INTERACTION OF RO-40-5967 AND VERAPAMIL WITH THE STABLY EXPRESSED ALPHA(1)-SUBUNIT OF THE CARDIAC L-TYPE CALCIUM-CHANNEL

The interaction of the nondihydropyridine calcium channel agonist Ro 40-5967 with the stably expressed class C alpha(1)-subunit of the cardiac L-type calcium channel was investigated and compared with that of verapamil by using the whole cell patch clamp configuration. Both compounds blocked the Ba++ inward current. The IC50 values at a holding potential of -80 or -40 mV were 4.9 and 1.4 mu M for Ro 40-5967 and 250 and 15.5 mu M for verapamil. Both Ro 40-5967 and verapamil induced a partial tonic block at a holding potential of -80 mV. The block increased with high depolarization rates. Both Ro 40-5967 and verapamil shifted the steady-state inactivation curve by more than 20 mV to hyperpolarized membrane potentials and decreased the inactivation rate constant. The effect of Ro 40-5967, but not that of verapamil, was attenuated by intracellular dialysis with GTP(gamma)S. The affinity for verapamil was not affected by replacing Ba++ by Ca++, but was increased by the coexpression of the beta(3)-subunit. These results indicate that both compounds interact with high affinity with the inactivated channel state, but may interact additionally with the open channel.