Beta-adrenoceptor blockade by atenolol, metoprolol and propranolol in the anaesthetized cat

The inhibitory effects of atenolol, metoprolol and propranolol on isoprenaline-induced tachycardia, broncho-relaxation and vasodilatation were investigated in the reserpinized and anaesthetized cat. In low doses all three antagonists inhibited the heart rate response to isoprenaline, the order of potency being propranolol greater than metoprolol greater than atenolol. While propranolol inhibited the bronchodilation and vasodilation responses to isoprenaline in the same dose range as it blocked the heart rate response, atenolol and metoprolol had to be given in considerably higher doses to block these effects. The results indicate that both metoprolol and atenolol, in contrast to propranolol, are selective beta1-adrenoceptor antagonist. No statistically significant difference in the degree of selectivity was found between metoprolol and atenolol. The three compounds were devoid of intrinsic beta-mimetic activity.