Effects of (-)-RO363 at human atrial beta-adrenoceptor subtypes, the human cloned beta 3-adrenoceptor and rodent intestinal beta 3-adrenoceptors

1. Chronic treatment of patients with beta-blockers causes atrial inotropic hyperresponsiveness through beta 2-adrenoceptors, 5-HT4 receptors and H2-receptors but apparently not through beta 1-adrenoceptors despite data claiming an increased beta 1-adrenoceptor density from homogenate binding studies. We have addressed the question of beta 1-adrenoceptor sensitivity by determining the inotropic potency and intrinsic activity of the beta 1-adrenoceptor selective partial agonist (-)-RO363 and by carrying out both homogenate binding and quantitative beta-adrenoceptor autoradiography in atria obtained from patients treated or not treated with beta-blockers. In the course of the experiments it became apparent that (-)-RO363 also may cause agonistic effects through the third atrial beta-adrenoceptor. To assess whether (-)-RO363 also caused agonistic effects through beta 3-adrenoceptors we studied its relaxant effects in rat colon and guinea-pig ileum, as well as receptor binding and adenylyl cyclase stimulation of chinese hamster ovary (CHO) cells expressing human beta 3-adrenoceptors. 2. beta-Adrenoceptors were labelled with (-)-[125I]-cyanopindolol. The density of both beta 1- and beta 2-adrenoceptors was unchanged in the 2 groups, as assessed with both quantitative receptor autoradiography and homogenate binding. The affinities of (-)-RO363 for beta 1-adrenoceptors (pKi = 8.0-7.7) and beta 2-adrenoceptors (pKi = 6.1-5.8) were not significantly different in the two groups. 3. (-)-RO363 increased atrial force with a pEC50 of 8.2 (beta-blocker treated) and 8.0 (non-beta-blocker treated) and intrinsic activity with respect to (-)-isoprenaline of 0.80 (beta-blocker treated) and 0.54 (non-beta-blocker treated) (P < 0.001) and with respect to Ca2+ (7 mM) of 0.65 (beta-blocker treated) and 0.45 (non-beta-blocker treated) (P < 0.01). The effects of (-)-RO363 were resistant to antagonism by the beta 2-adrenoceptor antagonist, ICI 118,551 (50 nM). The effects of 0.3-10 nM (-)-RO363 were antagonized by 3-10 nM of the beta 1-adrenoceptor selective antagonist CGP 20712A. The effects of 20-1000 nM (-)-RO363 were partially resistant to antagonism by 30-300 nM CGP 20712A. 4. (-)-RO363 relaxed the rat colon, partially precontracted by 30 mM KCl, with an intrinsic activity of 0.97 compared to (-)-isoprenaline. The concentration-effect curve to (-)-RO363 revealed 2 components, one antagonized by (-)-propranolol (200 nM) with pEC50 = 8.5 and fraction 0.66, the other resistant to (-)-propranolol (200 nM) with pEC50 = 5.6 and fraction 0.34 of maximal relaxation. 5. (-)-RO363 relaxed the longitudinal muscle of guinea-pig ileum, precontracted by 0.5 microM histamine, with intrinsic activity of 1.0 compared to (-)-isoprenaline and through 2 components, one antagonized by (-)-propranolol (200 nM) with pEC50 = 8.7 and fraction 0.67, the other resistant to (-)-propranolol with pEC50 = 4.9 and fraction 0.33 of maximal relaxation. 6. (-)-RO363 stimulated the adenylyl cyclase of CHO cells expressing human beta 3-adrenoceptors with pEC50 = 5.5 and intrinsic activity 0.74 with respect to (-)-isoprenaline (pEC50 = 5.9). (-)-RO363 competed for binding with [125I]-cyanopindolol at human beta 3-adrenoceptors transfected into CHO cells with pKi = 4.5. (-)-Isoprenaline (pKi = 5.2) and (-)-CGP 12177A (pKi = 6.1) also competed for binding at human beta 3-adrenoceptors. 7. We conclude that under conditions used in this study, (-)-RO363 is a potent partial agonist for human beta 1- and beta 3-adrenoceptors and appears also to activate the third human atrial beta-adrenoceptor. (-)-RO363 relaxes mammalian gut through both beta 1- and beta 3-adrenoceptors. (-)-RO363, used as a beta 1-adrenoceptor selective tool, confirms previous findings with (-)-noradrenaline that beta 1-adrenoceptor-mediated atrial effects are only slightly enhanced by chronic treatment of patients with beta-blockers. Chronic treatment with

Beta 2-adrenoceptor activation by zinterol causes protein phosphorylation, contractile effects and relaxant effects through a cAMP pathway in human atrium

