Stimulation of cyclic AMP-dependent protein kinase in rat atria by (-)-CGP 12177 through an atypical beta-adrenoceptor

Chronic activation of the low affinity site of β1-adrenoceptors stimulates haemodynamics but exacerbates pressure-overload cardiac remodelling

BACKGROUND AND PURPOSE

The β1-adrenoceptor has at least two binding sites, high and low affinity sites (β1H and β1L, respectively), which mediate cardiostimulation. While β1H-adrenoceptor can be blocked by all clinically used β-blockers, β1L-adrenoceptor is relatively resistant to blockade. Thus, chronic β1L-adrenoceptor activation may mediate persistent cardiostimulation, despite the concurrent blockade of β1H-adrenoceptors. Hence, it is important to determine the potential significance of β1L-adrenoceptors in vivo, particularly in pathological situations.

EXPERIMENTAL APPROACH

C57Bl/6 male mice were used. Chronic (4 or 8 weeks) β1L-adrenoceptor activation was achieved by treatment, via osmotic mini pumps, with (-)-CGP12177 (10 mg·kg(-1)·day(-1)). Cardiac function was assessed by echocardiography and micromanometry.

KEY RESULTS

(-)-CGP12177 treatment of healthy mice increased heart rate and left ventricular (LV) contractility. (-)-CGP12177 treatment of mice subjected to transverse aorta constriction (TAC), during weeks 4-8 or 4-12 after TAC, led to a positive inotropic effect and exacerbated fibrogenic signalling while cardiac hypertrophy tended to be more severe. (-)-CGP12177 treatment of mice with TAC also exacerbated the myocardial expression of hypertrophic, fibrogenic and inflammatory genes compared to untreated TAC mice. Washout of (-)-CGP12177 revealed a more pronounced cardiac dysfunction after 12 weeks of TAC.

CONCLUSIONS AND IMPLICATIONS

β1L-adrenoceptor activation provides functional support to the heart, in both normal and pathological (pressure overload) situations. Sustained β1L-adrenoceptor activation in the diseased heart exacerbates LV remodelling and therefore may promote disease progression from compensatory hypertrophy to heart failure.

Adrenoceptors and adaptive mechanisms in the heart during stress

Several cardiovascular disorders have been related to alterations in beta-adrenoceptor (beta-AR) signaling at or beyond the receptor level. During the stress reaction, the sympathetic-adrenal medullary system and the hypothalamus-pituitary-adrenal cortex axis are activated, causing beta-AR overstimulation and remodeling of the beta(1)/beta(2)/beta(3)-AR ratio in cardiomyocytes. In a model of foot-shock stress, we described decreased beta(1)-AR signaling occurring simultaneously with increased beta(2)-AR signaling, whereas the response to the nonconventional agonist, CGP12177, was not altered. These alterations may play an adaptive role to the increased sympathetic drive to the heart, protecting the cardiac tissue from the cardiotoxic effects mediated by beta(1)-ARs overstimulation without altering cardiac output, since this would be sustained by the beta(2)-AR, which would also protect myocytes from apoptosis. Moreover, the selective enhancement of the beta(2)-AR population might help to diminish the risk of overstimulation since this adrenoceptor subtype couples to both, stimulatory G (Gs) and inhibitory G (Gi) proteins. On the other hand, in the model of neurogenic hypertension, the decrease in beta(1)-AR-mediated response is not followed by increase in the beta(2)-AR-mediated response. However, the response to CGP12177, which was desensitized 48 h after the surgery, was normalized 7 days after that, when beta(1)-AR were downregulated. Therefore, both experimental models provided evidence that the classical isoform of beta(1)-AR and the recently described low-affinity isoform of beta(1)-AR show independent behavior and provide the heart with adaptive mechanisms to increased sympathetic stimulation during stress.

Phosphodiesterases PDE3 and PDE4 jointly control the inotropic effects but not chronotropic effects of (-)-CGP12177 despite PDE4-evoked sinoatrial bradycardia in rat atrium

