A comparison of the properties of prenalterol and corwin at beta 1- and beta 2-adrenoreceptors in vitro

Selective and full beta 1-adrenoceptor agonist action of a catechol derivative of denopamine (T-0509) in the guinea-pig cardiac muscle and trachea: comparison with denopamine, xamoterol and isoprenaline

1. The pharmacological actions of T-0509, a 3-hydroxy derivative of denopamine, were studied in various guinea-pig tissues; these effects were compared with those of isoprenaline, denopamine and xamoterol. 2. The intrinsic activities of the positive inotropic actions of T-0509, denopamine and xamoterol compared with isoprenaline (= 100%) in the papillary muscle were 99%, 83% and 28%, respectively, while their relative potencies (EC50 agonist EC50 isoprenaline) were 0.23, 33 and 1.4, respectively. The intrinsic activities of T-0509, denopamine and xamoterol as positive chronotropic agents in the right atria were 98%, 69% and 48%, respectively, and their equipotent concentrations (isoprenaline = 1) were 0.24, 50 and 4, respectively. 3. The positive chronotropic actions of T-0509 and denopamine were antagonized by bisoprolol (3 x 10(-8) M), but not by ICI 118,551 (3 x 10(-8) M). 4. The intrinsic activity of T-0509 in histamine-contracted tracheae was similar to that of isoprenaline, but its equipotent concentration was 38; the effects of both agents were antagonized by ICI 118,551 (3 x 10(-8) M), but not by bisoprolol (3 x 10(-8) M). Denopamine and xamoterol did not show any agonist activity on guinea-pig trachea. 5. Denopamine and xamoterol antagonized the positive chronotropic (pA2, denopamine: 6.98, xamoterol: 7.75) and tracheal relaxant (pA2, denopamine: 5.39, xamoterol: 6.25) effects of isoprenaline. 6. Isoprenaline, T-0509 and denopamine, but not xamoterol, contracted the guinea-pig aorta in a decreasing order in the presence of propranolol (10(-6) M).7. Based on the above studies, T-0509 appears to be a highly selective betaI-adrenoceptor agonist with full agonist properties, while denopamine and xamoterol appear to be selective, but partial betaI-adrenoceptor agonists.

Desensitization and functional antagonism by beta-adrenoceptor and muscarinic receptor agonists, respectively: a comparison with receptor alkylation for calculation of apparent agonist affinity

1. Apparent affinity constants (KD) for prenalterol, an agonist of low intrinsic efficacy at beta 1-adrenoceptors in rat left atria, have been determined by use of receptor desensitization and functional antagonism induced by isoprenaline and carbachol, respectively. The values obtained have been compared to those values estimated with the irreversible beta-adrenoceptor antagonist, bromoacetylalprenololmenthane (BAAM). 2. The -log KD values for prenalterol estimated by desensitization or irreversible antagonism ranged from 6.8-7.1 and 6.2-7.1, respectively. 3. Carbachol produced functional antagonism of the response to prenalterol even though it was removed before addition of prenalterol. This effect was mediated by M2-muscarinic receptors. Pretreatment of animals with pertussis toxin did not affect the functional antagonism elicited by carbachol. The apparent KD value obtained after pre-exposure to carbachol (6.8) was similar to those estimated by use of either alkylation with BAAM or desensitization with isoprenaline (see above). 4. It is concluded that acute desensitization or functional antagonism of responses to agonists of low intrinsic efficacy provides a means to estimate apparent KD constants. This approach could be useful to characterize receptors for which an irreversible antagonist may not be available.

Effects of xamoterol on inotropic and lusitropic properties of the human myocardium and on adenylate cyclase activity

The purpose of the present study was to characterize the effects of xamoterol in the human myocardium. In the presence of forskolin or milrinone, xamoterol increased isometric force of contraction, contraction velocity, and relaxation velocity in isolated, electrically driven preparations from human myocardium, but had no effect alone. There was no difference in the effect of xamoterol between right atrial myocardium and left ventricular myocardium from nonfailing (NF), moderately failing (NYHA II-III), and severely failing (NYHA IV) human hearts. The positive inotropic and lusitropic effects of isoprenaline were reduced depending on the severity of heart failure in left ventricular myocardium (i.e., NF greater than NYHA II-III greater than NYHA IV). In the presence of norepinephrine, xamoterol produced negative inotropic effects similar to those of the beta-adrenoceptor antagonists pindolol and propranolol. Xamoterol alone had no effects on force of contraction, whereas pindolol and propranolol markedly reduced contractile force. In NYHA class IV, isoprenaline stimulated adenylate cyclase about twofold but xamoterol, like pindolol or propranolol, had no effect. Experiments with the beta 1- and beta 2-selective antagonists CGP 207.12A and ICI 118.551, respectively, showed that the positive inotropic and lusitropic effects of xamoterol were mediated by beta 1-adrenoceptors. Consistently, xamoterol had a selectivity of 13.8 at beta 1-adrenoceptors as measured in radioligand binding experiments. It is concluded that xamoterol acts as a beta 1-adrenoceptor antagonist with a selectivity of 13.8 in human ventricular myocardium. The compound has an intrinsic sympathomimetic activity, as it produces beta 1-adrenoceptor-mediated positive inotropic and lusitropic effects in the presence of forskolin. The beneficial effects of xamoterol in patients with heart failure could be due to prevention of the detrimental effects of norepinephrine such as beta 1-adrenoceptor downregulation of an increase of Gi (inhibitory guanine-nucleotide binding protein).