Evidence from ventricular preparations of cat, sheep, rat and dog suggests that both beta 1-adrenoceptors (beta 1AR) and beta 2-adrenoceptors (beta 2AR) mediate positive inotropic effects but that only beta 1AR do it through activation of a cAMP pathway. On the other hand, our evidence has shown that both beta 1AR and beta 2AR hasten relaxation of isolated human myocardium consistent with a common cAMP pathway. We have now investigated in the isolated human right atrial appendage, a tissue whose beta-AR comprise around 2/3 of beta 1AR and 1/3 of beta 2AR, whether or not beta 2AR-mediated effects occur via activation of a cAMP pathway. We carried out experiments on atria obtained from patients without advanced heart failure undergoing open heart surgery. To activate beta 2AR, we used the beta 2AR-selective ligand zinterol. Experiments were carried out on paced atrial strips (1 Hz) and tissue homogenates and membrane particles. Zinterol caused positive inotropic and lusitropic (i.e. reduction of t1/2 of relaxation) effects with EC50 values of 3 and 2 nM, respectively. The zinterol-evoked effects were unaffected by the beta 1AR-selective antagonist CGP 20712A (300 nM) but blocked surmountably by the beta 2AR-selective antagonist ICI 118551 (50 nM) which reduced both EC50 values to 1 microM. Zinterol stimulated adenylyl cyclase activity with an EC50 of 30 nM and intrinsic activity of 0.75 with respect to (-)-isoprenaline (600 microM); the effects were resistant to blockade by CGP 20712A (300 nM) but antagonised surmountably by ICI 118551 (50 nM). Zinterol bound to membrane beta AR labelled with (-)-[125I] cyanopindolol with higher affinity for beta 2AR than for beta 1AR; the binding to beta 2AR but not to beta 1AR was reduced by GTP gamma S (10 microM). In the presence of CGP 20712A (300 nM) (-)-isoprenaline (400 microM) (to activate both beta 1AR and beta 2AR maximally) and zinterol (10 microM) increased contractile force 3.4-fold and 2.5-fold respectively and reduced relaxation t1/2 by 32% and 18% respectively. These effects of (-)-isoprenaline and zinterol were associated (5 min incubation) with phosphorylation (pmol P/mg supernatant protein) of troponin I and C-protein to values of 8.4 +/- 2.0 vs 12.4 +/- 2.3 and 10.1 +/- 2.5 vs 8.6 +/- 1.6 respectively. (-)-Isoprenaline and zinterol also caused phosphorylation of phospholamban (1.8 +/- 0.3 vs 0.4 +/- 0.1 pmol P/mg respectively) specifically at serine residues. We conclude that in human atrial myocardium activation of both beta 1AR and beta 2AR leads to cAMP-dependent phosphorylation of proteins involved in augmenting both contractility and relaxation.

Differences between the third cardiac beta-adrenoceptor and the colonic beta 3-adrenoceptor in the rat

1. The heart of several species including man contains atypical beta-adrenoceptors, in addition to coexisting beta 1- and beta 2-adrenoceptors. We now asked the question whether or not the third cardiac beta-adrenoceptor is identical to the putative beta 3-adrenoceptor. We compared the properties of the third cardiac beta-adrenoceptor with those of beta 3-adrenoceptors in isolated tissues of the rat. To study the third cardiac beta-adrenoceptor we used spontaneously beating right atria, paced left atria and paced left ventricular papillary muscles. As a likely model for putative beta 3-adrenoceptors we studied atypical beta-adrenoceptors of the colonic longitudinal muscle precontracted with 30 mM KCl. We used beta 3-adrenoceptor-selective agonists, antagonists and non-conventional partial agonists (ie high-affinity blockers of both beta 1- and beta 2-adrenoceptors know to exert also stimulant effects through beta 3-adrenoceptors). 2. The non-conventional partial agonist (-)-CGP 12177 caused positive chronotropic effects in right atria (pD2 = 7.3) and positive inotropic effects in left atria (pD2 = 7.5). The stimulant effects of (-)-CGP 12177 were resistant to blockade by 200 nM-2 microM (-)-propranolol and 3 microM ICI 118551 (a beta 2-selective antagonist) but antagonized by 1 microM (-)-bupranolol (pKB = 6.4-6.8), 3 microM CGP 20712A (a beta 1-selective antagonist) (pKB = 6.3-6.4) and 6.6 microM SR 59230A (a beta 3-selective antagonist, pKB = 5.1-5.4). 3. The non-conventional partial agonist cyanopindolol caused positive chronotropic effects in right atria (pD2 = 7.7) and positive inotropic effects in left atria (pD2 = 7.1). The stimulant effects of cyanopindolol were resistant to blockade by 200 nM (-)-propranolol but antagonized by 1 microM (-)-bupranolol (pKB = 6.8-7.1). 4. Neither (-)-CGP 12177 nor cyanopindolol caused stimulant effects in papillary muscles at concentrations between 0.2 nM and 20 microM. 5. In the presence of 200 nM (-)-propranolol the beta 3-adrenoceptor-selective agonists BRL 37344 (6 microM), SR 58611A (6 microM), ZD 2079 (60 microM) and CL 316243 (60 microM) did not cause stimulant effects or modify the potency and efficacy of the effects of (-)-CGP 12177 in right and left atria. The combination of 2 microM (-)-propranolol and 2 microM (-)-noradrenaline did not modify the chronotropic potency and efficacy of (-)-CGP 12177 compared to the potency and efficacy in the presence of 2 microM (-)-propranolol alone. 6. (-)-CGP 12177 relaxed the colon with a pD2 of 6.9 and a maximum effect of 55% compared to (-)-isoprenaline. The relaxant effects of (-)-CGP 12177 were resistant to blockade by 200 nM (-)-propranolol, 3 microM CGP 20712A, 3 microM ICI 118551 but blocked by 2 microM (-)-propranolol (pKB = 6.0), 1 microM (-)-bupranolol (pKB = 6.4) and 3 microM SR 59230A (pKB = 6.3). In the presence of 200 nM (-)-propranolol, (-)-CGP 12177 (20 microM) antagonized surmountably the relaxant effects of BRL 37344 (pKP = 7.3) (-)-noradrenaline (pKP = 7.0); and CL 316243 (pKP = 7.0). 7. Cyanopindolol in the presence of 200 nM (-)-propranolol relaxed the colon with a pD2 of 7.0 and a maximum effect of 40% compared to (-)-isoprenaline. As expected from a partial agonist, cyanopindolol antagonized the relaxant effects of both BRL 37344 and CL 316243 with a pKP = 7.6 and (-)-noradrenaline with a pKP = 7.4. 8. The following beta 3-adrenoceptor-selective agonists were potent colonic relaxants (pD2 values between parentheses): BRL 37344 (9.1), ZD 2079 (7.0), CL 316243 (9.0) and SR 58611A (8.2). The relaxant effects of these agonists were only marginally affected by 200 nM (-)-propranolol, not blocked by 3 microM CGP 20712A or 3 microM ICI 118551, and blocked by SR 59230A 3 microM (pKB = 6.9-7.5), 1 microM (-)-bupranolol (pKB = 6.2-6.4) and 2 microM (-)-propranolol (pKB = 6.3-6.5). 9...