Acting through a low-affinity site of the beta(1)-adrenoceptor (beta(1L)AR), CGP12177 causes sinoatrial tachycardia and positive inotropic effects in left atrium but not in the ventricle of the rat. However, inhibition of either PDE3 or PDE4 also uncovers positive inotropic effects of CGP12177 in ventricle, but whether these phosphodiesterases also control the atrial agonist effects of CGP12177 was unknown. We, therefore, investigated the effects of the PDE3-selective inhibitor cilostamide (300 nM) and PDE4 inhibitor rolipram (1 microM) on the (-)-CGP12177-evoked increases of sinoatrial beating rate and force of paced left atria of the rat. Rolipram (n = 8) increased basal sinoatrial rate by 27 +/- 5 bpm but cilostamide (n = 8) had no effect. The chronotropic potency of (-)-CGP12177 (-logEC(50)M = 7.5) was not changed by rolipram and cilostamide or their combination. (-)-CGP12177 increased left atrial force with intrinsic activity 0.25 compared to (-)-isoprenaline. Rolipram (n = 8) and cilostamide (n = 8) did not change basal force of left atria but concurrent rolipram + cilostamide (n = 8) increased force by 52 +/- 9% of the effect of 200 microM (-)-isoprenaline. Neither rolipram nor cilostamide affected the inotropic potency of (-)-CGP12177 (-logEC(50)M = 7.4) but concurrent rolipram + cilostamide caused potentiation (-logEC(50)M = 8.2) and converted (-)-CGP12177 into a full agonist compared to (-)-isoprenaline. Cyclic AMP appears to maintain sinoatrial rate and PDE4 elicits bradycardia through hydrolysis of cAMP in a compartment distinct from the beta(1L)AR-induced cAMP compartment through which (-)-CGP12177 causes tachycardia. In contrast to the (-)-CGP12177-evoked tachycardia, not controlled by PDE3 and PDE4, these isoenzymes jointly reduce (-)-CGP12177-evoked increases of left atrial contractility through beta(1L)AR.

Role of beta-adrenoceptor signaling in skeletal muscle: implications for muscle wasting and disease

The importance of beta-adrenergic signaling in the heart has been well documented, but it is only more recently that we have begun to understand the importance of this signaling pathway in skeletal muscle. There is considerable evidence regarding the stimulation of the beta-adrenergic system with beta-adrenoceptor agonists (beta-agonists). Although traditionally used for treating bronchospasm, it became apparent that some beta-agonists could increase skeletal muscle mass and decrease body fat. These so-called "repartitioning effects" proved desirable for the livestock industry trying to improve feed efficiency and meat quality. Studying beta-agonist effects on skeletal muscle has identified potential therapeutic applications for muscle wasting conditions such as sarcopenia, cancer cachexia, denervation, and neuromuscular diseases, aiming to attenuate (or potentially reverse) the muscle wasting and associated muscle weakness, and to enhance muscle growth and repair after injury. Some undesirable cardiovascular side effects of beta-agonists have so far limited their therapeutic potential. This review describes the physiological significance of beta-adrenergic signaling in skeletal muscle and examines the effects of beta-agonists on skeletal muscle structure and function. In addition, we examine the proposed beneficial effects of beta-agonist administration on skeletal muscle along with some of the less desirable cardiovascular effects. Understanding beta-adrenergic signaling in skeletal muscle is important for identifying new therapeutic targets and identifying novel approaches to attenuate the muscle wasting concomitant with many diseases.

The effects of both noradrenaline and CGP12177, mediated through human beta1 -adrenoceptors, are reduced by PDE3 in human atrium but PDE4 in CHO cells

(-)-Noradrenaline and (-)-CGP12177 activate beta(1)-adrenoceptors through a high (H)- and low-affinity (L) site, respectively. The positive inotropic effects of (-)-noradrenaline are blunted by phosphodiesterase4 (PDE4) but not PDE3, while both PDE isoenzymes, acting in concert, prevent the effects of (-)-CGP12177 through beta(1)-adrenoceptors in rat ventricle. We sought to unravel the role of PDE3 and PDE4 on signals through the H and L sites in human myocardium. The kinetics of matching positive inotropic effects of (-)-noradrenaline (20 nM) and (-)-CGP12177 (100 nM) were investigated on human atrial trabeculae in the absence and presence of the PDE3 inhibitor cilostamide (300 nM), PDE4 inhibitor rolipram (1 microM) or both. The influence of cilostamide and rolipram on agonist-evoked cyclic adenosine monophosphate (cAMP) increases were also compared in Chinese hamster ovary (CHO) cells expressing recombinant human beta1 -adrenoceptors. (-)-Noradrenaline and (-)-CGP12177 caused matching inotropic responses that faded during a 60-min time course. Cilostamide, but not rolipram, increased the positive inotropic effects and abolished the time dependent fade of both agonists. In CHO cells, rolipram, but not cilostamide, enhanced the cAMP signals caused by both (-)-noradrenaline and (-)-CGP12177. PDE3, but not PDE4, blunts the positive inotropic effects of both (-)-noradrenaline and (-)-CGP12177 through H and L sites, respectively, of human atrial beta1 -adrenoceptors. However, in CHO cells, PDE4 blunts the cAMP signals of both (-)-noradrenaline and (-)-CGP12177. Neither CHO cells nor the rat ventricle are appropriate models for the beta1 -adrenoceptor-evoked signalling to PDE3 observed in human atrium.

