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Pharmacological dissection of receptor-associated and voltage-sensitive ionic channels involved in catecholamine
Abstract:
The experiments were designed to quantify pharmacologically the degree of participation of channels associated with the nicotinic cholinoceptor compared with voltage-sensitive channels during the evoked release of [3H]noradrenaline from prelabelled 3-7-day old cultured bovine adrenal chromaffin cells. To achieve this purpose we studied (a) the release of [3H]noradrenaline evoked by secretagogues known to trigger the secretory response through activation of receptor-associated channels (acetylcholine, nicotine), voltage-sensitive Na+ (veratridine) and Ca2+ (high [K+] ) channels or direct, channel-independent promotion of Ca2+ entry (ionomycin); and (b) the selective blockade of some of those responses using ionic manipulations (Na+ deprivation, high Mg2+) or drugs known to block the activity of receptor-operated channels (imipramine, cocaine), voltage-dependent Na+ (tetrodotoxin) or Ca2+ (nitrendipine) channels. Inhibition by nitrendipine, a potent Ca2+ antagonist, of the secretory responses to both nicotine and high [K+] indicates a preferential Ca2+ entry through voltage-sensitive channels during the secretory process. Blockade by cocaine and imipramine of the release of [3H]noradrenaline evoked by acetylcholine and nicotine, without alteration of the responses to high [K+], veratridine or ionomycin, speaks in favor of a selective inactivation of the nicotinic receptor-associated channel. Since Na+ deprivation abolished [3H]noradrenaline release produced by nicotine, it seems that Na+ entry through the receptor-linked ionophore might be a primary event in the initiation of the secretory process; the fact that tetrodotoxin did not affect the release favors this view. However, veratridine induced a tetrodotoxin-sensitive secretory response, suggesting the presence of voltage-sensitive Na+ channels which might physiologically be used to propagate action potentials through gap junctions between adjacent chromaffin cells, only in the intact gland.(ABSTRACT TRUNCATED AT 250 WORDS)