(--)-Timolol is a more potent antagonist of the positive inotropic effects of (--)-adrenaline than of those of (--)-noradrenaline in human atrium

1. The affinity of (--)-timolol for beta 1- and beta 2-adrenoceptors was determined on isolated atrial preparations from patients undergoing open heart surgery. The times for onset and offset of antagonism of the positive inotropic effects of (--)-adrenaline and (--)-noradrenaline by (--)-timolol were measured. 2. The antagonism of the positive inotropic effects of (--)-adrenaline and (--)-noradrenaline by (--)-timolol (0.1-100 nM) was simple competitive in human atrium tissue. The slope of Schild-plots was not significantly different from 1.0 [0.93 +/- 0.09 for (--)-adrenaline, 0.97 +/- 0.09 for (--)-noradrenaline]. 3. The inotropic effects of (--)-adrenaline were antagonized significantly more by each concentration of (--)-timolol than those of (--)-noradrenaline. KB-values (-log M) were 10.10 +/- 0.09 against (--)-adrenaline and 9.43 +/- 0.07 against (--)-noradrenaline (P < 0.001). 4. Blocking kinetics of (--)-timolol for the beta-adrenoceptor were relatively slow. Half-times for the onset of blockade by 10 times KB of (--)-timolol were approximately 30 min for both (--)-adrenaline and (--)-noradrenaline; offset times were similar. 5. It is concluded that (--)-timolol has a higher affinity for the beta 2-adrenoceptor than for the beta 1-adrenoceptor in human atrium. This property may be beneficial clinically in protecting against the beta 2-adrenoceptor hypersensitivity induced by cardiac beta 1-adrenoceptor blockade, but also explain why severe asthma can occur after administration of very low intra-ocular doses of the drug.

Chronic beta 1-adrenoceptor blockade sensitises the H1 and H2 receptor systems in human atrium: rôle of cyclic nucleotides

We have reported that chronic treatment of patients with beta 1-adrenoceptor blockers sensitises isolated atrial preparations to adrenaline, noradrenaline and 5-Ht. We have now examined the effect of chronic treatment with beta-adrenoceptor blockers on responses to histamine of human right atrial appendages. We compared the effects of histamine on contractile force, cyclic AMP and cyclic GMP levels as well as cyclic AMP-dependent protein kinase (PKA) activity and explored the arrhythmogenic effects of histamine in preparations obtained from patients chronically treated or not treated with beta-adrenoceptor blockers. Histamine increased contractile force in paced preparations; the effects were blocked by the H2 receptor antagonist famotidine (0.1-30 mumol/l). The maximum inotropic response to histamine was doubled and the inotropic potency of histamine 0.4 log units greater in atria from beta-adrenoceptor blocker-treated compared to non beta-adrenoceptor blocker-treated patients. Histamine elicited frequency-dependent arrhythmias that were blocked by famotidine (30 mumol/l) but not by mepyramine (1 mumol/l). The incidence of arrhythmias was higher in atria from beta-adrenoceptor blocker-treated compared to untreated patients. Histamine increased both cyclic AMP and cyclic GMP levels, as well as PKA activity, significantly more in atria from beta-adrenoceptor blocker-treated compared to those from untreated patients. Mepyramine 1 mumol/l prevented the histamine-evoked increase in cyclic GMP levels, reduced the inotropic hyperresponsiveness and abolished the hyperresponsiveness in cyclic AMP levels and PKA activity observed in patients chronically treated with beta blockers. Sodium nitroprusside 10 mumol/l caused smaller increase of cyclic GMP levels than histamine and restored the contracile force depressed by mepyramine to its original level in atria from beta-adrenoceptor blocker-treated patients. The evidence is consistent with sensitisation of both the histamine H1 and histamine H2 receptor systems by chronic beta 1-adrenoceptor blockade. H1 receptor-mediated increases in cyclic GMP, enhanced through an as yet unknown mechanism by chronic beta 1-adrenoceptor blockade, may inhibit phosphodiesterase 3 activity, thereby causing enhanced histamine-evoked increases in cyclic AMP levels and PKA activity, and accounting partially for the increased inotropic responses to histamine through H2 receptors.

Comparison of the densities of 5-HT4 receptors, beta 1- and beta 2-adrenoceptors in human atrium: functional implications

We measured in human atrium the density of 5-HT4 receptors, labelled with [125I]-SB 207710 (1-butyl-4-piperidinyl) methyl 8-amino-7-iodo-1, 4-benzodioxan-5-carboxylate), and compared it with the density of beta1- and beta2-adrenoceptors, labelled with (-)-[125I]-cyanopindolol. [125I]-SB 207710 (5-1200 pmol/l) labelled a small population of saturable binding sites (Bmax approximately 4 fmol/mg protein) with a pK(D) of 9.7 and with 5-HT4 receptor characteristics, as assessed with competing ligands. The density of atrial binding sites with 5-HT4 receptor characteristics was 10 and 5 times lower, respectively, than the density of beta 1- and beta 2-adrenoceptors. We suggest that the small 5-HT4 receptor population may in part explain why the positive inotropic effects of 5-HT are smaller than those of catecholamines mediated through beta 1- and beta 2-adrenoceptors.

