intact) mesenteric arteries precontracted with serotonin. The relaxant effect of cyanopindolol was antagonized by bupranolol to about the same degree as in phenylephrine-contracted vessels. (7) In conclusion, beta2- and atypical beta-adrenoceptors (but not beta3-adrenoceptors) relax the rat mesenteric artery. The atypical beta-adrenoceptor, which is partially located endothelially, may differ from the low-affinity state of the beta1-adrenoceptor.

Atypical cardiostimulant beta-adrenoceptor in the rat heart: stereoselective antagonism by bupranolol but lack of effect by some bupranolol analogues

1. Atypical beta-adrenoceptors resistant to propranolol, but blocked by bupranolol, increase contractile force and/or frequency of the heart in humans and rats. We compared the potencies of the enantiomers of bupranolol and examined the possible effects of seven bupranolol analogues including bevantolol (BEV) at this receptor in pithed and vagotomized rats. 2. CGP 12177, an agonist of the atypical beta-adrenoceptor, increased heart rate dose-dependently. Its dose-response curve was shifted to the right by S-(-)-bupranolol 10 micro mol kg(-1) by a factor of 8.4, but not affected by the same dose of R-(+)-bupranolol. 3. Desmethylbupranolol and compounds BK-21, BK-22, BK-23 and BK-25 also increased heart rate dose-dependently. The beta(1)-adrenoceptor antagonist CGP 20712 given in combination with the beta(2)-adrenoceptor antagonist ICI 118,551 (0.1 micro mol kg(-1) each) reduced the positive chronotropic action of the five bupranolol analogues without affecting that of CGP 12177. The potencies of the bupranolol analogues to increase heart rate were correlated (r=0.91, P<0.05) with their affinities for beta(1)-adrenoceptor binding sites in rat brain cortex membranes labelled with [(3)H]CGP 12177 (in the presence of ICI 118,551). 4. BK-26 and BEV, 10 micro mol kg(-1) each, had only minor effects on heart rate by themselves and did not antagonize the effect of CGP 12177. However, at 1 micro mol kg(-1), they antagonized the increase in heart rate elicited by the beta(1)-adrenoceptor agonist prenalterol. 5. In conclusion, bupranolol is a stereoselective antagonist at the atypical cardiostimulant beta-adrenoceptor. The effects of the bupranolol analogues are related to the activation or blockade of beta(1)-adrenoceptors, but not of atypical beta-adrenoceptors.

Effects of chronic lesions of the anteroventral region of the third ventricle on cardiac beta-adrenoceptor function in conscious rats

This study examined whether cardiac beta-adrenoceptor (beta-AR) function was altered in conscious rats with lesions surrounding the anteroventral third ventricle (AV3V). The findings were: (1) beta(1,2)-AR-mediated tachycardia was similar in sham and AV3V-lesion rats, (2) beta(3)- and/or atypical beta-AR-mediated tachycardia elicited by isoproterenol (10 microg/kg, i.v.; ISO) was diminished in AV3V-lesion rats treated with beta(1,2)-AR antagonists, but was not in similarly-treated sham-lesion rats, and (3) the tachycardia elicited by the membrane permeable cAMP-analogue, 8-(4-chlorophenylthiol)-cAMP (10 micromol/kg, i.v.), was similar in AV3V- and sham-lesion rats. The possibility that increased plasma sodium/osmolality in AV3V-lesion rats down-regulated cardiac beta(3)- and/or atypical beta-ARs, but not beta(1,2)-ARs or intracellular cAMP signaling is discussed.

