title: Catecholamine secretion induced by nicotine is due to Ca++ channel but not Na+ channel activation in porcine adrenal chromaffin cells.
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++.
authors.affiliation: Department of Physiology, University of Wisconsin Medical School, Madison, USA.
authors.forename: Q
authors.initials: Q
authors.lastname: Li
authors.forename: E J
authors.initials: EJ
authors.lastname: Forsberg
mesh_terms: D000818:Animals; D015220:Calcium Channels / Q000187:drug effects*; D002395:Catecholamines / Q000378:metabolism*; D002838:Chromaffin System / Q000187:drug effects*; D008564:Membrane Potentials / Q000187:drug effects; D009538:Nicotine / Q000494:pharmacology*; D015222:Sodium Channels / Q000187:drug effects*; D013552:Swine
chemical_list: D015220:Calcium Channels; D002395:Catecholamines; D015222:Sodium Channels; D009538:Nicotine