Phosphodiesterase PDE3 blunts the positive inotropic and cyclic AMP enhancing effects of CGP12177 but not of noradrenaline in rat ventricle

1.--The cardiostimulant effects of CGP12177, mediated through a beta(1)-adrenoceptor site with low affinity for (-)-propranolol, are potentiated by the nonselective PDE inhibitor IBMX but the role of PDE isoenzymes is unknown. We studied the effects of the PDE3-selective inhibitor cilostamide (300 nM) and PDE4-selective inhibitor rolipram (1 microM) on the positive inotropic and cyclic AMP-enhancing effects of CGP12177 and noradrenaline in right ventricular strips of rat. 2.--CGP12177 (under (-)-propranolol 200 nM) only increased contractile force in the presence of either cilostamide or rolipram with -logEC(50)M 6.7 (E(max)=23% over basal) and 7.1 (E(max)=50%) respectively. The combination of cilostamide and rolipram caused CGP12177 to enhance contractile force with -logEC(50)M=7.7 and E(max)=178%. 3.--The positive inotropic effects of noradrenaline (-logEC(50)M=6.9) were potentiated by rolipram (-logEC(50)M=7.4) but not by cilostamide (-logEC(50)M=7.0). 4.--In the presence of rolipram and (-)-propranolol, noradrenaline (2 microM) and CGP12177 (10 microM) produced matching inotropic effects but failed to increase cyclic AMP levels. 20 microM (-)-noradrenaline increased cyclic AMP levels, a response further enhanced by rolipram. 5.--Both PDE3 and PDE4 of rat ventricle appear to hydrolyse cyclic AMP generated through the low-affinity beta(1)-adrenoceptor site, thereby preventing inotropic responses of CGP12177. When (-)-noradrenaline interacts with the beta(1)-adrenoceptor, the generated cyclic AMP is hydrolysed only by PDE4, thereby reducing cardiostimulation.

Effects of formoterol and BRL 37344 on human umbilical arteries in vitro in normotensive and pre-eclamptic pregnancy

Alterations in vascular responses to beta-adrenoceptor agonists in normotensive pregnancy and pre-eclampsia are not fully understood. Thus, we studied changes in vasodilator responses to beta(2)-adrenoceptor agonist formoterol and beta(3)-adrenoceptor agonist BRL 37344 on umbilical arteries isolated from normotensive (n=12) and pre-eclamptic (n=12) pregnant women. Changes in the relaxant effect of formoterol and BRL 37344 were investigated by measuring isometric tensions in endothelium-denuded strips of umbilical arteries in the presence or absence of metoprolol, ICI 118.551 and SR 59230A (beta(1), beta(2), beta(3)-adrenoceptor antagonists, respectively, 10(-6) mol/L). Effects of formoterol and BRL 37344 on cAMP levels of umbilical arteries were evaluated by radioimmunoassay kits. Formoterol (10(-10)-10(-4) mol/L) and BRL 37344 (10(-10)-10(-4) mol/L) caused concentration-dependent relaxation of the contraction induced by phenylephrine (10(-5) mol/L) in umbilical artery strips isolated from both groups. E(max) values of formoterol and BRL 37344 (for normotensive pregnant women: 87.33+/-0.87 and 53.25+/-1.17 vs. for pre-eclampsia: 73.68+/-1.58 and 43.64+/-1.19, n=12, P>0.05, respectively) were significantly smaller in strips from pre-eclamptic women (P<0.05), with no significant change in pD(2) values. E(max) values of formoterol were significantly higher than those of BRL 37344 in both tissue (P<0.05). ICI 118.551 and SR 59230A, but not metoprolol, antagonized the relaxant effects of formoterol and of BRL 37344 on umbilical artery strips isolated from normotensive and pre-eclamptic pregnant women. Formoterol and BRL 37344 increased cAMP levels in both groups, but less significant in pre-eclamptic strips (P<0.05). These results suggest that the relaxation caused in human umbilical arteries by formoterol and BRL 37344 is mediated by a mixed population of beta(2)- and beta(3)-adrenoceptor subtypes, with contribution of cAMP. Umbilical arteries from subjects with pre-eclampsia showed a weaker beta(2)- and beta(3)-receptor-mediated relaxation to formoterol and BRL 37344, suggesting that the reduced action of formoterol and BRL 37344 may be partly due to a decreased effect of cAMP.

Stress and cardiac beta adrenoceptors

Most modern theories about stress recognize that although stress is not a disease, it may be the trigger for the majority of diseases when allostatic overload has been generated. During stress, the glucocorticoids and catecholamines play a key role in the regulation of physiological parameters and homeostasis during stress. In the heart, positive chronotropic, inotropic, and lusitropic responses to catecholamines are mediated by various subtypes of adrenergic receptors (beta-ARs), mainly beta1- and beta2-adrenergic receptors. beta-ARs also control cardiomyocyte growth and death, thus contributing to cardiac remodelling. The structural basis of each beta-AR subtype, as well as their signalling pathways, and adaptive responses to stress are discussed. The participation of beta3- and putative beta4-ARs in the control of cardiac function is also discussed, with emphasis on low affinity beta-AR isoforms and the role they play in the response to the catecholamines under stress. The changes in beta-AR signalling under pathogenic conditions as well as under stress are reviewed.

