Mediation of the positive chronotropic effect of CGP 12177 and cyanopindolol in the pithed rat by atypical beta-adrenoceptors, different from beta 3-adrenoceptors

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.

Identification of key residues in transmembrane 4 responsible for the secondary, low-affinity conformation of the human β1-adrenoceptor

The β1-adrenoceptor exists in two agonist conformations/states: 1) a high-affinity state where responses to catecholamines and other agonists (e.g., cimaterol) are potently inhibited by β1-adrenoceptor antagonists, and 2) a low-affinity secondary conformation where agonist responses, particularly CGP12177 [(-)-4-(3-tert-butylamino-2-hydroxypropoxy)-benzimidazol-2-one] are relatively resistant to inhibition by β1-adrenoceptor antagonists. Although both states have been demonstrated in many species (including human), the precise nature of the secondary state is unknown and does not occur in the closely related β2-adrenoceptor. Here, using site-directed mutagenesis and functional measurements of production of a cyclic AMP response element upstream of a secreted placental alkaline phosphatase reporter gene and accumulation of (3)H-cAMP, we examined the pharmacological consequences of swapping transmembrane (TM) regions of the human β1- and β2-adrenoceptors, followed by single point mutations, to determine the key residues involved in the β1-adrenoceptor secondary conformation. We found that TM4 (particularly amino acids L195 and W199) had a major role in the generation of the secondary β1-adrenoceptor conformation. Thus, unlike at the human β1-wild-type adrenoceptor, at β1-TM4 mutant receptors, cimaterol and CGP12177 responses were both potently inhibited by antagonists. CGP12177 acted as a simple partial agonist with similar KB and EC50 values in the β1-TM4 but not β1-wild-type receptors. Furthermore pindolol switched from a biphasic concentration response at human β1-wild-type adrenoceptors to a monophasic concentration response in the β1-TM4 mutant receptors. Mutation of these amino acids to those found in the β2-adrenoceptor (L195Q and W199Y), or mutation of a single residue (W199D) in the human β1-adrenoceptor thus abolished this secondary conformation and created a β1-adrenoceptor with only one high-affinity agonist conformation.

β-Adrenergic Receptor-Stimulated Cardiac Myocyte Apoptosis: Role of β1 Integrins

Increased sympathetic nerve activity to the myocardium is a central feature in patients with heart failure. Accumulation of catecholamines plays an important role in the pathogenesis of heart disease. Acting via β-adrenergic receptors (β-AR), catecholamines (norepinephrine and isoproterenol) increase cardiac myocyte apoptosis in vitro and in vivo. Specifically, β₁-AR and β₂-AR coupled to Gαs exert a proapoptotic action, while β₂-AR coupled to Gi exerts an antiapoptotic action. β1 integrin signaling protects cardiac myocytes against β-AR-stimulated apoptosis in vitro and in vivo. Interaction of matrix metalloproteinase-2 (MMP-2) with β1 integrins interferes with the survival signals initiated by β1 integrins. This paper will discuss background information on β-AR and integrin signaling and summarize the role of β1 integrins in β-AR-stimulated cardiac myocyte apoptosis.

The discovery of drugs for obesity, the metabolic effects of leptin and variable receptor pharmacology: perspectives from beta3-adrenoceptor agonists

Although beta3-adrenoceptor (beta3AR) agonists have not become drugs for the treatment of obesity or diabetes, they offer perspectives on obesity drug discovery, the physiology of energy expenditure and receptor pharmacology. beta3AR agonists, some of which also stimulate other betaARs in humans, selectively stimulate fat oxidation in rodents and humans. This appears to be why they improve insulin sensitivity and reduce body fat whilst preserving lean body mass. Regulatory authorities ask that novel anti-obesity drugs improve insulin sensitivity and reduce mainly body fat. Drugs that act on different targets to stimulate fat oxidation may also offer these benefits. Stimulation of energy expenditure may be easy to detect only when the sympathetic nervous system is activated. Leptin resembles beta3AR agonists in that it increases fat oxidation, energy expenditure and insulin sensitivity. This is partly because it raises sympathetic activity, but it may also promote fat oxidation by directly stimulating muscle leptin receptors. The beta1AR and beta2AR can, like the beta3AR, display atypical pharmacologies. Moreover, the beta3AR can display variable pharmacologies of its own, depending on the radioligand used in binding studies or the functional response measured. Studies on the beta3AR demonstrate both the difficulties of predicting the in vivo effects of agonist drugs from in vitro data and that there may be opportunities for identifying drugs that act at a single receptor but have different profiles in vivo.

A study of antagonist affinities for the human histamine H2 receptor

BACKGROUND AND PURPOSE

Ligand affinity has been a fundamental concept in the field of pharmacology and has traditionally been considered to be constant for a given receptor-ligand interaction. Recent studies have demonstrated that this is not true for all three members of the G(s)-coupled beta-adrenoceptor family. This study evaluated antagonist affinity measurements at a different G(s)-coupled receptor, the histamine H(2) receptor, to determine whether antagonist affinity measurements made at a different family of GPCRs were constant.

EXPERIMENTAL APPROACH

CHO cells stably expressing the human histamine H(2) receptor and a CRE-SPAP reporter were used and antagonist affinity was assessed in short-term cAMP assays and longer term CRE gene transcription assays.

KEY RESULTS

Nine agonists and seven antagonists, of sufficient potency at the H(2) receptor to examine in detail, were identified. Measurements of antagonist affinity were the same regardless of the efficacy of the competing agonist, time of agonist incubation, cellular response measured or presence of a PDE inhibitor.

CONCLUSIONS AND IMPLICATIONS

Antagonist affinity at the G(s)-coupled histamine H(2) receptor obeys the accepted dogma for antagonism at GPCRs. This study further confirms that something unusual is indeed happening with the beta-adrenoceptors and is not an artefact related to the transfected cell system used. As the human histamine H(2) receptor does not behave in a similar manner to any of the human beta-adrenoceptors, it is clear that information gathered from one GPCR cannot be simply extrapolated to predict the behaviour of another GPCR. Each GPCR therefore requires careful and detailed evaluation on its own.

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.

Recruitment of functionally active heart beta2-adrenoceptors in the initial phase of endotoxic shock in pithed rats

A supersensitivity of the beta-adrenoceptor-mediated chronotropic response has been demonstrated in atria isolated from rats subjected to septic shock. Our study was undertaken to investigate whether bacterial endotoxin/LPS affects the increase in heart rate induced by beta-adrenoceptor agonists in the rat also in vivo. In pithed and vagotomized rats, the nonselective beta-adrenoceptor agonist isoprenaline (0.05-0.15 nmol/kg) and agonists at the high- and low-affinity state of beta1-adrenoceptors, that is, prenalterol (0.3-3 nmol/kg) and (+/-)-4-[3-[(1,1-dimethylethyl)amino]-2-hydroxypropoxy]-1,3-dihydro-2H-benzimidazole-2-one (CGP 12177; 3-6 nmol/kg), respectively, and at beta2-adrenoceptors, that is, fenoterol (1-5 nmol/kg), increased heart rate by 50 to 60 beats/min. Administration of LPS (0.4, 1, and 1.5 mg/kg), under continuous infusion of vasopressin, dose-dependently amplified the chronotropic response to isoprenaline, prenalterol, and fenoterol (by 80%, 50%, and 100%, respectively) but not to CGP 12177. The beta2-adrenoceptor antagonist erythro-(+/-)-1-(7-methylindan-4-yloxy)-3-isopropylaminobutan-2-ol (ICI 118551 0.1 mumol/kg) did not affect the chronotropic responses of isoprenaline, fenoterol, and prenalterol under non-endotoxic conditions, but abolished the potentiation of tachycardia produced by LPS (1.5 mg/kg). The beta1-adrenoceptor antagonist (+/-)-2-hydroxy-5-[2-[[2-hydroxy-3-[4-[1-methyl-4-(trifluoromethyl)-1H-imidazol-2-yl]-phenoxy]propyl]-amino]ethoxy]-benzamide CGP 20712A; 0.1 mumol/kg almost completely reduced the chronotropic effects of isoprenaline, fenoterol, and prenalterol both in control rats and in animals exposed to LPS (1.5 mg/kg). We conclude that LPS sensitizes cardiac beta-adrenoceptors by recruiting functionally active beta2-adrenoceptors, but the amplification of tachycardia occurs only when both beta1- and beta2-adrenoceptors are concomitantly activated. The pithed rat may serve as a model to examine the beta-adrenoceptor supersensitivity in vivo.

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.

