The effects of the d- and l-optical isomers of propranolol on the ventricular fibrillation threshold.

The minimum current required to induce ventricular fibrillation was determined in 22 sodium pentobarbital anesthetized open chest dogs in the presence of the d- or l-isomers of propranolol at concentrations of 0.2, 0.5, and 1.0 mg/kg. The hearts were paced from the right atrium at a constant basic cycle length of 350 msec and the current was delivered during the vulnerable period to the right ventricle as a 120 msec train of 4 msec square wave pulses at 100 Hz. The l-isomer of propranolol increased the ventricular fibrillation threshold (VFT) above control values at all concentrations; the maximum increase ranged from 17.8 to greater than 420.8% change from control. In contrast, the d-isomer caused only a small increase in the ventricular fibrillation threshold with the maximum increase ranging between 14.0 and 65.2% change from control. The maximum increase in VFT for the racemic mixture of propranolol was 44.7 to greater than 500.0% above control and was similar to the l-isomer alone. Since the beta-blocking potency of the l-isomer is 100 times the potency of the d-isomer we conclude that propranolol's action in increasing the ventricular fibrillation threshold is dependent on its beta-blocking effect.