Ca2+ influx stimulated phospholipase C activity in bovine adrenal chromaffin cells: responses to K+ depolarization and histamine

The role of Ca2+ influx in activating phospholipase C in bovine adrenal chr omaffin cells has been investigated. Phospholipase C activity in response t o K+ depolarization (56 mM) was blocked by the L-type Ca2+ channel antagoni st nifedipine and partially inhibited by the omega-conotoxins GVIA and MVII C. In contrast, phospholipase C activity in response to histamine receptor activation was unaffected by omega-conotoxin GVIA and partially inhibited b y omega-conotoxin MWC or nifedipine. This response was however markedly inh ibited by the non-selective Ca2+ channel antagonists La3+ or 1-[beta-[3-(4- Methoxyphenyl)propoxy]-4-methoyphenethyl]-H-imidazole (SKF-96365). Despite this Ca2+ dependence phospholipase C activity was not increased during peri ods of "capacitative" Ca2+ inflow generated by histamine-, caffeine- or tha psigargin-mediated depletion of internal Ca2+ stores. Thus, while Ca2+ infl ux in response to K+ depolarization or G-protein receptor activation can in crease phospholipase C activity in these cells, in the latter case it appea rs to be ineffective unless there is concurrent agonist occupation of the r eceptor. (C) 2000 Elsevier Science B.V. All rights reserved.