Positive inotropic and lusitropic effects mediated via the low-affinity state of beta1-adrenoceptors in pithed rats

1 Activation by CGP 12177 and cyanopindolol of the human and rat low-affinity state of beta(1)-adrenoceptors increases frequency and contractile force and hastens relaxation in isolated cardiac tissues, and probably relaxes isolated vessels. In order to identify the positive inotropic, positive lusitropic and vasodilator effects of both agonists also in vivo, we have determined their effects on the left ventricular systolic pressure (LVSP), the rate of intraventricular pressure rise (+dP dt(-1)(max)) and decline (-dP dt(-1)(max)), the diastolic blood pressure (DBP) and the mesenteric blood flow (MBF) in pithed and vagotomized rats. 2 CGP 12177 (0.1-100 nmol kg(-1)) and cyanopindolol (1-1000 nmol kg(-1)) dose-dependently enhanced all cardiac parameters. The nonselective beta-adrenoceptor antagonist bupranolol 10 micromol kg(-1) diminished the CGP 12177 (100 nmol kg(-1))-stimulated increases in LVSP from 26.3+/-8.2 to 13.1+/-1.8 mmHg (P<0.05), +dP dt(-1)(max) from 5287+/-290 to 2439+/-296 mmHg s(-1) (P<0.001) and -dP dt(-1)(max) from -3836+/-301 to -2187+/-443 mmHg s(-1) (P<0.05), respectively. The beta(1)-adrenoceptor antagonist CGP 20712A 10 micromol kg(-1) (known to block the low-affinity state of beta(1)-adrenoceptors at high doses) inhibited increases in +/-dP dt(-1)(max) elicited by the highest dose of CGP 12177. 3 The highest doses of CGP 12177 and cyanopindolol increased DBP by about 10 mmHg and MBF by 1.4+/-0.3 and 0.6+/-0.3 ml min(-1), respectively. The vascular effects of CGP 12177 were not affected by bupranolol and CGP 20712A. 4 In conclusion, activation of the low-affinity state of beta(1)-adrenoceptors by CGP 12177 and cyanopindolol in pithed rats causes a positive inotropic and lusitropic effect. By contrast, the vascular effects of CGP 12177 and cyanopindolol are not mediated by these receptors and have only marginal influence under in vivo conditions.

Direct demonstration of beta1- and evidence against beta2- and beta3-adrenoceptors, in smooth muscle cells of rat small mesenteric arteries

1 Recent evidence supports additional subtypes of vasodilator beta-adrenoceptor (beta-AR) besides the 'classical' beta(2). The aim of this study was to investigate the distribution of beta-ARs in the wall of rat mesenteric resistance artery (MRA), to establish the relative roles of beta-ARs in smooth muscle and other cell types in mediating vasodilatation and to analyse this in relation to the functional pharmacology. 2 We first examined the vasodilator beta-AR subtype using 'subtype-selective' agonists against the, commonly employed, phenylephrine-induced tone. Concentration-related relaxation was produced by isoprenaline (pEC(50): 7.70+/-0.1) (beta(1) and beta(2)). Salbutamol (beta(2)), BRL 37344 (beta(3)) and CGP 12177 (atypical beta) caused relaxation but were 144, 100 and 263 times less potent than isoprenaline; the 'beta(3)-adrenoceptor agonist' CL 316243 was ineffective. 3 In arteries precontracted with 5-HT or U 46619, isoprenaline produced concentration-related relaxation but salbutamol, BRL 37344, CGP 12177 and CL 316243 did not. SR 59230A, CGP 12177 and BRL 37344 caused a parallel rightward shift in the concentration-response curve to phenylephrine indicating competitive alpha(1)-AR antagonism, explaining the false-positive 'vasodilator' action against phenylephrine-induced tone. Endothelial denudation but not L-NAME slightly attenuated isoprenaline-mediated vasodilatation in phenylephrine and U 46619 precontracted MRA. 4 The beta-AR fluorescent ligand BODIPY TMR-CGP 12177 behaved as an irreversible beta(1)-AR antagonist in MRA and bound to the surface and inside vascular smooth muscle cells in intact vascular wall. Beta-ARs in smooth muscle cells were observed in a perinuclear location, consistent with the location of Golgi and endoplasmic reticulum. 5 Binding of BODIPY TMR-CGP 12177 was inhibited by BAAM (1 microM) in all three vascular tunics, confirming the presence of beta-ARs in adventitia, media and intima. Binding in adventitia was observed in both neuronal and non-neuronal cell types. Lack of co-localisation with a fluorescent ligand for alpha-ARs confirms the selectivity of BODIPY TMR-CGP 12177 for beta-ARs over alpha-ARs. 6 Our results support the presence of functional vasodilator beta(1)-ARs and show that they are mainly located in smooth muscle cells. Furthermore, we have demonstrated, for the first time, the usefulness of BODIPY TMR-CGP 12177 for identifying beta-AR distribution in the 'living' vascular wall.

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.

ALpha1-adrenoceptor antagonist properties of CGP 12177A and other beta-adrenoceptor ligands: evidence against beta(3)- or atypical beta-adrenoceptors in rat aorta

1. The alpha(1)-adrenoceptor antagonist properties of the beta-adrenoceptor nonconventional partial agonist, CGP 12177A, was investigated in functional assays in rat aorta and in radioligand binding assays in rat cerebral cortical membranes. In addition, binding affinities of other beta-adrenoceptor ligands were measured to investigate any correlation between alpha(1)-adrenoceptor affinity and relaxant potency in phenylephrine-constricted rings. 2. In functional studies, CGP 12177A produced parallel rightward shifts of the phenylephrine CRC with no reduction in the maximum responses. Schild regression analysis gave a straight line with a slope of 0.95 (95% CL: 0.87-1.04), suggesting reversible competitive antagonism, and gave a pK(B) value of 5.26. In contrast, CGP 12177A (<or=300 microm) had no effect on contraction induced by the thromboxane-mimetic, U46619. 3. In binding studies, CGP 12177A competed monophasically with [(3)H]prazosin binding (Hill slope, 0.95, 95% CL: 0.76-1.13), giving a pK(i) value of 5.48, in good agreement with the pK(B) from functional studies. 4. Competition experiments with various other beta-adrenoceptor ligands showed that they all displaced [(3)H]prazosin in a manner consistent with one-site competition. pK(i) values were as follows: SR 59230A, 6.25; cyanopindolol, 6.33; bupranolol, 6.35; alprenolol, 5.90; propranolol, 5.80; BRL 37344, 5.50; ICI 118551, 5.55; CGP 20712A, 5.26. The pK(i) values correlated well with the pEC(50) values for relaxation of phenylephrine-constricted rat aorta obtained previously (r(2)=0.984, P<0.0001). 5. In conclusion, relaxant effects of CGP 12177A and other beta-adrenoceptor ligands in phenylephrine-constricted rat aorta can be attributed to alpha(1)-adrenoceptor blockade and are unrelated to effects at beta(3)-adrenoceptors or atypical beta-adrenoceptors.

Overexpression of beta 1-adrenoceptors in adult rat ventricular myocytes enhances CGP 12177A cardiostimulation: implications for 'putative' beta 4-adrenoceptor pharmacology

1. CGP 12177A mediates cardiostimulation by activation of the 'putative' beta(4)-adrenoceptor; however, it has recently been reported that disruption of the beta(1)-adrenoceptor gene abolishes this effect. We have adenovirally overexpressed beta(1)-adrenoceptors in isolated, cultured adult rat ventricular cardiomyocytes and observed the inotropic potency of isoprenaline and CGP 12177A (in the presence of 1 microm propranolol). 2. Isoprenaline was a full inotropic agonist at rat ventricular myocytes (pD(2) 7.69+/-0.12). CGP 12177A was a nonconventional partial agonist (pD(2) 6.34+/-0.09), increasing inotropy and lusitropy, with an intrinsic activity of 0.34 and antagonised by bupranolol. 3. beta(1)-adrenoceptor overexpression enhanced the inotropic potency of isoprenaline by 11.7-fold (pD(2) 8.76+/-0.14) and CGP 12177A by 5.9-fold (7.11+/-0.10), respectively. Green fluorescent protein (GFP) overexpression did not alter the potency of isoprenaline or CGP 12177A (pD(2) 7.41+/-0.24 and pD(2) 6.60+/-0.50, respectively). 4. The cardiostimulant effects of CGP 12177A were enhanced by IBMX (phosphodiesterase inhibitor) and decreased by Rp-cAMPS (cAMP antagonist). CGP 12177A also increased cAMP levels. CGP 12177A but not isoprenaline initiated arrhythmias at lower concentrations following beta(1)-adrenoceptor overexpression. 5. (125)I-Cyanopindolol saturation binding in Adv.beta(1) myocytes demonstrated approximately 18-fold increase in beta(1)-adrenoceptors. (3)H-CGP 12177A saturation binding, in the presence of propranolol, increased approximately 5-fold following overexpression of beta(1)-adrenoceptors. 6. This study demonstrates enhanced cardiostimulation by CGP 12177A (in the presence of propranolol) in rat ventricular myocytes overexpressing beta(1)-adrenoceptors, mediated by a Gs/cAMP signalling pathway. 'Putative' beta(4)-adrenoceptor pharmacology appears to be mediated by activation of a novel affinity state of the beta(1)-adrenoceptor.

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.