Beta3-adrenoceptors in the cardiovascular system

beta-Adrenoceptors of the beta1 and beta2 subtypes classically mediate the effects of catecholamines on the contractility of cardiac muscle and the relaxation of vascular smooth muscle. Since the molecular characterization of the beta3-adrenoceptor in 1989, most studies of this adrenoceptor subtype have focused on its control of lipolysis in adipose tissues. However, more recent studies have investigated the involvement of beta3-adrenoceptors in the physiological control of cardiac and vascular contractility. In this article, the pharmacological and molecular evidence that supports the functional role of beta3-adrenoceptors in cardiovasculartissues of various species, including humans, will be discussed. These data might provide new insights into our understanding of the abnormal responsiveness of the cardiovascular system to catecholamines in heart failure and its treatment with beta3-adrenoceptor antagonists.

The negative inotropic action of catecholamines: Role of β3-adrenoceptors

There is now evidence for the involvement of four β-adrenoceptor populations in the regulation of cardiac function by catecholamines. β1- and β2-adrenoceptor stimulation classically produces an increase in contractility. A fourth β-adrenoceptor, as yet uncloned and designated provisionally as a β4-adrenoceptor, also mediates a positive inotropic effect. β3-adrenoceptors, which had been cloned at the end of the eighties, has been extensively studied as a potential target for antiobesity and antidiabetic drugs. Its characterization in the heart has opened new fields of investigations for the understanding of the cardiac adrenergic regulation. This review describes the cardiac electrical and mechanical effects induced by β3-adrenoceptor stimulation in different species (including human), as well as the signaling pathway. It also analyzes the role of these receptors in the abnormal responsiveness of catecholamines in heart failure.Key words: beta-adrenoceptor, heart, contractility, signaling pathway, heart failure.

In vivo evidence that isoproterenol may increase heart rate in the rat by mechanisms in addition to activation of cardiac beta(1)- or beta(2)-adrenoceptors

This study demonstrates that the tachycardia produced by systemic injections of the beta-adrenoceptor agonist, isoproterenol (10 microg/kg, i.v.), in conscious rats were not reduced after injection of the selective beta(1)-adrenoceptor antagonist, atenolol (1 mg/kg, i.v.), or after subsequent injection of the beta(1,2)-adrenoceptor antagonist, propranolol (1 mg/kg, i.v.). The hypotensive responses produced by isoproterenol were slightly diminished by atenolol and markedly diminished by propranolol. The tachycardia produced by catecholamines released for cardiac sympathetic nerve terminals were blocked by atenolol. These results suggest that the hypotensive actions of a 10 microg/kg dose of isoproterenol are mediated by activation of beta(1,2)-adrenoceptors whereas the increases in heart rate may be due to activation of another type of beta-adrenoceptor in cardiac pacemaker cells.

Long-term nitric oxide inhibition and chronotropic responses in rat isolated right atria

The long-term administration of nitric oxide synthesis inhibitors induces arterial hypertension accompanied by left ventricular hypertrophy and myocardial ischemic lesions. Because the enhancement of sympathetic drive has been implicated in these phenomena, the current study was performed to determine the potency of beta-adrenoceptor agonists and muscarinic agonists on the spontaneous rate of isolated right atria from rats given long-term treatment with the nitric oxide inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME). Atrial lesions induced by long-term treatment with L-NAME were also evaluated. Long-term L-NAME treatment caused a time-dependent, significant (P<0.05) increase in tail-cuff pressure compared with control animals. Our results showed that the potency of isoproterenol, norepinephrine, carbachol, and pilocarpine in isolated right atria from rats given long-term treatment with L-NAME for 7, 15, 30, and 60 days was not affected as compared with control animals. Addition of L-NAME in vitro (100 microl/L) affected neither basal rate nor chronotropic response for isoproterenol and norepinephrine in rat heart. Stereological analysis of the right atria at 15 and 30 days revealed a significant increase on amount of fibrous tissues in L-NAME-treated groups (27+/-2.3% and 28+/-1.3% for 15 and 30 days, respectively; P<0.05) as compared with the control group (22+/-1.1%). Our results indicate that nitric oxide does not to interfere with beta-adrenoceptor-mediated and muscarinic receptor-mediated chronotropic responses.