Cholinergic Stimulation of Inositol Phosphate Formation in Bovine Adrenal Chromaffin Cells: Distinct Nicotinic and Muscarinic Mechanisms

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Abstract
The ability of cholinergic agonists to activate phospholipase C in bovine adrenal chromaffin cells was examined by assaying the production of inositol phosphates in cells prelabeled with [3H]inositol. We found that both nicotinic and muscarinic agonists increased the accumulation of [3H]inositol phosphates (mainly inositol monophos‐phate) and that the effects mediated by the two types of receptors were independent of each other. The production of inositol phosphates by nicotinic stimulation required extracellular Ca2+ and was maximal at 0.2 mMCa2+. Increasing extracellular Ca2+ from 0.22 to 2.2 mM increased the sensitivity of inositol phosphates formation to stimulation by submaximal concentrations of 1,1‐dimethyl‐4‐phenyl‐piperazinium iodide (DMPP) but did not enhance the response to muscarine. Elevated K+ also stimulated Ca2+‐dependent [3H]inositol phosphate production, presumably by a non‐receptor‐mediated mechanism. The Ca2+ channel antagonists D600 and nifedipine inhibited the effects of DMPP and elevated K+ to a greater extent than that of muscarine. Ca2+ (0.3–10 μM) directly stimulated the release of inositol phosphates from digitonin‐permeabilized cells that had been prelabeled with [3H]inositol. Thus, cholinergic stimulation of bovine adrenal chromaffin cells results in the activation of phospholipase C by distinct muscarinic and nicotinic mechanisms. Nicotinic receptor stimulation and elevated K+ probably increased the accumulation of inositol phosphates through Ca2+ influx and a rise in cytosolic Ca2+. Because Ba2+ caused catechol‐amine secretion but did not enhance the formation of inositol phosphates, phospholipase C activation is not required for exocytosis. However, diglyceride and wyo‐inositol 1,4,5‐trisphosphate produced during cholinergic stimulation of chromaffin cells may modulate secretion and other cellular processes by activating protein kinase C and/or releasing Ca2+ from intracellular stores.

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