Calcium Dependency of Muscarinic and Nicotinic Agonist‐Induced ATP and Catecholamine Secretion from Porcine Adrenal Chromaffin Cells

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Abstract
The secretion of catecholamines and ATP induced by cholinergic agonists and its dependence on extracellular Ca2+ were studied in cultured porcine adrenal chromaffin cells. Both nicotine and methacholine (a selective muscarinic agonist) induced secretion and increases in cytosolic free Ca2+ concentration ([Ca2+]in), although the activation of nicotinic receptors produced responses that were larger than those produced by activation of muscarinic receptors. The secretion and the increase in [Ca2+]in evoked by nicotine were completely dependent on extracellular Ca2+ and were blocked by prior depolarization of the cells with high extracellular K+ levels. In addition, nicotine induced significant 45Ca2+ influx. In contrast, the secretion and the increase in [Ca2+]in evoked by methacholine were partially dependent on extracellular Ca2+; methacholine also induced 45Ca2+ influx. Prior depolarization of the cells with high extracellular K+ levels did not block methacholine‐induced secretion. In general, nicotinic responses were mediated by Ca2+ influx through voltage‐dependent pathways. In contrast, muscarinic responses were dependent on both Ca2+ influx through an unknown mechanism that could not be inactivated by high K+ concentration‐induced depolarization and presumably also intracellular Ca2+ mobilization.

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