(-)-CGP 12177-induced increase of human atrial contraction through a putative third beta-adrenoceptor

1. The inotropic effects of (-)-4-(3-t-butylamino-2-hydroxypropoxy)benzimidazol-2-one ((-)-CGP 12177), an antagonist for beta 1- and beta 2-adrenoceptors as well as an agonist for beta 3-adrenoceptors, were investigated on paced preparations of isolated right atrial appendages obtained from patients without advanced heart failure undergoing open heart surgery. 2. In the presence of (-)-propranolol (200 nM), (-)-CGP 12177 increased contractile force with a -log EC50, M, of 7.3. The maximum effects of (-)-CGP 12177 amounted to 15% and 11% of the effects of (-)-isoprenaline (400 microM) and of CaCl2 (6.75 mM) respectively. 3. (-)-Bupranolol 1 microM, an antagonist with a pKB of approximately 7.5 for beta 3-adrenoceptors, antagonized surmountably the positive inotropic effects of (-)-CGP 12177 (in the presence of 200 nM (-)-propranolol) with an apparent pKB of 7.3. 4. The potent positive inotropic effects of (-)-CGP 12177 and their resistance to blockade by (-)-propranolol but antagonism by (-)-bupranolol are consistent with the existence in human atrial myocardium of a minor third beta-adrenoceptor population, possibly related to beta 3-adrenoceptors.

Participation of 5-HT1-like and 5-HT2A receptors in the contraction of human temporal artery by 5-hydroxytryptamine and related drugs

1. We investigated the hypothesis that, as in some other large human arteries, 5-HT-induced contraction of the temporal artery is mediated through two co-existing receptor populations, 5-HT1-like- and 5-HT2A. Temporal arterial segments were obtained from patients undergoing brain surgery and rings prepared set up to contract with 5-HT and related agents. Fractions of maximal 5-HT responses mediated through 5-HT1-like and 5-HT2A receptors, f1 and f2 = 1-f1, were estimated by use of the 5-HT2A-selective antagonist ketanserin. 2. In rings with intact endothelium 5-HT evoked contractions with a -log EC50, M of 7.0. Ketanserin (10-1000 nM) antagonized part of the 5-HT-induced contractions. Ketanserin-resistant components of 5-HT-induced contractions were found with -log EC50, M of 6.9 and f1 of 0.17 (100 nM ketanserin) and -log EC50, M of 6.4 and f1 of 0.20 (1000 nM ketanserin). 3. In rings with endothelial function attenuated by enzymatic treatment, 5-HT caused contractions with a -log EC50, M of 7.2 that were partially blocked by ketanserin. Ketanserin-resistant components of 5-HT-induced contractions were found with -log EC50, M 7.4 and f1 of 0.16 (100 nM ketanserin) and -log EC50, M of 7.5 and f1 of 0.14 (1000 nM ketanserin). 4. The ketanserin-resistant component of 5-HT-evoked contraction was blocked by methiothepin (100-1000 nM) consistent with mediation through 5-HT1-like receptors. 5. In rings with intact endothelium the 5-HT1-like-selective agonist, sumatriptan, caused small contractions with a -log EC50, M of 6.5 and intrinsic activity of 0.21 with respect to 5-HT that were resistant to blockade by 1000 nM ketanserin but antagonized by 100 nM methiothepin. 6. In rings with intact endothelium the 5-HT2A receptor partial agonist SK&F 103829 (2,3,4,5-tetrahydro-8[methyl sulphonyl]-1H3-benzazepin-7-ol methensulphonate) contracted rings with a -log EC50, M of 5.0 and an intrinsic activity of 0.49 with respect to 5-HT; the effects were antagonized by ketanserin 1000 nM. 7. We conclude that 80-86% of the maximum 5-HT-evoked contraction of human temporal artery is mediated through 5-HT2A receptors, the remainder through 5-HT1-like-receptors, regardless of whether or not endothelium is functional. The 5-HT1-like-receptors are more likely to be 5-HT1D beta receptors than 5-HT1D alpha receptors and sumatriptan is a full agonist for these receptors. As found in arteries of other species, SK&F 103829 is a partial agonist for 5-HT2A receptors of human temporal artery.

Blockade of human and porcine myocardial 5-HT4 receptors by SB 203186

We investigated the blockade of the positive inotropic effects of 5-hydroxytryptamine (5-HT) by SB 203 186 (piperidinoethyl-indole-3-carboxylate hydrochloride) and its affinity for 5-HT4 receptors of human right atrium and piglet left atrium. We also compared the blocking effects of SB 203 186 against 5-HT-evoked tachycardia in anaesthetised adult Yucatan minipigs as well as new-born Camborough piglets. SB 203 186 caused competitive antagonism of the positive inotropic effects of 5-HT in electrically paced atrial preparations of man (pKB = 8.9) and piglet (pKB = 8.5) at concentrations (up to 0.3 micromol/l) which were devoid of depressant or stimulant effects. The affinity of SB 203 186 for atrial 5-HT4 receptors was 30-160 times higher than that of tropisetron. 5-HT caused tachycardia with similar potency and efficacy in Yucatan minipigs and new-born Camborough piglets. SB 203 186 (0.1-3 mg/kg, i.v.) surmountably antagonised 5-HT-evoked tachycardia in anaesthetised Yucatan minipigs or new-born Camborough piglets with similar potency. The blocking potency of SB 203 186 in Yucatan minipigs was 17 times higher than that of tropisetron. Intraduodenally administered SB 203 186 (0.3-3 mg/kg) to new-born Camborough piglets produced blockade of 5-HT-evoked tachycardia which was maximal after 20 min and lasted for more than 3 h with 0.3 mg/kg. The antagonism produced by the SB 203 186 administration in new-born Camborough piglets was dose-related and threefold greater through the intravenous route than through the intraduodenal route. We conclude that SB 203 186 is an antagonist with nanomolar affinity for both human and porcine atrial 5-HT4 receptor. The in vivo results demonstrate that the sinoatrial 5-HT4 receptors function is similar in new-born Camborough piglets and adult Yucatan minipigs. Both porcine breeds are valid models for human atrial 5-HT4 receptors as demonstrated with the antagonist SB 203 186.

