Relevance of selectivity and non-selectivity in beta-adrenoceptor blocking drugs

1 The heart rate responses to increasing doses of isoprenaline (n = 6) (during infusion with atropine 0.5 mg/min i.v.) and noradrenaline (n = 5), (during infusion with phentolamine 160 mg/h i.v.) were recorded before and after the intravenous administration of propranolol (15 mg) or metoprolol (15 mg). 2 In the doses used, metoprolol was less potent than propranolol in antagonizing isoprenaline-induced tachycardia, during atropine infusion. 3 In volunteers treated with phentolamine both metoprolol and propranolol produced similar inhibition of noradrenaline-induced tachycardia, indicating comparable beta 1-adrenoceptor blockade in the doses used. 4 The fact that the shift of the dose-response curve of isoprenaline-induced tachycardia was smaller after metoprolol than after propranolol supports the hypothesis that beta 2-adrenoceptors are present in the heart.