Inhibition of dihydropyridine-sensitive Ca++ channels by ethanol in undifferentiated and nerve growth factor-treated PC12 cells: interaction with the inactivated state.

The effects of acute exposure to ethanol on high-voltage activated calcium (Ca++) channels in undifferentiated (UND) pheochromocytoma (PC12) cells and in PC12 cells treated with nerve growth factor (NGF) were compared using the nystatin perforated-patch voltage-clamp technique. Both UND and NGF-treated cells expressed noninactivating, dihydropyridine-sensitive Ca++ current. Ethanol (10, 25 and 50 mM) reversibly inhibited noninactivating current in both UND and NGF-treated cells. However, the reduction of current was significantly greater in UND cells. NGF-treated cells expressed, in addition to dihydropyridine-sensitive current, omega-conotoxin GVIA-sensitive Ca++ current. The reduced ethanol inhibition in NGF-treated cells was not due to the contribution of these channels to the overall current since conotoxin did not alter the level of ethanol inhibition. In both cell types, ethanol had no significant effect after exposure of the cells to nifedipine, suggesting that these drugs act on the same dihydropyridine-sensitive Ca++ channel type. In both cell types, inhibition was greatest when voltage steps were made from depolarized holding potentials, suggesting that ethanol has a greater effect on the inactivated state than on the resting or activated state. Steady-state inactivation measurements indicated that ethanol caused a significant hyperpolarizing shift in the half-maximal inactivation potential for both cell types, supporting this proposition. The magnitude of this shift was correlated with the magnitude of ethanol inhibition of currents elicited from a holding potential of -40 mV. Thus, one mechanism by which ethanol inhibits Ca++ currents in PC12 cells is via a shift in steady-state inactivation An additional mechanism in UND cells must underlie the difference in ethanol sensitivity of Ca++ channels in the two cell types.