The selective action of beta-adrenoceptor blocking drugs and the nature of beta1 and beta2 adrenoceptors

Abstract

1 Purified membranes retaining a catecholamine responsive adenylate cyclase have prepared from rabbit heart, lung and (pseudo-pregnant) uterus. 2 These preparations have the characteristics of plasma membranes and both heart and lung respond to beta-adrenoceptor agonists in the order: (+/-)-isoprenaline greater than (-)-noradrenaline greater than (-)-adrenaline greater than (+)-isoprenaline greater than salbutamol. The sensitivity of the adenylate cyclase to beta-adrenoceptor stimulation is improved by pre-treatment of the animals with reserpine and syrosingopine. 3 Dose-ratios for several concentrations of propranolol (non-selective beta-adrenoceptor blocker), practolol and atenolol (cardio-selective beta-adrenoceptor blockers) have been measured on all three membrane preparations. Schild plots of log (dose ratio -1) vs. log dose were virtually coincident for heart and lung with a dissociation constant (Kb) for propranolol very close to the pharmacological value. The ratio of Kb values was 0.65 for practolol and 1.23 for atenolol compared with pharmacological cardio-selectivity ratios (measured on isolated atria and tracheal chain) of 67.6 and 110 respectively. The uterus/heart Kb ratio was 51.5 for atenolol. Inhibition of the uterus by practolol gave a Schild plot with slope significantly less than 1, indicating a different mechanism of action from the heart. 4 Kb values obtained by measuring adenylate cyclase stimulation in chopped tissue (including preparations of bronchial tree and alveolar tissue as well as whole lung) resembled the membrane values rather than those found in whole organs. 5 The results show that the pharmacological selectivity of practolol and atenolol is maintained at the receptor-adenylate cyclase level, at least as far as heart and uterus are concerned, though the smaller selectivity ratios in the biochemical system suggest that receptor differences is not the only factor and that phase distribution of the drug may also be important. Membranes prepared from whole lung show that phase distribution of the drug may also be important. Membranes prepared from whole lung show an overall beta1 response which may simply reflect the predominance of beta1 cell types containing beta1-adrenoceptors over bronchial smooth muscle.