Calcium antagonist blockade of slow action potentials in cultured chick heart cells.

The effects of four Ca antagonists, bepridil, diltiazem, nifedipine, and verapamil, on slow channels were studied in cultured cell reaggregates prepared from 14-day-old chick embryonic hearts. The cell membrane was partially depolarized to about -45 mV by using 22 mM KCl to inactivate the fast Na+ channels. Slow action potentials were induced by 10(-6) M isoproterenol with electrical stimulation. Cumulative dose-response curves for the effect of the four drugs on the blocking of slow action potentials (using Vmax as the indicator) were analyzed by Hill plots. The dose values for 50% of maximal effect, at a stimulation frequency of 60/min, were (in order of decreasing potencies) as follows: 5.2 X 10(-9) M for nifedipine, 3.1 X 10(-7) M for diltiazem, 1.2 X 10(-6) M for verapamil, and 5.1 X 10(-6) M for bepridil. The effect of all four Ca antagonists showed use (or frequency)-dependency, i.e., the drugs were more effective at higher stimulation rates. This may reflect a blocking action of the drugs on the nonresting states of the channels and (or) a slowing of the recovery kinetics of the channels from the inactivated state back to the resting state. In a separate type of experiment utilizing a 5-min rest period in the presence of the drugs, nifedipine blocked and bepridil exhibited some depression of the first action potential elicited, i.e., use-independent effect, indicating that these drugs may also act on resting channels. Thus, these four Ca antagonists have a prominent use-dependent component in their actions, and one or two may also have a use-independent component.