Cardiostimulant and cardiodepressant effects through overexpressed human beta2-adrenoceptors in murine heart: regional differences and functional role of beta1-adrenoceptors

(-)-Isoprenaline enhances cardiac contractility through beta-adrenoceptors. However, in cardiac tissue from transgenic mice with a 200-400-fold cardiac overexpression of the human beta(2)-adrenoceptor (TG4) we observed a pronounced cardiodepression at high (-)-isoprenaline concentrations. Here, we investigated the functional role of the coexisting beta(1)-, beta(2)-, and beta(3)-adrenoceptor subtypes in several regions of the TG4 heart, and in particular their contribution to the negative inotropic effect. In paced TG4 left atria, (-)-isoprenaline produced bell-shaped concentration-effect curves increasing (-logEC(50)M=9.0) and decreasing (-logIC(50)M=6.4) contractile force. These effects were unaffected by the beta(1)-selective CGP 20712A (300 nM). The beta(2)-selective inverse agonist ICI 118,551 (30-1,000 nM) antagonised in surmountable manner both the positive and negative inotropic effects of (-)-isoprenaline with similar concentration-dependence, consistent with an exclusive mediation through beta(2)-adrenoceptors. The beta(3)-adrenoceptor-selective agonist BRL37344 (1 nM-10 microM) failed to produce significant inotropic effects in TG4 left atria. Subsequently, we measured left atrial action potentials accompanying the inotropic changes induced by (-)-isoprenaline. Action potentials tended to have shorter duration in left atria from TG4 mice than from non-transgenic littermate mice. However, (-)-isoprenaline prolonged the duration of 30% repolarisation in atria from non-transgenic littermate but not from TG4 mice, while 90% repolarisation was abbreviated in both groups of atria. Negative inotropic effects of (-)-isoprenaline were also observed in right ventricular preparations. Pertussis toxin-treatment of the mice abolished the negative inotropic effects in left atria and reduced cardiodepression in right ventricle, indicating an involvement of beta(2)-adrenoceptor coupling to PTX-sensitive G-proteins. In additional experiments, designed to study the native murine beta(1)-adrenoceptor function, we used the physiological beta(1)-adrenoceptor agonist (-)-noradrenaline. In the presence of 600 nM ICI 118,551 we failed to find a functional role of the beta(1)-adrenoceptors in left atria, and detected only a marginal contribution to the positive chronotropic effect in right atria. We also investigated the effects of the non-conventional partial agonist (-)-CGP 12177 (0.2 nM-6 microM), which in wild-type mice causes tachycardia through beta(1)-adrenoceptors. In TG4 right atria, however, (-)-CGP 12177-evoked tachycardia was resistant to blockade by CGP 20712A but antagonised by ICI 118,551, consistent with mediation through human beta(2)-adrenoceptors. The results from TG4 mice suggest that the positive and negative inotropic effects of (-)-isoprenaline are mediated through human overexpressed beta(2)-adrenoceptors coupled to G(s) protein and G(i) protein, respectively. The (-)-isoprenaline-evoked shortening of the atrial action potential combined with reduced responses of L-type Ca(2+) current may contribute to the negative inotropic effects. The function of murine cardiac beta(1)-adrenoceptors is suppressed by overexpressed human beta(2)-adrenoceptors.

Evidence against beta 3-adrenoceptors or low affinity state of beta 1-adrenoceptors mediating relaxation in rat isolated aorta

1 The presence of beta(3)-adrenoceptors and the low affinity state of the beta(1)-adrenoceptor (formerly "putative beta(4)-adrenoceptor") was investigated in ring preparations of rat isolated aorta preconstricted with phenylephrine or prostaglandin F(2alpha) (PGF(2alpha)). Relaxant responses to isoprenaline, selective beta(3)-adrenoceptor agonists (BRL 37344, SR 58611A, CL 316243) and non-conventional partial agonists (CGP 12177A, cyanopindolol, pindolol) were obtained. 2 In phenylephrine-constricted, but not PGF(2alpha)-constricted rings, relaxations to isoprenaline showed a propranolol-resistant component. 3 In phenylephrine-constricted rings, relaxations to BRL 37344 (pEC(50), 4.64) and SR 58611A (pEC(50), 4.94) were not antagonized by the selective beta(3)-adrenoceptor antagonist SR 59230A (< or =1 microM). CL 316243 (< or =100 microM) failed to produce relaxation. In PGF(2alpha)-constricted rings only SR 58611A produced relaxation, which was not affected by SR 59230A (< or =3 microM). 4 Non-conventional partial agonists produced relaxation in phenylephrine-constricted but not PGF(2alpha)-constricted rings. The relaxation to CGP 12177A was unaffected by SR 59230A (< or =1 microM) or by CGP 20712A (10 microM), reported to block the low affinity state of the beta(1)-adrenoceptor. 5 beta-adrenoceptor antagonists also produced relaxation in phenylephrine-constricted rings with an order of potency of (pEC(50) values): bupranolol (5.5) approximately 38;SR 59230A (5.47) approximately 38;cyanopindolol (5.47)>pindolol (5.30)>alprenolol (5.10)>propranolol (4.83)>ICI 118551 (4.60)>CGP 12177A (4.38) approximately 38;CGP 20712A (4.35). Bupranolol (100 microM), alprenolol (30 microM), propranolol (100 microM) and SR 59230A (10 microM) produced no relaxation in PGF(2alpha)-constricted rings. 6 These results provide no evidence for the presence of functional beta(3)-adrenoceptors or the low affinity state of the beta(1)-adrenoceptor in rat aorta.

The beta2- and beta3-adrenoceptor-mediated relaxation induced by fenoterol in guinea pig taenia caecum

Fenoterol, a beta2-adrenoceptor selective agonist, belongs to the arylethanolamine class. To understand the receptor subtypes responsible for beta-adrenoceptor-mediated relaxation of guinea pig taenia caecum, we investigated the effect of fenoterol. Fenoterol caused concentration-dependent relaxation of the guinea pig taenia caecum. Propranolol, bupranolol and butoxamine produced shifts of the concentration-response curve for fenoterol. Schild regression analyses carried out for propranolol, butoxamine and bupranolol against fenoterol gave pA2 values of 8.41, 6.33 and 8.44, respectively. However, in the presence of 3 x 10(-4) M atenolol, 10(-4) M butoxamine and 10(-6) M phentolamine to block the beta1-, beta2- and a-adrenoceptor effects, respectively, Schild regression analysis carried out for bupranolol against fenoterol gave pA2 values of 5.80. These results suggest that the relaxant response to fenoterol in the guinea pig taenia caecum is mediated by both the beta2- and the beta3-adrenoceptors.

Pharmacological approaches for the treatment of obesity

The high incidence of obesity, its multifactorial nature, the complexity and lack of knowledge of the bodyweight control system, and the scarcity of adequate therapeutics have fuelled anti-obesity drug development during a considerable number of years. Irrespective of the efforts invested by researchers and companies, few products have reached a minimum level of effectiveness, and even fewer are available in medical practice. As a consequence of anti-obesity research, our knowledge of the bodyweight control system increased but, despite this, the pharmacological approaches to the treatment of obesity have not resulted yet in effective drugs. This review provides a panoramic of the multiple different approaches developed to obtain workable drugs. These approaches, however, rely in only four main lines of action: control of energy intake, mainly through modification of appetite;control of energy expenditure, essentially through the increase of thermogenesis;control of the availability of substrates to cells and tissues through hormonal and other metabolic factors controlling the fate of the available energy substrates; andcontrol of fat reserves through modulation of lipogenesis and lipolysis in white adipose tissue. A large proportion of current research is centred on neuropeptidic control of appetite, followed by the development of drugs controlling thermogenic mechanisms and analysis of the factors controlling adipocyte growth and fat storage. The adipocyte is also a fundamental source of metabolic signals, signals that can be intercepted, modulated and used to force the brain to adjust the mass of fat with the physiological means available. The large variety of different approaches used in the search for effective anti-obesity drugs show both the deep involvement of researchers on this field and the large amount of resources devoted to this problem by pharmaceutical companies. Future trends in anti-obesity drug research follow closely the approaches outlined; however, the increasing mass of information on the molecular basis of bodyweight control and obesity will in the end prevail in our search for effective and harmless anti-obesity drugs.

Bucindolol exerts agonistic activity on the propranolol-insensitive state of beta1-adrenoceptors in human myocardium

In congestive heart failure patients, treatment with beta-adrenoceptor antagonists improves symptoms and decreases mortality. However, intrinsic sympathomimetic activity of beta-adrenoceptor antagonists might be disadvantageous in chronic heart failure. The nonselective beta1- and beta2-adrenoceptor antagonist bucindolol has failed to decrease mortality in clinical trials. A putative beta4-adrenoceptor, which mediates positive inotropic effects by activation of the adenylate cyclase has been described. Recently, this putative beta4-adrenoceptor has been identified to be a propranolol-insensitive state of the beta1-adrenoceptor. The present study aimed to characterize whether bucindolol exhibits agonistic activity on this atypical beta1-adrenoceptor state as one possible reason for clinical inefficiency. For comparison (S)-4-(3'-t-butylamino-1'-hydroxypropoxy)-benzimidozole-2 (CGP 12177), metoprolol, and nebivolol were investigated. Bucindolol did not reveal intrinsic sympathomimetic activity in electrically driven (1 Hz, 37 degrees C), forskolin-stimulated, left ventricular papillary muscle strips (donor hearts, nonfailing; n = 5) and in right auricular trabeculae (bypass operation; n = 4). Functional studies on the propranolol-insensitive state of beta1-adrenoceptors were performed in isolated muscle preparations after beta1- and beta2-adrenoceptor antagonism (propranolol, 1 microM), inhibition of beta3-mediated inotropic effects (N-nitro-L-arginine, 100 microM) and forskolin treatment (0.3 microM). Positive inotropic response to stimulation of atypical state beta1-adrenoceptors could be demonstrated in right auricular as well as left ventricular human myocardium (CGP 12177 treatment, 10 microM). Under these conditions, also bucindolol, but not metoprolol and nebivolol, significantly increased contractility (all 10 microM). In conclusion, bucindolol but not metoprolol or nebivolol mediate positive inotropic effects in human myocardium due to activation of atypical state beta1-adrenoceptors. Thus, the agonistic activity of bucindolol may influence outcome in heart failure patients.

