Effects of verapamil and its optical isomers on repetitive slow responses induced by electrical depolarization in canine ventricular myocardium.

The electrophysiological effects of verapamil (racemic compounds) and its optical (d- and 1-) isomers on canine ventricular myocardial fibers were investigated in current clamp conditions using single sucrose gap chamber and microelectrodes. The current-voltage (I-V) relationships were obtained in normal and low Na (12 mM)--low Ca (0.45 mM) solutions with and without the drugs. Verapamil and its optical isomers blocked repetitive action potential discharges (slow responses) induced by depolarizing DC-currents. However, l-isomer was more potent than d-isomer in suppressing these responses. The difference in the potency was attributed to their different actions on the steady state I-V relationships. Namely, l-isomer increased time independent membrane conductance to potassium ions (probably gk1), while d-isomer did not. This effect of l-isomer may favor the suppression of phase 4 depolarization and hence reduce the frequency of repetitive action potential discharges in depolarized ventricular muscle more effectively than d-isomer.