The selectivity of beta-adrenoceptor antagonists on isoprenaline-induced changes in heart rate, blood pressure, soleus muscle contractility and airways function in anaesthetized cats.

Abstract

The beta-adrenoceptor antagonist of propranolol, metoprolol, atenolol and butoxamine in anaesthetized cats has been measured and compared with the activity of four synthetic phenylethanolamine derivatives. The effects of isoprenaline on four parameters in the anaesthetized cat: heart rate, blood pressure, soleus muscle contractility and airway reactance, were measured and the modification of the isoprenaline dose-response relation by each of the antagonist drugs assessed. Parallel shifts in log dose-response curves for isoprenaline were caused by propranolol for all parameters, by metoprolol and atenolol for each parameter except blood pressure, and butoxamine for each except soleus muscle and heart rate. Selectivity of action of the antagonists between different organs was measured by comparing DR10 values, computed from isoprenaline dose-ratios. Propranolol was the most potent antagonist and showed slight selectivity of action on soleus muscle compared with heart. Atenolol and metoprolol were approximately equipotent and were cardioselective at low doses only. Butoxamine was the least potent antagonist and possessed non-beta-adrenoceptor effects on the parameters measured. Each of the new compounds, 4'-bromo-2'-methoxy-N-isopropyl phenylethanolamine, the 4'-chloro- and 4'-methyl analogues, and 4'-methoxy-N-t-butyl phenylethanolamine, was a potent antagonist but did not exhibit any selectivity of action. The results suggest no clear separation of beta-adrenoceptors into beta 1- and beta 2-subclasses in organs of the cat. There is no apparent separation of beta-adrenoceptor-mediated effects on skeletal muscle and airways.