Influx of 22Na through acetylcholine receptor-associated Na channels: relationship between 22Na influx, 45Ca influx and secretion of catecholamines in cultured bovine adrenal medulla cells.

The effects of carbachol, veratridine and high K on the influx of 22Na were investigated in relation to the influx of 45Ca and the secretion of catecholamines in cultured bovine adrenal medulla cells, in which stimulation of nicotinic but not muscarinic acetylcholine receptor causes the secretory response. (1) Carbachol caused a rapid influx of 22Na, influx of 45Ca and secretion of catecholamines, all of which occurred within 1 min and leveled off thereafter. Influx of 45Ca and secretion of catecholamines caused by carbachol were not inhibited by tetrodotoxin, but were greatly reduced in Na-free medium. Nicotine evoked an influx of 22Na and it was antagonized by hexamethonium and d-tubocurarine but not by tetrodotoxin. Muscarine had no effect on 22Na influx. The concentration-response curve of carbachol for 22Na influx was quite similar to that for 45Ca influx. (2) Veratridine induced a sustained influx of 22Na, influx of 45Ca and secretion of catecholamines, all of which were antagonized by tetrodotoxin. Influx of 45Ca and secretion of catecholamines due to veratridine were not observed in Na-free medium. (3) High K caused an influx of 45Ca and secretion of catecholamines but did not cause an influx of 22Na. High K-induced influx of 45Ca and secretion of catecholamines were not inhibited by tetrodotoxin nor by Na removal. (4) Magnesium, an inhibitor of voltage-dependent Ca channels, inhibited the influx of 45Ca and secretion of catecholamines caused by carbachol, veratridine and high K. These results indicate that cultured bovine adrenal medulla cells have at least three distinct ion channels: (1) nicotinic acetylcholine receptor-associated Na channels which are not inhibited by tetrodotoxin, (2) voltage-dependent Na channels which are kept activated by veratridine and inhibited by tetrodotoxin and (3) voltage-dependent Ca channels. Influx of Ca through voltage-dependent Ca channels is the common ionic event for the secretion of catecholamines caused by either carbachol, veratridine or high K. It seems that the influx of Na through acetylcholine receptor-associated Na channels as well as voltage-dependent Na channels, activates voltage-dependent Ca channels which triggers the secretion of catecholamines.