Multiple signalling pathways involved in beta2-adrenoceptor-mediated glucose uptake in rat skeletal muscle cells

1. Beta-adrenoceptor (AR) agonists increase 2-deoxy-[3H]-D-glucose uptake (GU) via beta2-AR in rat L6 cells. The beta-AR agonists, zinterol (beta2-AR) and (-)-isoprenaline, increased cAMP accumulation in a concentration-dependent manner (pEC50=9.1+/-0.02 and 7.8+/-0.02). Cholera toxin (% max increase 141.8+/-2.5) and the cAMP analogues, 8-bromo-cAMP (8Br-cAMP) and dibutyryl cAMP (dbcAMP), also increased GU (196.8+/-13.5 and 196.4+/-17.3%). 2. The adenylate cyclase inhibitor, 2',5'-dideoxyadenosine (50 microM), significantly reduced cAMP accumulation to zinterol (100 nM) (109.7+35.0 to 21.6+4.5 pmol well(-1)), or forskolin (10 microM) (230.1+/-58.0 to 107.2+/-26.3 pmol well(-1)), and partially inhibited zinterol-stimulated GU (217+/-26.3 to 176.1+/-20.4%). The protein kinase A (PKA) inhibitor, 4-cyano-3-methylisoquinoline (100 nM), did not inhibit zinterol-stimulated GU. The PDE4 inhibitor, rolipram (10 microM), increased cAMP accumulation to zinterol or forskolin, and sensitised the GU response to zinterol, indicating a stimulatory role of cAMP in GU. 3. cAMP accumulation studies indicated that the beta2-AR was desensitised by prolonged stimulation with zinterol, but not forskolin, whereas GU responses to zinterol increased with time, suggesting that receptor desensitisation may be involved in GU. Receptor desensitisation was not reversed by inhibition of PKA or Gi. 4. PTX pretreatment (100 ng ml(-1)) inhibited insulin or zinterol-stimulated but not 8Br-cAMP or dbcAMP-stimulated GU. The PI3K inhibitor, LY294002 (1 microM), inhibited insulin- (174.9+/-5.9 to 142.7+/-2.7%) and zinterol- (166.9+/-7.6 to 141.1+/-8.1%) but not 8 Br-cAMP-stimulated GU. In contrast to insulin, zinterol did not cause phosphorylation of Akt. 5. The results suggest that GU in L6 cells involves three mechanisms: (1) an insulin-dependent pathway involving PI3K, (2) a beta2-AR-mediated pathway involving both cAMP and PI3K, and (3) a receptor-independent pathway suggested by cAMP analogues that increase GU independently of PI3K. PKA appears to negatively regulate beta2-AR-mediated GU.

Evidence for two atypical conformations of beta-adrenoceptors and their interaction with Gi proteins

In this study, we investigated whether the responses of right atria from sinoaortic denervated rats to CGP12177 (4(3-t-butylamino-2-hydroxypropoxy benzidimidazole-2 one, hydrochloride)), isoprenaline and norepinephrine desensitized in parallel and whether CGP12177 interacted with distinct conformations of beta-adrenoceptors. Right atria from rats 48 h after sinoaortic denervation were subsensitive to isoprenaline, norepinephrine and CGP12177. One week after sinoaortic denervation, the sensitivity to CGP12177 had recovered whereas the responses to isoprenaline and norepinephrine were still subsensitive, suggesting that the binding sites for these molecules showed independent behavior. In atria from 48 h sinoaortic-denervated rats, propranolol or 3 microM CGP20712A (2-hydroxy-5(2-((2-hydroxy-3-(4-((methyl-4-trifluormethyl)1H imidazole-2-yl)-phenoxypropyl) amino) ethoxy)-benzamide monomethane sulphonate)) blocked the responses to 10 nM-1 microM CGP12177 and steepened the curves. The concentration-response curves to CGP12177 in the presence of ICI118,551 (erythro-DL-1(-methylindan-4-yloxy)-3-isopropylamino-butan-2-ol) were biphasic, suggesting that CGP12177 interacted with at least two conformations of beta-adrenoceptors that showed negative cooperativism, one acting through beta(2)-adrenoceptor-Gi and the other via beta(1)-adrenoceptor-Gs. This hypothesis was confirmed in right atria from sinoaortic-denervated rats treated with pertussis toxin.