Adrenergic regulation of cardiac myocyte apoptosis

The direct effects of catecholamines on cardiac myocytes may contribute to both normal physiologic adaptation and pathologic remodeling, and may be associated with cellular hypertrophy, apoptosis, and alterations in contractile function. Norepinephrine (NE) signals via alpha- and beta-adrenergic receptors (AR) that are coupled to G-proteins. Pharmacologic studies of cardiac myocytes in vitro demonstrate that stimulation of beta1-AR induces apoptosis which is cAMP-dependent and involves the voltage-dependent calcium influx channel. In contrast, stimulation of beta2-AR exerts an anti-apoptotic effect which appears to be mediated by a pertussis toxin-sensitive G protein. Stimulation of alpha1-AR causes myocyte hypertrophy and may exert an anti-apoptotic action. In transgenic mice, myocardial overexpression of either beta1-AR or G(alpha)s is associated with myocyte apoptosis and the development of dilated cardiomyopathy. Myocardial overexpression of beta2-AR at low levels results in improved cardiac function, whereas expression at high levels leads to dilated cardiomyopathy. Overexpression of wildtype alpha1B-AR does not result in apoptosis, whereas overexpression of G(alpha)q results in myocyte hypertrophy and/or apoptosis depending on the level of expression. Differential activation of the members of the mitogen-activated protein kinase (MAPK) superfamily and production of reactive oxygen species appear to play a key role in mediating the actions of adrenergic pathways on myocyte apoptosis and hypertrophy. This review summarizes current knowledge about the molecular and cellular mechanisms involved in the regulation of cardiac myocyte apoptosis via stimulation of adrenergic receptors and their coupled effector pathways.

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.

Chronotropic response to (±)-CGP12177 in right atria of stressed rats

Foot-shock stress changes the sensitivity of the rat right atria to β1- and β2-adrenoceptor (AR) agonists. We investigated whether the same stress protocol also changes the atrial sensitivity to the non conventional agonist, (±)-CGP12177. Concentration-response curves to (±)-CGP12177, a β1- and β2-adrenoceptor antagonist with agonist properties at the putative β4-adrenoceptors, were obtained in the absence and presence of propranolol (200 nM or 2 µM), CGP20712A 10 nM plus ICI118,551 50 nM, or CGP20712A (1 µM or 3 µM), in right atria from rats submitted to three daily foot-shock sessions (120 mA pulses of 1.0 s duration applied at random intervals of 5-25 s over 30 min) and killed after the third session. The pD2 for (±)-CGP12177 was not influenced by foot-shock stress. The stimulant effect of (±)-CGP12177 was resistant to blockade by 200 nM and 2 µM (±)-propranolol, and to combined blockade by CGP20712A and ICI118,551. However, in right atria from stressed rats given 200 nM propranolol, the concentration-response curve to the agonist was shifted 2.0-fold to the right. CGP20712A shifted the concentration-response curve to (±)-CGP12177 to the right by 4.6- (1 µM) and 19-fold (3 µM) in atria of control rats, and by 2.2- (1 µM) and 43-fold (3 µM) in atria of stressed rats. Maximum response to CGP12177 was not affected by propranolol or CGP20712A in concentrations ranging from 0.1 nM to 10 µM. These results show that the chronotropic effect of (±)-CGP12177 is mediated by atypical β4-adrenoceptors. In constrast with to β1-and (or) β2-AR, this receptor is resistant to the effects of foot-shock stress, suggesting that the putative β4-AR is a different receptor from a low affinity state of β1-adrenoceptor, as previously proposed, unless both proposed isoforms of β1-adrenoceptor show independent stress-induced behavior.Key words: putative β4-adrenoceptor, low affinity β1-adrenoceptor isoform, stress, right atria, chronotropic response.

Agonistic activity of SR59230A at atypical beta-adrenoceptors in guinea pig gastric fundus and duodenum

We have recently suggested that atypical beta-adrenoceptors are present in guinea pig gastric fundus and duodenum. In the present study, we have shown that SR59230A (3-(2-ethylphenoxy)-1-[(1S)-1,2,3,4-tetrahydronaphth-1-ylamino]-(2S)-2-propanol oxalate), a selective beta(3)-adrenoceptor antagonist, possesses agonistic activities at atypical beta-adrenoceptors in these tissues. SR59230A caused concentration-dependent relaxations. However, (+/-)-propranolol (1 microM) did not affect SR59230A-induced relaxations. Pretreatment of with a combination of (+/-)-propranolol (1 microM) and the non-selective beta(1)-, beta(2)-, beta(3)- and beta(4)-adrenoceptor antagonist, (+/-)-bupranolol (30 microM), significantly antagonized the relaxant effects induced by SR59230A. The results clearly indicate that SR59230A acts as an atypical beta-adrenoceptor agonist on guinea pig gastric fundus and duodenum.

(+/-)-Pindolol acts as a partial agonist at atypical beta-adrenoceptors in the guinea pig duodenum

The agonistic and antagonistic effects of (+/-)-pindolol (1-(1H-indol-4-yloxy)-3-[(1-methylethyl)amino]-2-propanol) were estimated to clarify whether (+/-)-pindolol acts as a partial agonist on atypical beta-adrenoceptors in the guinea pig duodenum. (+/-)-Pindolol induced concentration-dependent relaxation with a pD2 value of 5.10 +/- 0.03 and an intrinsic activity of 0.83 +/- 0.03. However, the relaxations to (+/-)-pindolol were not antagonized by the non-selective beta1- and beta2-adrenoceptor antagonist (+/-)-propranolol (1 microM). In the presence of (+/-)-propranolol (1 microM), the non-selective beta1-, beta2- and beta3-adrenoceptor antagonist (+/-)-bupranolol (30 microM) induced a rightward shift of the concentration-response curves for (+/-)-pindolol (apparent pA2 = 5.41 +/- 0.06). In the presence of (+/-)-propranolol, (+/-)-pindolol (10 microM) weakly but significantly antagonized the relaxant effects to catecholamines ((-)-isoprenaline, (-)-noradrenaline and (-)-adrenaline), a selective beta3-adrenoceptor agonist BRL37344 ((R*,R*)-(+/-)-4-[2-[(2-(3-chlorophenyl)-2-hydroxyethyl) amino]propyl]phenoxyacetic acid sodium salt) and a non-conventional partial beta3-adrenoceptor agonist (+/-)-CGP12177A([4-[3-[(1,1-dimethylethyl)amino]-2-hydroxypropoxy]-1,3-dihydro-2H-benzimidazol-2-one] hydrochloride). These results demonstrate that (+/-)-pindolol possesses both agonistic and antagonistic effects on atypical beta-adrenoceptors in the guinea pig duodenum.

Acetyl-RYYRIK-NH(2) is a highly efficacious OP(4) receptor agonist in the cardiovascular system of anesthetized rats

Nociceptin, the endogenous ligand of the OP(4) or ORL(1) (opioid receptor-like(1)) receptor, decreases blood pressure and heart rate in anesthetized rats. Since the OP(4) receptor antagonist [Phe(1)Psi(CH(2)-NH)Gly(2)]-nociceptin(1-13)NH(2) possesses an agonistic effect in this model, we examined whether other purported OP(4) receptor antagonists, acetyl-RYYRIK-NH(2) and naloxone benzoylhydrazone, antagonize the depressant effects of nociceptin. Acetyl-RYYRIK-NH(2), like nociceptin and [Phe(1)Psi(CH(2)-NH)Gly(2)]-nociceptin(1-13)NH(2) and unlike naloxone benzoylhydrazone, decreased diastolic blood pressure and heart rate (rank order of potencies: nociceptin approximately equal to acetyl-RYYRIK-NH(2) >> [Phe(1)Psi(CH(2)-NH)Gly(2)]-nociceptin(1-13)NH(2)). The depressant effects were insensitive to the OP(1-3) receptor antagonist naloxone but diminished by naloxone benzoylhydrazone. In conclusion, the hypotensive and bradycardic effects of nociceptin in the anesthetized rat are mediated via OP(4) receptors, at which acetyl-RYYRIK-NH(2) is a highly potent and efficacious agonist.

