Nitric oxide modulation of agonist-evoked intracellular Ca2+ release in neurosecretory PC-12 cells: inhibition of phospholipase C activity via cyclic GMP-dependent protein kinase I.

Nitric oxide is a signaling molecule involved in events crucial to neuronal cell function, such as neurotransmitter release, gene transcription, and neurotoxicity, i.e., a number of processes in which a key role appears to be played by increases in intracellular Ca2+ concentration. In the neurosecretory/neuronal cell line PC-12, we have investigated the role of nitric oxide in the modulation of Ca2+ release from intracellular stores elicited by activation of three different receptors coupled to phosphatidyl-inositol-4,5-bisphosphate hydrolysis, i.e., the purinergic P2U, muscarinic M3, and bradykinin B2 receptors. The results obtained show that nitric oxide donors have an inhibitory effect on agonist-evoked Ca2+ release. This effect is not due to nitric oxide-induced modifications of Ca2+ storage, because the total releasable Ca2+ pool, measured as the radioactivity released by thapsigargin and ionomycin in cells loaded at equilibrium with 45Ca2+, was unchanged. In contrast, nitric oxide donors decreased agonist-evoked inositol-1,4,5-trisphosphate generation and total inositol phosphate accumulation. Similarly, nitric oxide inhibited total inositol phosphate accumulation stimulated by either aluminium fluoride or Ca2+. All of these effects were mimicked by the cGMP analogue 8-bromo-cGMP. When cells were incubated with nitric oxide synthase inhibitors, the results observed were opposite those produced by nitric oxide donors. All of the effects of nitric oxide were abolished when cells were treated with the cGMP-dependent protein kinase I inhibitor KT5823. Furthermore, KT5823 mimicked the effects of nitric oxide synthase inhibitors. We conclude that nitric oxide and Ca2+ signaling pathways are interconnected in PC-12 cells. Modulation of inositol phosphate generation and Ca2+ release by nitric oxide appears to be exerted primarily at the level of phospholipase C functioning and to be mediated by the activation of cGMP-dependent protein kinase I.