Characterization of intrinsic sympathomimetic activity of carteolol in rat cardiovascular preparations

We evaluated in vitro, in myocardial and vascular preparations isolated from reserpine-treated rats, the intrinsic sympathomimetic activity (ISA) of carteolol, a beta(1)/beta(2)-adrenoceptor blocking agent used in cardiovascular and non-cardiovascular diseases. In spontaneously beating atria, carteolol, at low concentrations (0.01 and 0.1 microM), antagonized the positive inotropic effect of isoprenaline, whereas at higher concentrations (1 microM to 1 mM), it caused an increase in the force of contraction (EC(50): 4.6 +/- 0.1 microM, E(max): 17.1 +/- 1.1%, with respect to the maximum isoprenaline response) and a slight increase (7.8 +/- 1.9% over basal values) in the heart rate. The positive inotropic effect of carteolol was abolished by concentrations of propranolol or timolol (10 microM) much higher than those blocking isoprenaline effects in the same preparations. A similar positive inotropic effect was also observed in electrically driven left atrium and in Langendorff perfused hearts. Functional and biochemical evidences supported the involvement of cAMP in the cardiac action of carteolol. In peripheral arteries (femoral and tail) pre-contracted with phenylephrine, carteolol exerted ISA-related relaxing effects, independent of the presence of endothelium and sensitive to high concentrations (10 microM) of conventional beta-blockers. On the basis of these results, we propose to categorize carteolol as a non-conventional partial agonist of both cardiac and vascular beta-adrenoceptors.

Impairment of the low-affinity state beta1-adrenoceptor-induced relaxation in spontaneously hypertensive rats

1 In hypertension, a decrease of the vascular beta-adrenergic relaxation has been described. However, the specific involvement of each beta-adrenoceptor (beta-AR) subtype, in particular the low-affinity state of beta1-AR, has not yet been evaluated. We investigated whether the low-affinity state of beta1-AR-induced relaxation was impaired in Spontaneously Hypertensive Rats (SHR). 2 The relaxant responses to CGP 12177 and cyanopindolol, low-affinity state beta1-AR agonists (with beta1-/beta2-AR antagonistic and partial beta3-AR agonistic properties) were evaluated on thoracic aortic rings isolated from 12-weeks-old Wistar Kyoto rats (WKY) and SHR. 3 In WKY, CGP 12177 and cyanopindolol produced an endothelium and nitric oxide (NO)-independent relaxation. CGP 12177-induced endothelium-independent relaxation was not modified either by beta1-, beta2-AR (nadolol) or beta3-AR (L-748337 or SR 59230A) antagonists but was significantly reduced by high concentrations of CGP 20712A (P<0.05). This relaxation was also reduced by adenylyl cyclase inhibitors, SQ 22536 or MDL 12330A. 4 In SHR, CGP 12177 produced mainly an endothelium and NO-dependent relaxation. This effect was not modified by nadolol, but was strongly reduced by beta3-AR blockade. Endothelium-independent relaxation to CGP 12177 was not altered by adenylyl cyclase inhibition, but was amplified in preparations from pertussis toxin-pretreated SHR. 5 The immunohistochemical analysis revealed an upregulation of beta3-AR in the endothelial layer of SHR aorta, whereas the beta3-AR-induced relaxation was not modified. 6 In conclusion, we demonstrated an impaired low-affinity state of the beta1-AR-induced relaxation and an upregulation of the beta3-AR in hypertension. Some clinical implications of those findings are discussed.

Binding of (-)-[3H]-CGP12177 at two sites in recombinant human beta 1-adrenoceptors and interaction with beta-blockers

To verify the hypothesis that the non-conventional partial agonist (-)-CGP12177 binds at two beta(1)-adrenoceptor sites, human beta(1)-adrenoceptors, expressed in CHO cells, were labelled with (-)-[(3)H]-CGP12177. We compared the binding affinity and antagonist potency of 12 clinically used beta-blockers against the cyclic AMP-enhancing effects of (-)-isoprenaline and (-)-CGP12177.(-)-[(3)H]-CGP12177 bound to a high affinity site (H; K(H)=0.47 nM) and low affinity site (L); K(L)=235 nM). (-)-[(3)H]-CGP12177 dissociated from the beta(1)-adrenoceptors with a fast component (k(off)=0.45 min(-1)), consistent with the L-site, and a slow component (k(off)=0.017-0.033 min(-1)), consistent with the H-site. (-)-Isoprenaline and (-)-CGP12177 caused 96-fold and 12-fold maximal increases in cyclic AMP levels with -logEC(50)M of 8.2 and 7.6. (-)-CGP12177 antagonised the effects of (-)-isoprenaline with a pK(B) of 9.9. The beta-blockers antagonised the effects of (-)-isoprenaline more than the effects of (-)-CGP12177 with potency ratios: (-)-atenolol 1,000, (+/-)-metropolol 676, (-)-pindolol 631, (-)-timolol 589, (+/-)-carvedilol 204, (+/-)-oxprenolol 138, (+/-)-sotalol 132, (-)-propranolol 120, (+/-)-bisoprolol 95, (+/-)-alprenolol 81, (+/-)-nadolol 68 and (-)-bupranolol 56. In intact cells the binding constants of beta-blockers, estimated from competition with 3-5 nM (-)-[(3)H]-CGP12177 (binding to the H-site), correlated with the corresponding affinities estimated from antagonism of the (-)-isoprenaline effects. We conclude that (-)-[(3)H]-CGP12177 binds at two sites in the recombinant beta(1)-adrenoceptor. (-)-CGP12177 is an antagonist of catecholamine effects through the H-site and a non-conventional partial agonist through the L-site. beta-blockers are more potent antagonists through the H-site than the L-site.

