Effects of propranolol and alprenolol on intra-atrial conduction in the dog

Abstract

Intra-atrial conduction and its modification by two β-adrenoceptor blocking agents have been investigated in the dog. Premature atrial stimuli (St 2) were applied at variable intervals following regular stimuli (St 1) ensuring atrial pacing. A significant intra-atrial supernormal conduction was spontaneously observed at coupling intervals of 280−180 msec in 9 out of 10 animals. This phenomenon was still observed after propranolol or alprenolol administration but it occurred significantly earlier. With intervals shorter than 180 msec, atrioventricular conduction was decreased and both drugs further decreased it while simultaneously increasing nodoventricular effective and atrial functional refractory periods. In 1 dog, alprenolol induced a typical gap phenomenon in atrio-nodal conduction: as the St 1−St 2 interval became shorter, a point was reached where A 2 (atrial premature depolarization) was no longer conducted through the A-V node. If the St 1−St 2 interval was further decreased, the intra-atrial conduction delay increased sufficiently to allow A 2 to be conducted through the A-V node once again.