Modification of beta-adrenoceptor signal transduction pathway by genetic manipulation and heart failure

The beta-adrenoceptor (beta-AR) mediated signal transduction pathway in cardiomyocytes is known to involve beta1- and beta2-ARs, stimulatory (Gs) and inhibitory (Gi) guanine nucleotide binding proteins, adenylyl cyclase (AC) and cAMP-dependent protein kinase (PKA). The activation of beta1- and beta2-ARs has been shown to increase heart function by increasing Ca2+ -movements across the sarcolemmal membrane and sarcoplasmic reticulum through the stimulation of Gs-proteins, activation of AC and PKA enzymes and phosphorylation of the target sites. The activation of PKA has also been reported to increase phosphorylation of some myofibrillar proteins (for promoting cardiac relaxation) and nuclear proteins (for cardiac hypertrophy). The activation of beta2-AR has also been shown to affect Gi-proteins, stimulate mitogen activated protein kinase and increase protein synthesis by enhancing gene expression. Beta1- and beta2-ARs as well as AC are considered to be regulated by PKA- and protein kinase C (PKC)-mediated phosphorylations directly; both PKA and PKC also regulate beta-AR indirectly through the involvement of beta-AR kinase (betaARK), beta-arrestins and Gbeta gamma-protein subunits. Genetic manipulation of different components and regulators of beta-AR signal transduction pathway by employing transgenic and knockout mouse models has provided insight into their functional and regulatory characteristics in cardiomyocytes. The genetic studies have also helped in understanding the pathophysiological role of PARK in heart dysfunction and therapeutic role of betaARK inhibitors in the treatment of heart failure. Varying degrees of defects in the beta-AR signal transduction system have been identified in different types of heart failure to explain the attenuated response of the failing heart to sympathetic stimulation or catecholamine infusion. A decrease in beta1-AR density, an increase in the level of G1-proteins and overexpression of betaARK are usually associated with heart failure; however, these attenuations have been shown to be dependent upon the type and stage of heart failure as well as region of the heart. Both local and circulating renin-angiotensin systems, sympathetic nervous system and endothelial cell function appears to regulate the status of beta-AR signal transduction pathway in the failing heart. Thus different components and regulators of the beta-AR signal transduction pathway appears to represent important targets for the development of therapeutic interventions for the treatment of heart failure.

Partial agonistic effects of carteolol on atypical beta-adrenoceptors in the guinea pig gastric fundus

The properties of the beta1-/beta2-adrenoceptor partial agonist carteolol were investigated in atypical beta-adrenoceptors on the guinea pig gastric fundus. Carteolol induced concentration-dependent relaxation in this tissue (pD2 = 5.55, intrinsic activity = 0.94). However, a combination of the selective beta1-adrenoceptor antagonist atenolol (100 microM) and the selective beta2-adrenoceptor antagonist butoxamine (100 microM) produced only small rightward shifts in the concentration-response curves of carteolol in the gastric fundus (pD2 = 4.91, intrinsic activity = 0.94). In the presence of both atenolol (100 microM) and butoxamine (100 microM), the non-selective beta1-, beta2- and beta3-adrenoceptor antagonist (+/-)-bupranolol (10-100 microM) caused a concentration-dependent right-ward shift of the concentration-response curves for carteolol in the guinea pig gastric fundus. Schild plot analyses of the effects of (+/-)-bupranolol against carteolol gave the pA2 value of 5.29 and the Schild slope was not significantly different from unity. Furthermore, carteolol (10 microM) weakly but significantly antagonized the relaxant responses to catecholamines ((-)-isoprenaline, (-)-noradrenaline and (-)-adrenaline), a selective beta3-adrenoceptor agonist BRL37344 ((R*,R*)-(+/-)-4-[2-[(2-(3-chlorophenyl)-2-hydroxyethyl)amino]propyl]phenoxy-acetic acid sodium salt) and a non-conventional partial beta3-adrenoceptor agonist (+/-)-CGP12177A ([4-[3-[(1,1dimethylethyl)amino]-2-hydroxypropoxy]-1,3-dihydro-2H-benzimidazol-2-one] hydrochloride) in the guinea pig gastric fundus. These results suggest that the partial agonistic effects of carteolol are mediated by atypical beta-adrenoceptors in the guinea pig gastric fundus.

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.

Beta 1-, beta 2- and atypical beta-adrenoceptor-mediated relaxation in rat isolated aorta

beta-adrenoceptor-mediated relaxation was investigated in ring preparations of rat isolated thoracic aorta. Rings were pre-constricted with a sub-maximal concentration of noradrenaline (1 microM) and relaxant responses to cumulative concentrations of beta-adrenoceptor agonists obtained. The concentration-response curve (CRC) to isoprenaline was shifted to the right by propranolol (0.3 microM) with a steepening of the slope. Estimation of the magnitude of the shift from EC(50) values gave a pA(2) of 7.6. Selective beta(1)- and beta(2)-adrenoceptor antagonists, CGP 20712A (0.1 microM) and ICI 118551 (0.1 microM), respectively, produced 4 and 14 fold shifts of the isoprenaline CRC. Atypical beta-adrenoceptor agonists also produced concentration-dependent relaxation of aortic rings. The order of potency of the beta-adrenoceptor agonists was (-log EC(50)): isoprenaline (6. 25)>cyanopindolol (5.59)>isoprenaline+propranolol (5.11)>CGP 12177A (4.40)>ZD 2079 (4.24)>ZM 215001 (4.07)>BRL 37344 (3.89). Relaxation to CGP 12177A and ZM 215001 was unaffected by propranolol (0.3 microM). SR 59230A ( Collapse

Key Words

• atypical β-adrenoceptors
• β₃-adrenoceptors
• β₄-adrenoceptors
• β₁-adrenoceptors
• β₂-adrenoceptors
• vascular smooth muscle
• rat aorta

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MESH Headings

• Acetylcholine/pharmacology
• Adrenergic alpha-Agonists/pharmacology
• Adrenergic beta-Agonists/pharmacology
• Adrenergic beta-Antagonists/pharmacology
• Animals
• Aorta, Thoracic/drug effects
• Aorta, Thoracic/physiology
• Bupranolol/pharmacology
• Dose-Response Relationship, Drug
• Imidazoles/pharmacology
• In Vitro Techniques
• Isoproterenol/pharmacology
• Male
• Muscle Relaxation/drug effects
• Muscle Relaxation/physiology
• Muscle, Smooth, Vascular/drug effects
• Muscle, Smooth, Vascular/physiology
• Norepinephrine/pharmacology
• Pindolol/analogs & derivatives
• Pindolol/pharmacology
• Propanolamines/pharmacology
• Propranolol/pharmacology
• Rats
• Rats, Wistar
• Receptors, Adrenergic, beta/classification
• Receptors, Adrenergic, beta/physiology
• Receptors, Adrenergic, beta-1/physiology
• Receptors, Adrenergic, beta-2/physiology
• Substrate Specificity
• Vasodilator Agents/pharmacology

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Grants Collapse

Affiliation(s)

L Brawley School of Biological and Biomedical Sciences, Glasgow Caledonian University, Glasgow G4 0BA
A M Shaw School of Biological and Biomedical Sciences, Glasgow Caledonian University, Glasgow G4 0BA
A MacDonald School of Biological and Biomedical Sciences, Glasgow Caledonian University, Glasgow G4 0BA

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Putative beta 4-adrenoceptors in rat ventricle mediate increases in contractile force and cell Ca2+: comparison with atrial receptors and relationship to (-)-[3H]-CGP 12177 binding

1. We identified putative beta4-adrenoceptors by radioligand binding, measured increases in ventricular contractile force by (-)-CGP 12177 and (+/-)-cyanopindolol and demonstrated increased Ca2+ transients by (-)-CGP 12177 in rat cardiomyocytes. 2. (-)-[3H]-CGP 12177 labelled 13 - 22 fmol mg-1 protein ventricular beta1, beta2-adrenoceptors (pKD approximately 9.0) and 50 - 90 fmol mg-1 protein putative beta4-adrenoceptors (pKD approximately 7.3). The affinity values (pKi) for (beta1,beta2-) and putative beta4-adrenoceptors, estimated from binding inhibition, were (-)-propranolol 8.4, 5.7; (-)-bupranolol 9.7, 5.8; (+/-)-cyanopindolol 10.0,7.4. 3. In left ventricular papillary muscle, in the presence of 30 microM 3-isobutyl-1-methylxanthine, (-)-CGP 12177 and (+/-)-cyanopindolol caused positive inotropic effects, (pEC50, (-)-CGP 12177, 7.6; (+/-)-cyanopindolol, 7.0) which were antagonized by (-)-bupranolol (pKB 6.7 - 7.0) and (-)-CGP 20712A (pKB 6.3 - 6.6). The cardiostimulant effects of (-)-CGP 12177 in papillary muscle, left and right atrium were antagonized by (+/-)-cyanopindolol (pKP 7.0 - 7.4). 4. (-)-CGP 12177 (1 microM) in the presence of 200 nM (-)-propranolol increased Ca2+ transient amplitude by 56% in atrial myocytes, but only caused a marginal increase in ventricular myocytes. In the presence of 1 microM 3-isobutyl-1-methylxanthine and 200 nM (-)-propranolol, 1 microM (-)-CGP 12177 caused a 73% increase in Ca2+ transient amplitude in ventricular myocytes. (-)-CGP 12177 elicited arrhythmic transients in some atrial and ventricular myocytes. 5. Probably by preventing cyclic AMP hydrolysis, 3-isobutyl-1-methylxanthine facilitates the inotropic function of ventricular putative beta4-adrenoceptors, suggesting coupling to Gs protein-adenylyl cyclase. The receptor-mediated increases in contractile force are related to increases of Ca2+ in atrial and ventricular myocytes. The agreement of binding affinities of agonists with cardiostimulant potencies is consistent with mediation through putative beta4-adrenoceptors labelled with (-)-[3H]-CGP 12177.

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.

