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

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 [ 3 H]inositol. We found that both nicotinic and muscarinic agonists increased the accumulation of [ 3 H]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 Ca 2+ and was maximal at 0.2 m M Ca 2+ . Increasing extracellular Ca 2+ from 0.22 to 2.2 m M 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 Ca 2+ -dependent [ 3 H]inositol phosphate production, presumably by a non-receptor-mediated mechanism. The Ca 2+ channel antagonists D600 and nifedipine inhibited the effects of DMPP and elevated K + to a greater extent than that of muscarine. Ca 2+ (0.3–10 Μ M ) directly stimulated the release of inositol phosphates from digitonin-permeabilized cells that had been prelabeled with [ 3 H]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 Ca 2+ influx and a rise in cytosolic Ca 2+ . Because Ba 2+ 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 Ca 2+ from intracellular stores.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65738/1/j.1471-4159.1987.tb01037.x.pd