NITRIC-OXIDE MODULATES AGONIST-EVOKED CA2-64 CELLS( RELEASE AND INFLUX RESPONSES IN PC12)

Nitric oxide (NO) is a signalling molecule involved in events crucial to neuronal cell function such as neurotransmitter release, gene trans cription, and neurotoxicity. In these, as well as in many other neuron al processes, a key role may be played by the increases of the intrace llular Ca2+ concentration ([Ca2+]i) occurring in response to activatio n of plasma membrane receptors coupled to phosphatidylinositol 4,5-bis phosphate hydrolysis. Such a [Ca2+](i) increases are sustained by rele ase of the cation from intracellular stores and stimulation of influx through specific Ca2+ channels. We have investigated the role of NO in modulating the two above Ca2+ processes occuring subsequently to musc arinic receptor activation in a selected clone (PC12-64) of PC12 cells , a neurosecretory/neuronal cell model. Analysis of [Ca2+](i) variatio ns in fura-2-loaded cells, exposed to different NO synthase inhibitors or NO donors, showed that Ca2+ release from intracellular stores was moderately inhibited and stimulated by these two groups of drugs, resp ectively, while Ca2+ influx through the channels directly coupled to m uscarinic receptors was found to be insensitive to NO action. In contr ast, Ca2+ influx activated by muscarinic receptor-induced store deplet ion (investigated also by Mn2+ quenching of the fura-2 signal) was inc reased by NO generation and inhibited by NO synthase blockade. Incubat ion of the cells with 8-bromo cGMP did not mimick the action of NO, su ggesting that the effect of the messenger on Ca2+ influx is exerted th rough a signalling pathway different from cGMP generation.