Markedly reduced effects of (-)-isoprenaline but not of (-)-CGP12177 and unchanged affinity of beta-blockers at Gly389-beta1-adrenoceptors compared to Arg389-beta1-adrenoceptors

1. Substitution of arginine by glycine at position 389, a frequent beta(1)-adrenoceptor polymorphism, reduces adenylyl cyclase stimulation by (-)-isoprenaline. beta(1)-Adrenoceptors mediate the effects of catecholamines and nonconventional partial agonists ((-)-CGP12177) through different sites. We investigated the influence of the 389 polymorphism on beta blocker affinity, as well as on the responses to (-)-isoprenaline and the nonconventional partial agonist (-)-CGP12177 on cyclic AMP levels in CHO cells expressing recombinant Arg389-beta(1)-adrenoceptors (101 fmol mg(-1) protein) or Gly389-beta(1)-adrenoceptors (94 fmol mg(-1)). 2. The affinity of beta-blockers and partial agonists, estimated from competition binding with (-)-[(125)I]-cyanopindolol, was not different for Arg389-beta(1)-adrenoceptors and Gly389-beta(1)-adrenoceptors. 3. The maximum cAMP increases by (-)-isoprenaline and (-)-CGP12177 at Gly389-beta(1)-adrenoceptors were reduced by 97 and 46%, but the potencies enhanced 2 and 0.5 log units, respectively, compared to Arg389-beta(1)-adrenoceptors. The intrinsic activity of (-)-CGP12177 with respect to the (-)-isoprenaline was 0.057 at Arg389-beta(1)-adrenoceptors and 1.05 at Gly389-beta(1)-adrenoceptors. 4. We confirm in intact CHO cells that responses to (-)-isoprenaline are markedly reduced at Gly389-beta(1)-adrenoceptors compared to Arg389-beta(1)-adrenoceptors. However, the 389 polymorphism reduces considerably less the agonist responses to (-)-CGP12177, indicating that coupling to G(s) protein is different for beta(1)-adrenoceptors activated by catecholamines than for receptors activated by nonconventional partial agonists. The affinity of beta-blockers is conserved across the Arg389Gly polymorphism.

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.

The beta1-adrenoceptor site activated by CGP12177 varies in behavior according to the estrous cycle phase and stress

The aim of this work was to assess whether stress and estrous cycle phases affected the beta1-adrenoceptor (beta1-AR) site activated by CGP12177 in the right atria of rats. The chronotropic response to CGP12177 in the absence or presence of antagonists was determined in atria from rats submitted to one daily foot-shock session for 3 consecutive days. Blood was collected for hormonal assays. The pD2 for CGP12177 in atria from females was lower than in atria from males and was unaltered by stress or the estrous cycle. Propranolol (200 nM) or CGP20712A (3 microM) shifted the concentration-response curves to CGP12177 to the right in control and stressed estrus or control diestrus rats. Atria from stressed diestrus rats were resistant to blockade by propranolol or CGP20712A, indicating that the effect of beta-adrenoceptor antagonists on the response to CGP12177 is influenced by estrous cycle phases. The stress-induced increase in serum corticosterone levels was independent of the estrous cycle or gender, but the estradiol/progesterone ratio was affected differently in the two groups of female rats. In the diestrus group, serum estradiol levels decreased after the first foot-shock session and remained low until the day of sacrifice, whereas in the estrus group the serum levels of estradiol did not decrease after stress and peaked on the second day, which corresponded to proestrus. These data do not indicate whether there is a direct or indirect effect of stress hormones and (or) sex steroids on cardiac beta1-AR sensitivity. However, they do show that the classic and low-affinity binding sites of the beta1-AR are independently regulated and that the beta1-AR atypical site affinity for antagonists depends on the estrous cycle.

Atypical beta-adrenergic effects on insulin signaling and action in beta(3)-adrenoceptor-deficient brown adipocytes

