Abstract

Secretion induced by nicotinic agonists in adrenal chromaffin cells depends on membrane depolarization produced by the opening of nicotinic receptor channels. It is generally believed that membrane depolarization activates voltage-gated Na+ channels, leading to the generation of action potentials and the subsequent activation of voltage-gated Ca++ channels. However, our results indicate that, in cultured porcine chromaffin cells, Na+ channels and action potentials play little role in nicotine-induced secretion. Although removal of extracellular Na+ blocked secretion produced by nicotine, tetrodotoxin, which abolished voltage-activated Na+ currents, had no effect on nicotine-induced secretion, even at low nicotine concentrations. The blocking effect of Na+ removal on nicotine-induced secretion could be reversed by adding excess extracellular Ca++ (20 mM), a reversal which was inhibited by the dihydropyridine Ca++ channel blocker, nimodipine (2 microM). Nimodipine also blocked nicotine-induced secretion under normal ionic conditions, but had little effect on nicotine-induced depolarization. When measured using a perforated patch (nystatin), current clamp technique, nicotine produced a rapid and sustained depolarization which included an initial volley of 1 to 15 action potentials. In contrast, when measured using a standard whole-cell, current clamp configuration, nicotine produced a slower depolarization and numerous action potentials. These results suggest that voltage-gated Ca++ channels in porcine chromaffin cells are activated directly by persistent depolarization produced by Na+ entry through the nicotinic receptor channel under normal ionic conditions, and by Ca++ entry through the nicotinic receptor channel in the absence of Na+, but the presence of high extracellular Ca++.