In rat pinealocytes the cyclic GMP response to NO is regulated by Ca2+ and protein kinase C.

There is ample evidence that beta-adrenergic stimulation of cyclic GMP formation is potentiated by alpha1-adrenergic mechanisms, the latter leading to elevation of intracellular Ca2+ concentration ([Ca2+]i) and protein kinase C (PKC) activation. Recent studies have shown that nitric oxide synthase (NOS) and nitric oxide (NO) are a component of the adrenoceptor-cyclic GMP signalling pathway. The aim of the present investigation was to study the roles of alpha1-adrenergic mechanisms, Ca2+ and PKC on NO-stimulated cyclic GMP formation. To this end suspension cultures of rat pinealocytes were treated with the NO donor sodium nitroprusside (SNP) in the presence of alpha1-adrenergic agonists, [Ca2+]i-elevating substances, PKC inhibitors, followed by measurement of cyclic GMP accumulation. It was found that alpha1-adrenergic stimulation did not affect NO-activated cyclic GMP synthesis. Therefore alpha1-mechanisms act prior to NO induction of cyclic GMP. Agents, which elevate [Ca2+]i depressed NO-induced cyclic GMP formation. Since literature data show that Ca2+ stimulates pineal NO formation it is apparent that Ca2+ has antagonistic effects in the pineal adrenoceptor-cyclic GMP signalling pathway. The inhibitory effect of Ca2+ was unchanged after inhibition of phosphodiesterases suggesting that it may interfere with cytosolic guanylyl cyclase activation. Inhibition of PKC, but not of other protein kinases, decreased NO-activated cyclic GMP formation. Therefore it appears that non-alpha1-adrenergic-regulated PKC possesses a regulatory rote in NO-induced cyclic GMP formation.