Cross talk between adrenergic and insulin signaling systems may represent a fundamental molecular basis of insulin resistance. We have characterized a newly established beta(3)-adrenoceptor-deficient (beta(3)-KO) brown adipocyte cell line and have used it to selectively investigate the potential role of novel-state and typical beta-adrenoceptors (beta-AR) on insulin signaling and action. The novel-state beta(1)-AR agonist CGP-12177 strongly induced uncoupling protein-1 in beta(3)-KO brown adipocytes as opposed to the beta(3)-selective agonist CL-316,243. Furthermore, CGP-12177 potently reduced insulin-induced glucose uptake and glycogen synthesis. Neither the selective beta(1)- and beta(2)-antagonists metoprolol and ICI-118,551 nor the nonselective antagonist propranolol blocked these effects. The classical beta(1)-AR agonist dobutamine and the beta(2)-AR agonist clenbuterol also considerably diminished insulin-induced glucose uptake. In contrast to CGP-12177 treatment, these negative effects were completely abrogated by metoprolol and ICI-118,551. Stimulation with CGP-12177 did not impair insulin receptor kinase activity but decreased insulin receptor substrate-1 binding to phosphatidylinositol (PI) 3-kinase and activation of protein kinase B. Thus the present study characterizes a novel cell system to selectively analyze molecular and functional interactions between novel and classical beta-adrenoceptor types with insulin action. Furthermore, it indicates insulin receptor-independent, but PI 3-kinase-dependent, potent negative effects of the novel beta(1)-adrenoceptor state on diverse biological end points of insulin action.

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.

The use of high-throughput synthesis and purification in the preparation of a directed library of adrenergic agents

A library of potential agonists and antagonists for adrenergic receptors was prepared using high-throughput solution-phase parallel synthesis. Traditional solution-phase reductive amination reactions followed by rapid purification by ion exchange chromatography yielded products with near-analytical purity. An array of ketones and amines, arranged in an 8 x 12 matrix, were combined to form 96 individual compounds.

Desensitization and resensitization of beta 1- and putative beta 4-adrenoceptor mediated responses occur in parallel in a rat model of cardiac failure

1. Cardiostimulant effects of the non-conventional partial agonist, CGP 12177A, are mediated by a receptor distinct from the beta3-adrenoceptor and termed the putative beta4-adrenoceptor. Using a rat model of cardiac failure, induced by myocardial infarction (MI), we compared the desensitization and resensitization of responses to CGP 12177A with those to isoprenaline and RO 363 in left (LA) and right atria (RA). We also examined the ability of beta-adrenoceptor antagonists to block responses to CGP 12177A. 2. MI reduced the maximum inotropic response to isoprenaline by 48% (sham 4.1+/-0.6 mN, n=10; MI 2.1+/-0.4 mN, n=8, P<0.02), RO 363 by 61% (sham 4.2+/-0.5 mN, n=10; MI 1.8+/-0.3 mN, n=8, P<0.005) and CGP 12177A by 49% (sham 1.4+/-0.1 mN, n=5; MI 0.7+/-0.2 mN, n=7, P<0.05) in electrically stimulated LA. MI also reduced the sensitivity to isoprenaline (pEC50: sham 8.79+/-0.08, n=10; MI 8.30+/-0.10, n=8; P=0.001) and RO 363 (pEC50: sham 8.69+/-0.07, n=10; MI 8.33+/-0.10, n=8; P<0.01). The maximum chronotropic responses to isoprenaline, RO 363 and CGP 12177A in RA were unaffected. 3. Pertussis toxin treatment (10 microg kg-1, i.p.) restored the maximum inotropic response and sensitivity to isoprenaline (sham 3.5+/-0.5 mN, n=9; MI 3.2+/-0.6 mN, n=11, P=0.702) and CGP 12177A (sham 1.6+/-0.3 mN, n=6; MI 1.9+/-0.4 mN, n=7, P=0.537) in MI animals to levels similar to those in the sham group. 4. CGP 20712A (pKB: LA 6.7+/-0.2, n=6; RA 7. 1+/-0.1, n=4), ICI 118,551 (pKB: LA 6.4+/-0.1, n=5; RA 6.3+/-0.1, n=6), propranolol (pKB: LA 6.6+/-0.1, n=5; RA 6.8+/-0.1, n=6) and bupranolol (pKB: LA 7.2+/-0.1, n=6; RA 7.7+/-0.1, n=8), showed moderate affinity for the putative beta4-adrenoceptor. 5. Desensitization after MI and resensitization (after pertussis toxin treatment) to isoprenaline and CGP 12177A therefore occur in parallel, suggesting that the beta1- and putative beta4-adrenoceptor use the same signalling pathway. Antagonist affinity studies confirmed that drugs acting at beta1-adrenoceptors also interact with putative beta4-adrenoceptors with approximately 100 times lower affinity. We suggest that CGP 12177A produces its cardiac effects by interacting with a low affinity state of the beta1-adrenoceptor.

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.

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.

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.