Role of the beta(3)-adrenergic receptor and/or a putative beta(4)-adrenergic receptor on the expression of uncoupling proteins and peroxisome proliferator-activated receptor-gamma coactivator-1

Administration of beta-adrenergic receptor (beta-AR) agonists, especially beta(3)-AR agonists, is well known to increase thermogenesis in rodents and humans. In this work we studied the role of the beta(3)-AR in regulating mRNA expression of genes involved in thermogenesis, i.e., mitochondrial uncoupling proteins UCP2 and UCP3, and peroxisome proliferator-activated receptor-gamma coactivator-1 (PGC-1), in mouse skeletal muscle. For this purpose, different beta(3)-AR agonists were administered acutely to both wild type mice and mice whose beta(3)-AR gene has been disrupted (beta(3)-AR KO mice). CL 316243 increased the expression of UCP2, UCP3 and PGC-1 in wild type mice only. By contrast, BRL 37344 and CGP 12177 increased the expression of UCP2 and UCP3 in both wild type and beta(3)-AR KO mice, whereas they increased the expression of PGC-1 in wild type mice only. Finally, acute (3 h) cold exposure increased the expression of UCP2 and UCP3, but not PGC-1, in skeletal muscle of both wild type and beta(3)-AR KO mice. These results show that selective stimulation of the beta(3)-AR affects the expression of UCP2, UCP3 and PGC-1 in skeletal muscle. This effect is probably indirect, as muscle does not seem to express beta(3)-AR. In addition, our data suggest that BRL 37344 and CGP 12177 act, in part, through an as yet unidentified receptor, possibly a beta(4)-AR.

Effect of bupranolol for BRL37344 and noradrenaline-induced relaxations mediating atypical beta/beta3-adrenoceptor in rat oesophageal muscularis mucosae

We previously suggested that the existence of atypical beta/beta3-adrenoceptor with pA2-values for bupranolol, a non-selective beta-adrenoceptor antagonist, against BRL37344 and noradrenaline were 5.79 and 5.53 in guinea pig taenia caecum, respectively. We furthermore determined the affinity of bupranolol to subclassify atypical beta/beta3-adrenoceptor in rat oesophageal muscularis mucosae, because it is rich in atypical beta/beta3-adrenoceptor. BRL37344 and noradrenaline produced a concentration-dependent relaxation of rat oesophageal muscularis mucosae. The responses to BRL37344 and noradrenaline were resistant to 3x10(-6) M propranolol, 10(-4) M atenolol, and 10(-4) M butoxamine. However, bupranolol antagonized the responses to BRL37344 and noradrenaline in a concentration-dependent manner. Schild plot analyses of bupranolol against BRL37344 and noradrenaline gave pA2-values of 7.06 and 6.96, respectively. These results suggest that bupranolol can distinguish the difference in affinity between atypical beta/beta3-adrenoceptors in rat oesophageal muscularis mucosae and guinea pig taenia caecum. The difference in behavior of bupranolol confirms the existence of some atypical beta/beta3-adrenoceptors subtypes.

Pharmacological evidence for β2-adrenoceptor in right atria from stressed female rats

The purpose of the present study was to demonstrate a physiological response to TA2005, a potent β2-adrenoceptor (β2-AR) selective agonist, in right atria isolated from stressed female rats under the influence of the estrous cycle. We obtained concentration-response curves to the agonist in the presence and in the absence of selective antagonists in right atria isolated from female rats submitted to three daily foot-shock sessions (30 min duration, 120 pulses of 1.0 mA, 1.0 s, applied at random intervals of 5-25 s) and sacrificed at estrus or diestrus. Our results showed that the pD2 values of TA2005 were not influenced by estrous cycle phase or foot-shock stress. However, in right atria from stressed rats sacrificed during diestrus, the concentration-response curve to TA2005 was biphasic, with a response being obtained at concentrations of 0.1 nM, whereas during estrus no response was observed at doses lower than 3 nM. ICI118,551, a β2-AR antagonist, abolished the response to nanomolar concentrations of TA2005 in right atria from stressed rats at diestrus, with no changes in agonist pD2 values in right atria from control rats (7.47 ± 0.09, p > 0.05) but a 3-fold decrease in pD2 values of TA2005 in right atria from foot shock stressed rats (7.90 ± 0.07, p [Formula: see text] 0.05). Concentration-response curves to TA2005 in the presence of ICI118,551 were best fitted by a one-site model equation. The β1-AR antagonist, CGP20712A, shifted to the right only the second part of the concentration-response curves to the agonist, unmasking the putative β2-AR-mediated response to the agonist in tissues isolated from stressed rats at diestrus. Under this condition, concentration-response curves to the agonist were best fitted by a two-site model equation. pD2 and maximum response of TA2005 interaction with β1- and putative β2-adrenoceptor components were calculated. Schild analyses gave a pKB value for CGP20712A that was typical for the interaction with β1-AR in each experimental group. pKB values for ICI118,551 could not be obtained in stressed rats sacrificed at diestrus since Schild plot slopes were lower than 1.0. In right atria from control rats, ICI118,551 pKB values were similar to reported values for the interaction of the antagonist with β1-AR. These results confirm that a heterogenous β-AR population mediating the chronotropic response to catecholamines can be demonstrated in right atria from foot shock stressed female rats sacrificed at diestrus. The stress-induced response seems to be mediated by the β2-AR subtype. Right atria from rats sacrificed during estrus are protected against stress-induced alterations on the homogeneity of β-AR population.Key words: foot-shock stress, TA2005, ICI118,551, CGP20712A, estrus, diestrus.

Peripheral cardiovascular actions of SR 58611 A, a beta 3-adrenoceptor agonist, in the dog: lack of central effect

In order to investigate the putative role of beta3-adrenoceptors in central and peripheral cardiovascular regulations, the effects of intracisternal (i.c.) and intravenous (i.v.) injections of SR 58611 A (10, 50, 100 and 200 nmol kg-1), a selective beta3-adrenoceptor agonist, were investigated in chloralose anaesthetized dogs. In normal dogs, i.v. SR 58611 A (100 and 200 nmol kg-1) induced a dose-dependent increase in heart rate with no change in blood pressure. After i.c. injection, SR 58611 A failed to modify blood pressure and heart rate (except at the highest dose 200 nmol kg-1 which induced a positive chronotropic effect). The positive chronotropic effect of SR 58611 A (200 nmol kg-1) appeared earlier and was significantly more pronounced after i.v. than i.c. administration. The positive chronotropic effect of i.v. SR 58611 A (200 nmol kg-1) was reduced by pretreatment with beta-adrenoceptor antagonists [propranolol, nadolol, bupranolol or the beta3-adrenoceptor selective antagonist, SR 59230 A (2 mg kg-1 i.v.)] and suppressed after sinoaortic denervation (i.e. after removal of vagal tone to the heart). These experiments do not show evidence for a primary central cardiovascular effect of SR 58611 A. The positive chronotropic effect of i.v. SR 58611 A is mainly of peripheral origin and can be attributed to a baroreceptor-mediated reflex due to the beta3-adrenoceptor mediated vasodilation with an increase in sympathetic tone and a reduction in vagal tone to the heart.

Atypical beta-adrenoceptors of rat thoracic aorta

The possible existence of atypical beta-adrenoceptors in vascular smooth muscle of rat isolated thoracic aorta was investigated. Isoprenaline (10(-8)-10(-4) M) produced concentration-dependent relaxation of phenylephrine (10(-5) M) precontracted rings of endothelium-denuded rat aorta in vitro. Isoprenaline-induced relaxation was resistant to blockade by atenolol (10(-6) M). But, propranolol (2 x 10(-7) M) caused a non-competitive inhibition and SR 59230A (6.6 x 10(-6) M), a beta3-adrenoceptor selective antagonist, failed to produce additional antagonism in presence of propranolol. BRL 37344 (10(-8)-10(-4) M), a beta3-selective agonist, did not relax ring segments precontracted with phenylephrine (10(-5) M) in the absence of endothelium. The non-conventional partial agonist (-)-cyanopindolol (5 x 10(-6)-10(-4) M) induced a marked relaxation in phenylephrine (10(-5)M) precontracted aortic rings without endothelium. This vasodilation was resistant to blockade by propranolol (2 x 10(-7) M) and SR 59230A (10(-5) M). Salbutamol (10(-8)-10(-4) M) produced concentration-dependent relaxation in isolated endothelium-denuded aortic rings precontracted with phenylephrine (10(-5) M). Propranolol (2 x 10(-7) M), but not atenolol (10(-6) M), inhibited this relaxant response. It is concluded that in endothelium-denuded thoracic aorta, salbutamol acts through beta2-adrenoceptors whereas isoprenaline seems to activate both beta2-adrenoceptors and an atypical beta-adrenergic receptor. This atypical beta-adrenoceptor is distinct from putative beta3-adrenoceptor and maybe resembles the reported fourth cardiac beta-adrenoceptor.