Sensitization of human atrial 5-HT4 receptors by chronic beta-blocker treatment

BACKGROUND

Chronic treatment of patients with beta-blockers induces beta 2-adrenergic receptor hyperresponsiveness in atrium and sinoatrial node. To investigate whether other atrial Gs protein-coupled receptors also become hyperresponsive after chronic treatment with beta-blockers, we investigated 5-HT4 receptors in tissues and myocytes, which mediate serotonin-evoked increases of both contractile force and cAMP levels.

METHODS AND RESULTS

Isolated right atrial strips from patients who had been chronically treated or not treated with a beta-blocker were set up to contract. In tissues from beta-blocker-treated patients (n = 27), the maximum inotropic response to serotonin was 56 +/- 3% (mean +/- SEM) of the effect elicited by (-)-isoproterenol (200 mumol/L) compared with a response of 19 +/- 6% in tissues from non-beta-blocker-treated patients (n = 13) (P < .001). The responsiveness of the tissues to Ca2+ was unchanged by chronic beta-blocker treatment. Serotonin (1 and 10 mumol/L) increased tissue cAMP levels, the increase with 10 mumol/L being significantly greater (P < .05) in tissues from beta-blocker-treated (n = 9) than in non-beta-blocker-treated (n = 7) patients. In paced atrial myocytes, serotonin caused concentration-dependent increases in contraction. Myocytes obtained from atria of beta-blocker-treated patients were more sensitive (P < .01) to the effects of serotonin (-log EC50, 7.9 +/- 0.2 mol/L; n = 12) than myocytes obtained from non-beta-blocker-treated patients (-log EC50, 7.3 +/- 0.2 mol/L, n = 12). Chronic beta-blocker treatment had no effect on forskolin-evoked myocyte responses. Carbachol (1 mumol/L) suppressed the effects of both serotonin (n = 6) and (-)-isoproterenol (n = 6) in the same atrial myocyte.

CONCLUSIONS

Chronic treatment of patients with beta-blockers causes atrial 5-HT4 receptor inotropic hyperresponsiveness and enhanced serotonin-evoked increases in cAMP levels. This may be due to modified cross talk between 5-HT4 receptors, beta-adrenergic receptors, and muscarinic receptors.

Labelling with [125I]-SB 207710 of a small 5-HT4 receptor population in piglet right atrium: functional relevance

1. We investigated the affinity of SB 207710 for sinoatrial 5-HT4 receptors and the density of right atrial 5-HT4 receptors with [125I]-SB 207710 in right atria of new-born piglets. 2. SB 207710 (1-100 nM) antagonized the 5-HT-evoked tachycardia surmountably with a pKB of 9.8. 3. [125I]-SB 207710 (5-1500 pM) labelled a small population of saturable binding sites with a pKD of 10.1 and with 5-HT4 receptor characteristics. The density of atrial binding sites with 5-HT4 receptor characteristics was 174 and 22 times lower respectively than those of atrial beta 1- and beta 2-adrenoceptors, labelled with (-)-[125I]-cyanopindolol. 4. We suggest that the small 5-HT4 receptor population may in part explain why the maximal tachycardia caused by 5-HT is smaller than that caused by catecholamines.

Beta-adrenoceptor subtypes in human coronary artery: desensitization of beta 2-adrenergic vasorelaxation by chronic beta 1-adrenergic stimulation in vitro

We previously demonstrated that right atrial strips from patients treated with beta 1-selective antagonists exhibit sensitization of beta 2-adrenergic responses in vitro. We also showed that cardiac beta 2-adrenergic sensitization can be induced in normal subjects prospectively by beta 1-blocker treatment. To determine whether such cross-talk could be induced in vitro, we studied beta-adrenoceptor-mediated vasorelaxation in deendothelialized rings of human coronary artery from patients undergoing cardiac transplantation. After incubation with 10 microM phenoxybenzamine for 1 h, concentration-effect curves were determined to norepinephrine (NE) and epinephrine (EPI), with or without 300 nM CGP 20712A (a beta 1-selective antagonist), 50 nM ICI 118551 (a beta 2-selective antagonist), or both antagonists. Both beta 1- and beta 2-adrenergic components to vasorelaxation were detected. Other rings were incubated for 16 h with either 1 microM NE (a selective beta 1-adrenoceptor agonist) or 300 nM CGP 20712A, or both. After washout, concentration-effect curves were determined to EPI in the presence of 300 nM CGP 20712A (beta 2-adrenergic responses). No differences in beta 2-adrenergic vasorelaxation were noted after prolonged incubation with either CGP 20712A or the combination of CGP 20712A and NE.(ABSTRACT TRUNCATED AT 250 WORDS)

Do human atrial 5-HT4 receptors mediate arrhythmias?

Since their first description five years ago, knowledge about human atrial 5-HT4 receptors has increased considerably. Progress has been facilitated by the advent of selective antagonists with high affinity for human atrial 5-HT4 receptors. The receptors have been detected in both right and left atrium where they mediate increases in contractile force. Human sinoatrial 5-HT4 receptors may mediate the tachycardia caused by 5-HT and cisapride, and 5-HT elicits arrhythmias via 5-HT4 receptors in human atrium. In this article, Alberto Kaumann suggests that 5-HT may be involved in the genesis of atrial fibrillation and associated thromboembolic stroke and that both the arrhythmia and stroke could be prevented by inhibiting 5-HT4 receptors.