Metabolic response to various beta-adrenoceptor agonists in beta3-adrenoceptor knockout mice: evidence for a new beta-adrenergic receptor in brown adipose tissue

The beta3-adrenoceptor plays an important role in the adrenergic response of brown and white adipose tissues (BAT and WAT). In this study, in vitro metabolic responses to beta-adrenoceptor stimulation were compared in adipose tissues of beta3-adrenoceptor knockout and wild type mice. The measured parameters were BAT fragment oxygen uptake (MO2) and isolated white adipocyte lipolysis. In BAT of wild type mice (-)-norepinephrine maximally stimulated MO2 4.1+/-0.8 fold. Similar maximal stimulations were obtained with beta1-, beta2- or beta3-adrenoceptor selective agonists (dobutamine 5.1+/-0.3, terbutaline 5.3+/-0.3 and CL 316,243 4.8+/-0.9 fold, respectively); in BAT of beta3-adrenoceptor knockout mice, the beta1- and beta2-responses were fully conserved. In BAT of wild type mice, the beta1/beta2-antagonist and beta3-partial agonist CGP 12177 elicited a maximal MO2 response (4.7+/-0.4 fold). In beta3-adrenoceptor knockout BAT, this response was fully conserved despite an absence of response to CL 316,243. This unexpected result suggests that an atypical beta-adrenoceptor, distinct from the beta1-, beta2- and beta3-subtypes and referred to as a putative beta4-adrenoceptor is present in BAT and that it can mediate in vitro a maximal MO2 stimulation. In isolated white adipocytes of wild type mice, (-)-epinephrine maximally stimulated lipolysis 12.1+/-2.6 fold. Similar maximal stimulations were obtained with beta1-, beta2- or beta3-adrenoceptor selective agonists (TO509 12+/-2, procaterol 11+/-3, CL 316,243 11+/-3 fold, respectively) or with CGP 12177 (7.1+/-1.5 fold). In isolated white adipocytes of beta3-adrenoceptor knockout mice, the lipolytic responses to (-)epinephrine, to the beta1-, beta2-, beta3-adrenoceptor selective agonists and to CGP 12177 were almost or totally depressed, whereas those to ACTH, forskolin and dibutyryl cyclic AMP were conserved.

Atypical adrenoceptor-mediated relaxation of canine pulmonary artery through a cAMP-dependent pathway

We studied the existence of beta 3-adrenoceptors in canine pulmonary artery smooth muscle under isometric conditions in vitro. A rank order potency of vascular relaxation was isoproterenol > salbutamol > selective beta 3-adrenoceptor agonists, CL 316243 and BRL 37344. A marked desensitization to salbutamol occurred by pretreatment with salbutamol but not with CL 316243. When beta 1-adrenoceptors were blocked, the relaxant responses to salbutamol were competitively antagonized by the beta 2-adrenoceptor antagonist ICI 118551, whereas the response to CL 316243 was not. Cyanopindolol, a non-selective beta-adrenoceptor antagonist, antagonized CL 316243-induced relaxation in a competitive manner with a pA2 of 6.10, and this value was lower than that when salbutamol was used as an agonist (6.69). Intracellular cAMP levels were increased by CL 316243, an effect that was not altered by ICI 118551. Therefore, beta 3-adrenoceptors may be present and functioning in canine pulmonary artery.

(-)-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'.

Proposal for the interaction of non-conventional partial agonists and catecholamines with the 'putative beta 4-adrenoceptor' in mammalian heart

1. Evidence for a 'putative beta 4-adrenoceptor' originated over 20 years ago when cardiostimulant effects were observed to non-conventional partial agonists. These agonists were originally described as beta 1- and beta 2-adrenoceptor antagonists; however, they cause cardiostimulant effects at much higher concentrations than those required to block beta 1- and beta 2-adrenoceptors. Cardiostimulant effects of non-conventional partial agonists have been observed in mouse, rat, guinea-pig, cat, ferret and human heart tissues. 2. The receptor is expressed in several heart regions, including the sinoatrial node, atrium and ventricle. 3. The receptor is resistant to blockade by most antagonists that possess high affinity for beta 1- and beta 2-adrenoceptors, but is blocked with moderate affinity by (-)-bupranolol and CGP 20712A. 4. The receptor is pharmacologically distinct from the beta 3-adrenoceptor. Micromolar concentrations of beta 3-adrenoceptor agonists have no agonist or blocking activity. The receptor is also resistant to blockade by a beta 3-adrenoceptor-selective antagonist. 5. The receptor mediates increases in cAMP levels and cAMP-dependent protein kinase (PK) A activity in cardiac tissues. Phosphodiesterase inhibition potentiates the positive chronotropic and inotropic effects of non-conventional partial agonists. 6. The receptor mediates hastening of atrial and ventricular relaxation, which is consistent with involvement of a cAMP-dependent pathway. 7. The non-conventional partial agonist (-)-[3H]-CGP 12177A labels the cardiac putative beta 4-adrenoceptor. Non-conventional partial agonists compete for binding with affinities that are closely similar to their agonist potencies. Catecholamines compete for binding in a stereoselective manner with a rank order of affinity of (-)-RO363 > (-)-isoprenaline > (-)-noradrenaline > or = (-)-adrenaline >> (+)-isoprenaline, suggesting that catecholamines can interact with the receptor. 8. The putative beta 4-adrenoceptor appears to be coupled to the Gs-adenylyl cyclase system, which could serve as a guide to its future cloning. Activation of the receptor may plausibly improve diastolic function but could also mediate arrhythmias.