Comparison of the effects of propranolol, pindolol, oxprenolol and acebutolol on atrioventricular conduction in unanaesthetized dogs.

1 Atrio-ventricular conduction (AVC) in the unanaesthetized dog (17 animals) was studied using atrial pacing at progressively increasing frequencies applied via electrodes implanted in the right atrium. In animals habituated to the experimental conditions, beta-adrenoceptor blocking drugs devoid of intrinsic sympathomimetic activity (ISA) modified neither heart-rate (HR) nor AVC, though 4 mg/kg i.v. propranolol transiently raised HR and facilitated AVC. Due to their ISA, pindolol and oxprenolol raised HR and facilitated AVC at high doses. 2 Thus, when not under anaesthesia, the resting dog, in contrast to man, possesses no tonic adrenergic influence which might be suppressed by beta-adrenoceptor blockings drugs. 3 After atropine or pentobarbitone, which considerably increased HR, beta-adrenoceptor blocking effects became clearly visible, and the hitherto reported depressant effects on HR and AVC were observed. However, the ISA of oxprenolol, and especially of pindolol, can partly or completely mask their beta-adrenoceptor blocking action at high doses under these experimental conditions.