Variable participation of 5-HT1-like receptors and 5-HT2 receptors in serotonin-induced contraction of human isolated coronary arteries. 5-HT1-like receptors resemble cloned 5-HT1D beta receptors

BACKGROUND

Serotonin may contract human large coronary arteries through two 5-hydroxytryptamine (5-HT) receptors, 5-HT1-like and 5-HT2. These 5-HT1-like receptors resemble both cloned 5-HT1D receptor subtypes, 5-HT1D alpha and 5-HT 1D beta. Although these subtypes have similar pharmacology, 5-HT1D beta receptors appear to have lower affinity for ketanserin than 5-HT1D alpha receptors. We assessed the relative participation of 5-HT1-like and 5-HT2 receptors and attempted to identify whether vasoconstrictor 5-HT1-like receptors are 5-HT1D alpha or 5-HT1D beta.

METHODS AND RESULTS

Epicardial coronary arteries were dissected from the hearts of 29 patients (including 1 healthy (donor) undergoing heart transplant operation. Endothelium-denuded strips were set up to contract at 37 degrees C. To assess the relative contributions of 5-HT1-like and 5-HT2 receptors, we blocked the latter with ketanserin (0.1 to 1.0 mumol/L) and ketanserin-resistant receptors with methiothepin (0.1 mumol/L). Concentration-effect curves for 5-HT, in the absence and presence of ketanserin, were analyzed by using a model for two receptor subtypes. The fractional contributions of 5-HT1-like and 5-HT2 receptors to the maximum effect of 5-HT, f1 and f2, were estimated in arteries from 28 patients: f1 (0.71 +/- 0.20, mean +/- SD) was significantly larger than f2 (0.29 +/- 0.20) (P < .0001). Using [3H]-serotonin to label transfected and expressed receptors, we verified that ketanserin has lower affinity for 5-HT1D beta (pKi [-log Ki, mol/L] less than 5.0) than for 5-HT1D alpha (pKi = 7.1 +/- 0.1) receptors. A concentration of ketanserin (1 mumol/L) that would occupy more than 90% of 5-HT1D alpha receptors failed to block 5-HT-induced contractions (4 patients). The 5-HT1-like receptor stimulant sumatriptan evoked maximal contractions that matched f1 and was equipotent with 5-HT through 5-HT1-like receptors (8 patients). No systematic influence of disease, atheroma, or therapy on f1 and f2 was detected.

CONCLUSIONS

Coronary artery contractile 5-HT1-like receptors resemble cloned 5-HT1D beta receptors and predominate over 5-HT2 receptors in mediating serotonin-evoked contractions. Sumatriptan contracts coronary arteries as a full agonist through 5-HT1-like receptors.

Blockade of human atrial 5-HT4 receptors by SB 207710, a selective and high affinity 5-HT4 receptor antagonist

The mode of antagonism of 5-hydroxytryptamine-induced positive inotropic effects by the highly selective 5-HT4 receptor antagonist SB 207710 (1-butyl-4-piperidinyl) methyl 8-amino-7-iodo-1,4-benzodioxan-5-carboxylate) was investigated on isolated preparations of human right atrial appendage. SB 207710 caused concentration-dependent (0.1-10 nmol/l) surmountable antagonism of the effects of 5-hydroxytryptamine with a pKB (mol/l) of 10.1. Due to its high selectivity and affinity, SB 207710 could be a powerful tool for the comparison of human atrial 5-HT4 receptors with 5-HT4 receptors of other organs of man and other species.

5-Hydroxytryptamine causes rate-dependent arrhythmias through 5-HT4 receptors in human atrium: facilitation by chronic beta-adrenoceptor blockade

We have investigated the ability of 5-hydroxytryptamine (5-HT) to elicit arrhythmic contractions in isolated human atrial strips as a function of pacing rate (0.1-2 Hz) using a method recently introduced by us (Kaumann and Sanders, this journal, 1993 b) and examined the nature of the 5-HT receptors involved. Right atrial appendage tissue was obtained from 14 patients undergoing cardiac surgery. None of the patients had advanced heart failure. 5-HT (0.6-20 mumol/l) induced arrhythmic contractions during pacing in 4/11 atrial strips from 3/4 patients who had not received beta blockers and in 21/27 atrial strips from 9/10 patients who had been chronically treated with beta blockers (primarily beta 1-selective). The incidence of arrhythmic contractions evoked by 5-HT did not reach statistical significance in the atrial tissue from the non-beta blocked patients but was highly significant in the atrial tissue from the chronically beta blocked patients. The arrhythmic contractions usually occurred more frequently at low than at high pacing rates and were observed at the physiological frequency of 1 Hz in 1/4 atrial strips from 1/4 of the non-beta blocked patients and 6/11 strips from 5/10 of the beta blocked patients. The 5-HT-evoked arrhythmic contractions were observed during blockade of beta 1-adrenoceptors, beta 2-adrenoceptors and 5-HT3 receptors, ruling out the participation of these receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Potentiation of responses to sympathetic nerve stimulation and vasoconstrictor agents by SK&F 103829 in the feline mesenteric circulation