Characterisation of the atypical beta-adrenoceptor in rabbit isolated jejunum using BRL 37344, cyanopindolol and SR 59230A

The present study was carried out to further investigate the nature of the beta-adrenoceptor in rabbit jejunum using BRL 37344, a selective beta3-adrenoceptor agonist, cyanopindolol, a beta-adrenoceptor antagonist with blocking activity at beta3-adrenoceptors and SR 59230A, a new selective beta3-adrenoceptor antagonist. Isoprenaline produced a concentration-dependent inhibition of the spontaneous contractions of rabbit jejunum with a pD2 of 7.14. Propranolol (1 microM) shifted the isoprenaline concentration-response curve (CRC) to the right with a concentration-ratio of 5.85, considerably less than would be expected for an action at classical beta-adrenoceptors (estimated pA2 6.66). BRL 37344 also produced a concentration-dependent inhibition of spontaneous contractions with a pD2 of 7.41. The BRL 37344 CRC was unaffected by propranolol (1 microM). In the presence of propranolol (1 microM), cyanopindolol (1 microM) shifted the isoprenaline CRC to the right (concentration-ratio of 21). Cyanopindolol also shifted the BRL 37344 CRC to the right (concentration-ratio of 38). These shifts are consistent with the affinity of cyanopindolol for beta3-adrenoceptors (estimated pA2 values of 7.27 and 7.38 against isoprenaline and BRL 37344, respectively). In the presence of propranolol (1 microM), SR 59230A produced a concentration-dependent rightward shift of the isoprenaline CRC. The Schild plot gave a pA2 value of 7.16, although the slope of the regression line was significantly different from unity (0.65). SR 59230A also produced a concentration-dependent shift of the BRL 37344 CRC. The Schild plot gave a pA2 of 7.58 with the slope of the regression line not significantly different from unity (0.81). The presence of beta3-adrenoceptors mediating relaxation of spontaneous contractions in rabbit jejunum is supported by the relatively poor antagonism of isoprenaline by propranolol, the relaxant effect of BRL 37344 and the antagonism of isoprenaline and BRL 37344 by cyanopindolol and SR 59230A. The lack of simple competitive antagonism of isoprenaline, but not BRL 37344, by SR 59230A may suggest more than one population of atypical beta-adrenoceptor.

Effects of beta2-adrenergic stimulation on single-channel gating of rat cardiac L-type Ca2+ channels

Cardiac L-type Ca2+ channels can be stimulated by activation of beta2-adrenoceptors. We intended to determine how the gating behavior at the single-channel level (cell-attached configuration) is affected after selective stimulation of beta2-adrenoceptors. Rat cardiomyocytes were exposed to zinterol, a beta2-agonist (n = 7), isoproterenol (n = 6), a nonselective agonist, 8-bromo-cAMP (n = 6), and a combination of isoproterenol and ICI-118551 (n = 8), a selective beta2-receptor antagonist, or isoproterenol and CGP-20712A, a beta1-selective antagonist (n = 7). In all groups the ensemble-average current and the availability of the channels to open on depolarization were increased in a similar fashion. In addition, the open probability (Po) within active sweeps was elevated. However, zinterol exerted this effect in a unique manner. It elevated Po not by shortening closed times but solely by reducing active sweeps with very low Po and a short burst duration. All zinterol effects were abolished by ICI-118551 (n = 5) and mimicked by isoproterenol plus CGP-20712A (n = 7). We conclude that beta2-adrenoceptor activation of L-type channels differs qualitatively from the classical cAMP-dependent mechanism.

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

1. The in vitro activity of four aryl propanolamines was compared to two prototypic beta3 receptor agonists, CGP 12177 and CL316243 at the human beta3 receptor, the rat beta3 receptor in the stomach fundus and receptors mediating atrial tachycardia. 2. L-739,574 was the most potent (EC50 = 9 nM) and selective agonist at the human beta3 receptor with high maximal response (74% of the maximal response to isoproterenol). 3. A phenol-biaryl ether analogue possessed modest affinity for the human beta3 receptor (EC50 = 246 nM), but was highly efficacious with a maximal response 82% of the maximal response to isoproterenol. The other derivatives were intermediate in potency with low maximal responses. 4. These agonists at the human beta3 receptor did not activate the rat beta3 receptor in the rat stomach fundus. In fact, the aryl propanolamines (10(-6) M) inhibited CL316243-induced activation of the rat beta3 receptor. Thus, agonist activity at the human beta3 receptor translated into antagonist activity at the rat beta3 receptor. 5. L739,574 and the phenol biaryl ether increased heart rate via beta1 receptors. 6. Although CGP12177 produced atrial tachycardia, neither the indole sulphonamide nor biphenyl biaryl ether did, although both had high affinity for the human beta3 receptor. Thus, the atrial tachycardic receptor was not identical to the human beta3 receptor. 7. These studies (a) characterized four aryl propanolamines with high affinity at the human beta3 receptor, (b) found that they were antagonists at the rat beta3 receptor, an observation with profound implications for in vivo rat data, and (c) established that the rodent atrial non-beta1, beta2 or beta3 tachycardic receptor was also unrelated to the human beta3 receptor.

The negative inotropic effect of beta3-adrenoceptor stimulation is mediated by activation of a nitric oxide synthase pathway in human ventricle

Beta1- and beta2-adrenoceptors in heart muscle cells mediate the catecholamine-induced increase in the force and frequency of cardiac contraction. Recently, in addition, we demonstrated the functional expression of beta3-adrenoceptors in the human heart. Their stimulation, in marked contrast with that of beta1- and beta2-adrenoceptors, induces a decrease in contractility through presently unknown mechanisms. In the present study, we examined the role of a nitric oxide (NO) synthase pathway in mediating the beta3-adrenoceptor effect on the contractility of human endomyocardial biopsies. The negative inotropic effects of a beta3-adrenoceptor agonist, BRL 37344, and also of norepinephrine in the presence of alpha- and beta1-2-blockade were inhibited both by a nonspecific blocker of NO, methylene blue, and two NO synthase (NOS) inhibitors, L-N-monomethyl-arginine and L-nitroarginine-methyl ester. The effect of the NOS inhibitors was reversed by an excess of L-arginine, the natural substrate of NOS, but not by D-arginine. Moreover, the effects of the beta3-adrenoceptor agonist on contractility were associated with parallel increases in the production of NO and intracellular cGMP, which were also inhibited by NOS inhibitors. Immunohistochemical staining of human ventricular biopsies showed the expression of the endothelial constitutive (eNOS), but not the inducible (iNOS) isoform of NOS in both ventricular myocytes and endothelial cells. These results demonstrate that beta3-adrenoceptor stimulation decreases cardiac contractility through activation of an NOS pathway. Changes in the expression of this pathway may alter the balance between positive and negative inotropic effects of catecholamines on the heart potentially leading to myocardial dysfunction.

Beta3-adrenoceptor mechanisms in guinea-pig taenia caecum

Beta-adrenoceptor-mediated relaxation of guinea-pig taenia caecum was investigated by studying the effects of BRL37344, CGP12177 and norepinephrine. These drugs caused graded relaxation of the guinea-pig taenia caecum. The concentration-response curves for these drugs were unaffected by propranolol, atenolol, butoxamine, prazosin, yohimbine and phentolamine. Bupranolol produced shifts of the concentration-response curves for these drugs. Schild regression analyses carried out for bupranolol against BRL37344, CGP12177 and norepinephrine gave pA2 values of 5.79, 5.61 and 5.53, respectively. CGP12177 and norepinephrine significantly increased cyclic AMP levels in this preparation. Bupranolol significantly decreased cyclic AMP levels elicited by CGP12177 and norepinephrine, whereas propranolol produced no effect. These results suggest that the relaxant responses to BRL37344, CGP12177 and norepinephrine in the guinea-pig taenia caecum are mediated by beta3-adrenoceptors.

(-)-CGP 12177 causes cardiostimulation and binds to cardiac putative beta 4-adrenoceptors in both wild-type and beta 3-adrenoceptor knockout mice

Some blockers of beta1- and beta2-adrenoceptors cause cardiostimulant effects through an atypical beta-adrenoceptor (putative beta4-adrenoceptor) that resembles the beta3-adrenoceptor. It is likely but not proven that the putative beta4-adrenoceptor is genetically distinct from the beta3-adrenoceptor. We therefore investigated whether or not the cardiac atypical beta-adrenoceptor could mediate agonist effects in mice lacking a functional beta3-adrenoceptor gene (beta3 KO). (-)-CGP 12177, a beta1- and beta2-adrenoceptor blocker that causes agonist effects through both beta3-adrenoceptors and cardiac putative beta4-adrenoceptors, caused cardiostimulant effects that were not different in atria from wild-type (WT) mice and beta3 KO mice. The effects of (-)-CGP 12177 were resistant to blockade by (-)-propranolol (200 nM) but were blocked by (-)-bupranolol (1 microM) with an equilibrium dissociation constant of 15 nM in WT and 17 nM in beta3 KO. (-)-[3H]CGP 12177 labeled a similar density of the putative beta4-adrenoceptor in ventricular membranes from the hearts of both WT (Bmax = 52 fmol/mg protein) and beta3 KO (Bmax = 53 fmol/mg protein) mice. The affinity of (-)-[3H]CGP 12177 for the cardiac putative beta4-adrenoceptor was not different between WT (Kd = 46 nM) and beta3 KO (Kd= 40 nM). These results provide definitive evidence that the cardiac putative beta4-adrenoceptor is distinct from the beta3-adrenoceptor.