1. The amplification of vasoconstrictor effects of several agonists and sympathetic nerve stimulation, caused by 5-HT2 receptor activation, was studied in the autoperfused mesenteric circulation of anaesthetized cats. To produce long lasting and selective 5-HT2 receptor stimulation we used SK&F 103829 (2,3,4,5 tetrahydro-8[methyl-sulphonyl]-1H3-benzazepin-7-ol methensulphonate). We assessed that SK&F 103829 was a strong contractile partial agonist in isolated preparations of rat tail artery and calf pulmonary artery. 2. The intrinsic activity of SK&F 103829 with respect to 5-hydroxytryptamine (5-HT) was 0.8 in rat tail artery and 0.6 in calf pulmonary artery. SK&F 103829-induced contractile responses were surmountably antagonized by ketanserin with a potency expected from its affinity for 5-HT2 receptors. SK&F 103829 surmountably antagonized the effects of 5-HT in rat tail artery with a pKp of 5.8. 3. Concentrations of SK&F 103829 causing greater than threshold constrictions enhanced vasoconstrictor responses of sympathetic nerve stimulation, noradrenaline, angiotensin II, methoxamine and alpha, beta-methylene ATP in the mesenteric arterial bed. Increases in mesenteric arterial pressure by noradrenaline, observed in the presence of prazosin, were also potentiated by SK&F 103829. 4. Ketanserin prevented both the constrictor effect of SK&F 103829 and the SK&F 103829-evoked potentiation of the responses to noradrenaline and angiotensin II in the mesenteric arterial bed. Ketanserin, however, failed to abolish (once established) the SK&F 103829-evoked potentiation of the constrictor effects caused by both noradrenaline and angiotensin II. 5. Short lasting constrictor effects of 5-HT were reversed to dilator effects by SK&F 103829 in both the mesenteric arterial and venous bed, thereby revealing the existence of vasodilator 5-HT receptors.6. The main finding is consistent with a sensitization of the mesenteric arterial bed to vasoconstrictor responses mediated through alpha 1- and alpha2-adrenoceptors, angiotensin II receptors and purinoceptors by SK&F 103829-evoked activation of 5-HT2 receptors. This property, together with the direct constrictor effect of the mesenteric arterial bed suggest that SK&F 103829 can reduce portal venous flow thereby being a useful therapeutic principle for the treatment of portal hypertension.

Studies on the intrinsic activity (efficacy) of human adrenergic receptors. Co-expression of beta 1 and beta 2 reveals a lower efficacy for the beta 1 receptor

Through co-expression of the human beta 1 and beta 2 adrenergic receptors in a single tester cell (the murine L fibroblast) and through assaying the effect of the beta 1 and beta 2 selective blockers CGP 20712A and ICI 118551 on isoproterenol-stimulated adenylyl cyclase, it is shown that the maximal stimulation achievable with a given cell density of beta 1 adrenergic receptor is less than that obtained with the same density of the beta 2 adrenergic receptor. It is concluded that the efficacy of the 2 receptors differs in that the beta 1 adrenergic receptor has a lower efficacy (or intrinsic activity) than does the beta 2 adrenergic receptor.

Efficacy of beta 1-adrenergic receptors is lower than that of beta 2-adrenergic receptors

We investigated the relative activity at which fully occupied human beta 1- and beta 2-adrenergic receptors (beta 1AR and beta 2AR) activate the stimulatory G protein (Gs)/adenylyl cyclase (AC) system in isolated membranes. The receptors were cloned and coexpressed in permanent cell lines at beta 1/beta 2 ratios that varied from 1:2 to 3:1 and at total receptor abundance that ranged from 8 to 2200 fmol/mg of membrane protein. Cell lines expressing beta 1AR or beta 2AR alone were also obtained. Competitive inhibition of isoproterenol-stimulated AC activity by the beta 2-selective antagonist ICI 118551 showed in all cases that maximal stimulation elicited by beta 1AR was lower than when it was elicited by equivalent densities of beta 2AR. This was especially noticeable at limiting concentrations of receptor, where the beta 1AR-mediated effect was < 10% of that mediated by beta 2AR. At receptor concentrations > 1000 fmol/mg of protein, stimulation by beta 2AR appeared to reach a maximum, while stimulation by beta 1AR continued to increase, so that at 3200 fmol/mg, beta 1AR-stimulated activity was 80% of beta 2AR-stimulated activity. It is clear that the degree to which a given receptor system is able to activate the Gs/AC system depends not only on its abundance but also on an activity parameter determined by the nature of the receptor, which we refer to as receptor efficacy. For human beta ARs, this efficacy parameter is much lower for the beta 1 subtype than for its beta 2 counterpart. The more effective stimulation of AC through beta 2AR than through beta 1AR is an inherent property of the receptor and not the cell in which it is expressed.

Blockade of human atrial 5-HT4 receptors by GR 113808

1. The mode of antagonism of 5-hydroxytryptamine (5-HT)-induced positive inotropic effects by the highly selective 5-HT4 receptor antagonist GR 113808 ([1-[2-methylsulphonylamino ethyl]-4-piperidinyl]methyl 1-methyl-1H-indole-3-carboxylate) was investigated on isolated preparations of human right atrium. 2. GR 113808 caused concentration-dependent (2-100 nM) surmountable antagonism of the effects of 5-HT with a pKB (M) of 8.8. 3. The affinity of GR 113808 for human atrial 5-HT4 receptors, together with its high selectivity for 5-HT4 receptors comprise useful properties for investigating the question of 5-HT4 receptor subtypes.

Characterization of the 5-HT4 receptor mediating tachycardia in piglet isolated right atrium

1. In order to explore whether 5-HT4 receptor subtypes exist, we have characterized further the 5-HT4 receptor that mediates tachycardia in the piglet isolated right atrium. All experiments were carried out in the presence of propranolol (400 nM) and cocaine (6 microM). We used tryptamine derivatives, substituted benzamides and benzimidazolone derivatives as pharmacological tools. 2. Tachycardia responses to 5-hydroxytryptamine (5-HT) were mimicked by other tryptamine derivatives with the following order of potency: 5-HT > 5-methoxytryptamine alpha-methyl-5-HT = bufotenine bufotenine > 5-carboxamidotryptamine = tryptamine (after treatment with pargyline) > 5-methoxy-N,N-dimethyltryptamine > 2-methyl-5-HT. 3. The substituted benzamides were all partial agonists relative to 5-HT except (-)-zacopride which was a full agonist. The stimulant potency order was renzapride > cisapride = (-)-zacopride > metoclopramide > (+)-zacopride. 4. The benzimidazolone derivatives had contrasting effects. BIMU 8 (endo-N-(8-methyl-8-azabicyclo[3.2.1]oct-3-yl)-2,3-dihydro-(1-methyl(eth yl- 2-oxo-1H-benzimidazole-1-carboxamide hydrochloride) was a full agonist relative to 5-HT whilst BIMU 1 (endo-N-(8-methyl-8-azabicyclo[3.2.1]oct-3-yl)-2,3-dihydro-3-ethyl-2-oxo - 1H-benzimidazole-1-carboxamide hydrochloride) was a partial agonist with low intrinsic activity compared to 5-HT but had similar potency. We estimated a pKB of 7.9 for BIMU 1 antagonism of 5-HT-induced tachycardia. DAU 6215 (N-endo-8-methyl-8-azabicyclo[3.2.1]-oct-3-yl)-2,3-dihydro-2-oxo-lH-benzimidazole-l-carboxamide, hydrochloride) had no chronotropic activity and was found to be a simple competitive antagonist with a pKB of 7.15.SB 203186 (1-piperidinyl)ethyl lH-indole 3-carboxylate) was a potent antagonist with a pKB of 8.3.The affinity of SB 203186 was approximately 20 times higher than that of tropisetron (ICS 205-930;pKB= 6.9) and DAU 6215 (pKB= 7.0). GR1 13808 (([1-[2-[methylsulphonyl amino]ethyl]-4-piperidinyl]methyl 1-methyl-1H-indole-3-carboxylate) and SDZ 205-557 ((2-diethylaminoethyl)2-methoxy-4-amino-5-chloro-benzoate) also antagonized 5-HT-induced tachycardia but not by simple competitive blockade.6. The sinoatrial 5-HT4 receptor in the piglet has a pharmacological profile that correlates well with 5-HT4 receptors characterized in rat oesophagus, guinea-pig ileum and colon, mouse embryonic colliculi neurones and human atrium.

Both beta 1- and beta 2-adrenoceptors mediate catecholamine-evoked arrhythmias in isolated human right atrium

The involvement of beta 1- and beta 2-adrenoceptors in catecholamine-evoked arrhythmias was investigated in isolated human right atrial appendages obtained from 22 patients chronically treated with beta blockers (usually beta 1-selective) and 9 patients not treated with beta blockers. A simple experimental model that assesses the incidence of arrhythmic contractions as a function of heart rate (pacing) is introduced. beta 1-adrenoceptors were activated by (-)-noradrenaline during beta 2-adrenoceptor blockade with 50 nmol/l ICI 118551. beta 2-adrenoceptors were activated by (-)-adrenaline during beta 1-adrenoceptor blockade with 300 nmol/l CGP 20712A. Both (-)-noradrenaline and (-)-adrenaline caused arrhythmic contractions whose incidence was greater at low than at high pacing rates. CGP 20712A (300 nmol/l) blocked the (-)-noradrenaline-evoked contractions in 1/1 atrial strip from 1/1 patient not treated with a beta blocker and 17/17 atrial strips from 15/15 patients chronically treated with beta blockers. ICI 118551 (50 nmol/l) blocked the (-)-adrenaline-evoked contractions in 3/4 atrial strips from 3/4 patients not treated with beta blockers and 17/20 atrial strips from 15/18 patients chronically treated with beta blockers. The incidence of arrhythmic contractions evoked by both (-)-noradrenaline and (-)-adrenaline was higher in chronically beta blocked patients than in non beta blocked patients. We conclude that both beta 1- and beta 2-adrenoceptors mediate atrial arrhythmias and that the generation of these arrhythmias is facilitated by chronic beta 1-adrenoceptor blockade.

Coexistence of functioning beta 1- and beta 2-adrenoceptors in single myocytes from human ventricle

BACKGROUND

Both beta 1- and beta 2-adrenoceptors (beta 1 AR and beta 2 AR) are present in human ventricle. This study was designed to determine whether the two subtypes contribute to contraction in single myocytes from human heart.

METHODS AND RESULTS

(-)-Epinephrine increased the contraction amplitude and velocity of single myocytes isolated from the ventricles of failing and nonfailing human hearts. Concentration-response curves to (-)-epinephrine were constructed in the presence and absence of selective antagonists for beta 1 AR (CGP 20712A) and beta 2 AR (ICI 118,551). Responses to (-)-epinephrine were antagonized to a variable degree by the blockers, suggesting heterogeneous contribution of beta 1AR and beta 2AR among cells. The most common response in single myocytes was that ICI 118,551 (50 nmol/L) shifted the concentration-response curve less than 10-fold: this was lower than the 100-fold shift expected for a pure beta 2AR effect. Inclusion of CGP 20712A (300 nmol/L) with ICI 118,551 shifted the (-)-epinephrine curve still further. These observations suggest that both beta 1AR and beta 2AR contribute to the increase in contraction amplitude with (-)-epinephrine in this group of myocytes. When 300 nmol/L CGP 20712A was present as the sole antagonist, only a marginal shift of the concentration-response curve for (-)-epinephrine was usually observed, indicating that beta 1AR were not mediating the effect of these low concentrations of (-)-epinephrine. Both beta 1AR and beta 2AR mediated a considerable abbreviation of the time to peak contraction and time to 50% relaxation in the single cells.

CONCLUSIONS

beta 1AR and beta 2AR coexist and function on human ventricular myocytes. At low (-)-epinephrine concentrations, contractile responses are predominantly mediated by beta 2AR rather than beta 1AR in myocytes from failing hearts.