Validity of (-)-[3H]-CGP 12177A as a radioligand for the 'putative beta4-adrenoceptor' in rat atrium

1. We have recently suggested the existence in the heart of a 'putative beta4-adrenoceptor' based on the cardiostimulant effects of non-conventional partial agonists, compounds that cause cardiostimulant effects at greater concentrations than those required to block beta1- and beta2-adrenoceptors. We sought to obtain further evidence by establishing and validating a radioligand binding assay for this receptor with (-)-[3H]-CGP 12177A ([-]-4-(3-tertiarybutylamino-2-hydroxypropoxy) benzimidazol-2-one) in rat atrium. We investigated (-)-[3H]-CGP 12177A for this purpose for two reasons, because it is a non-conventional partial agonist and also because it is a hydrophilic radioligand. 2. Increasing concentrations of (-)-[3H]-CGP 12177A, in the absence or presence of 20 microM (-)-CGP 12177A to define non-specific binding, resulted in a biphasic saturation isotherm. Low concentrations bound to beta1- and beta2-adrenoceptors (pKD 9.4+/-0.1, Bmax 26.9+/-3.1 fmol mg(-1) protein) and higher concentrations bound to the 'putative beta4-adrenoceptor' (pKD 7.5+/-0.1, Bmax 47.7+/-4.9 fmol mg(-1) protein). In other experiments designed to exclude beta1- and beta2-adrenoceptors, (-)-[3H]-CGP 12177A (1-200 nM) binding in the presence of 500 nM (-)-propranolol was also saturable (pKD 7.6+/-0.1, Bmax 50.8+/-7.4 fmol mg(-1) protein). 3. The non-conventional partial agonists (-)-CGP 12177A (pKi 7.3+/-0.2), (+/-)-cyanopindolol (pKi 7.6+/-0.2), [-]-pindolol (pK1 6.6+/-0.1) and (+/-)-carazolol (pKi 7.2+/-0.2) and the antagonist (-)-bupranolol (pKi 6.6+/-0.2), all competed for (-)-[3H]-CGP 12177A binding in the presence of 500 nM (-)-propranolol at the 'putative beta4-adrenoceptor', with affinities closely similar to potencies and affinities determined in organ bath studies. 4. The catecholamines competed with (-)-[3H]-CGP 12177A at the 'putative beta 4-adrenoceptor' in a stereoselective manner, (-)-noradrenaline (pKiH 6.3+/-0.3, pKiL 3.5+/-0.1), (-)-adrenaline (pKiH 6.5+/-0.2, pKiL 2.9+/-0.1), (-)-isoprenaline (pKiH 6.2+/-0.5, pKiL 3.4+/-0.1), (+)-isoprenaline (pKi< 1.7), (-)-RO363 ((-)-(1-(3,4-dimethoxyphenethylamino)-3-(3,4-dihydroxyphenoxy++ +)-2-propranol)oxalate, pKi 5.5+/-0.1). 5. The inclusion of guanosine 5-triphosphate (GTP 0.1 mM) had no effect on binding of (-)-CGP 12177A or (-)-isoprenaline to the 'putative beta4-adrenoceptor'. In competition binding studies, (-)-CGP 12177A competed with (-)-[3H]-CGP 12177A for one receptor state in the absence (pKi 7.3+/-0.2) or presence of GTP (pKi 7.3+/-0.2). (-)-Isoprenaline competed with (-)-[3H]-CGP 12177A for two states in the absence (pKiH 6.6+/-0.3, pKiL 3.5+/-0.1; % H 25+/-7) or presence of GTP (pKiH 6.2+/-0.5, pKiL 3.4+/-0.1; % H 37+/-6). In contrast, at beta1-adrenoceptors, GTP stabilized the low affinity state of the receptor for (-)-isoprenaline. 6. The specificity of binding to the 'putative beta 4-adrenoceptor' was tested with compounds active at other receptors. High concentrations of the beta 3-adrenoceptor agonists, BRL 37344 ((RR+SS)[4-[2-[[2-(3-chlorophenyl)-2-hydroxy-ethyl]amino]propyl]phenoxy]acetic acid, 6 microM), SR 58611A (ethyl{(7S)-7-[(2R)-2-(3-chlorophenyl)-2-hydroxyethylamino]-5,6,7,8-tetrahydronaphtyl2-yloxy} acetate hydrochloride, 6 microM), ZD 2079 ((+/-)-1-phenyl-2-(2-4-carboxymethylphenoxy)-ethylamino)-ethan-1-ol, 60 microM), CL 316243 (disodium (R,R)-5-[2-[2-(3-chlorophenyl)-2-hydroxyethyl-amino]propyl]- 1,3-benzodioxole-2,2-dicarboxylate, 60 microM) and antagonist SR 59230A (3-(2-ethylphenoxy)-1-[(1S)-1,2,3,4-tetrahydronaphth-1-ylamino]-2S-2-propanol oxalate, 6 microM) caused less than 22% inhibition of (-)[3H]-CGP 12177A binding in the presence of 500 nM (-)propranolol. Histamine (1mM), atropine (1 microM), phentolamine (10 microM), 5-HT (100 microM) and the 5-HT4 receptor antagonist SB 207710 ((1-butyl-4-piperidinyl)-methyl 8-amino-7-iodo-1,4-benzodioxan-5-carboxylate, 10nM) caused less than 26% inhibition of binding. 7.Non-conventional partial agonists, the antagonist (-)bupranolol and catecholamines all competed for (-)[3H]-CGP 12177A binding in the absence of (-)propranolol at beta1-adrenoceptors, with affinities (pKi) ranging from 1.6-3.6 log orders greater than at the 'putative beta 4-adrenoceptor'. 8.We have established and validated a radioligand binding assay in rat atrium for the 'putative beta 4-adrenoceptor' which is distinct from beta1-, beta2- and beta 3-adrenoceptors. The stereoselective interaction with the catecholamines provides further support for the classification of the receptor as 'putative beta 4-adrenoceptor'.

Further evidence for differences between cardiac atypical beta-adrenoceptors and brown adipose tissue beta3-adrenoceptors in the pithed rat

1. We have previously shown (Malinowska & Schlicker, 1996) that the atypical beta-adrenoceptor involved in the positive chronotropic effect of the so-called non-conventional partial beta-adrenoceptor agonists CGP 12177 and cyanopindolol in the pithed rat possesses properties markedly different from those observed for beta3-adrenoceptors in the literature. In the present study, we have directly compared the pharmacological properties of the atypical cardiostimulant beta-adrenoceptor and of the beta3-adrenoceptor mediating the thermogenic response in the brown adipose tissue in pithed and vagotomized rats. 2. Heart rate was dose-dependently increased by CGP 12177 and cyanopindolol by maximally 150 and 100 beats min(-1), yielding pED50 values of 8.0 and 7.3, respectively (pED50, -log10 of the dose in mol kg(-1) body weight i.v. causing the half-maximum effect), but not affected by the selective beta3-adrenoceptor agonist CL 316243 (pED50 > 6.0). 3. CGP 12177, cyanopindolol and CL 316243 increased temperature in the brown adipose tissue by maximally 1 degree C (pED50 values 7.4, 6.3 and 8.6, respectively). 4. The beta1-adrenoceptor antagonist CGP 20712 10 micromol kg(-1), attenuated the cardiostimulatory effect of CGP 12177 and, at a still higher dose (30 micromol kg(-1)), also antagonized its thermogenic effect. The -log10 values of the doses causing a two fold shift of the dose-response curves (DRCs) of CGP 12177 to the right were 6.1 and 5.2, respectively, and were much lower than the corresponding value for the antagonism of CGP 20712 against the beta1-adrenoceptor-mediated positive chronotropic effect which was 8.6. 5. The cardiostimulant and the thermogenic effect of CGP 12177 were not affected by the beta2-adrenoceptor antagonist ICI 118551 10 micromol kg(-1). 6. The beta3-adrenoceptor antagonist SR 59230A (which, by itself, caused a beta1-adrenoceptor-mediated increase in heart rate and, for this reason, was studied after administration of a low dose of CGP 20712) attenuated the cardiostimulant and the thermogenic effect of CGP 12177 to a similar extent. The -log10 values of the doses causing two fold rightward shifts of the DRCs of CGP 12177 were 5.9 and 5.7, respectively. 7. The non-selective beta-adrenoceptor antagonist bupranolol diminished the cardiostimulant and thermogenic response to a very similar extent. The -log10 values causing two fold rightward shifts of the DRCs of CGP 12177 were 5.6 and 5.7, respectively, and were much lower than the corresponding values for the antagonism of bupranolol against the beta1-adrenoceptor-mediated positive chronotropic effect and the beta2-adrenoceptor-mediated decrease in diastolic blood pressure which were 7.6 and 8.3, respectively. 8. The rank order of agonistic potencies for the cardiostimulant effect (CGP 12177 > cyanopindolol > CL 316243) differs from that for the thermogenic response in the brown adipose tissue (CL 316243 > CGP 12177 > cyanopindolol); furthermore, there is a difference with respect to the rank orders of antagonistic potencies for cardiostimulation (CGP 20712 > or = SR 59230A > or = bupranolol > ICI 118551) and thermogenesis (SR 59230A = bupranolol > CGP 20712 > ICI 118551). 9. In conclusion, our study provides further evidence that the atypical cardiostimulant beta-adrenoceptors (causing an increase in heart rate) and beta3-adrenoceptors are pharmacologically different.