IN RAT PINEALOCYTES THE CYCLIC-GMP RESPONSE TO NO IS REGULATED BY CA2-KINASE-C( AND PROTEIN)

There is ample evidence that beta-adrenergic stimulation of cyclic GMP formation is potentiated by alpha(1)-adrenergic mechanisms, the latte r 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 alpha(1)-adrenergic mechanism s, Ca2+ and PKC on NO-stimulated cyclic GMP formation. To this end sus pension cultures of rat pinealocytes were treated with the NO donor so dium nitroprusside (SNP) in the presence of alpha(1)-adrenergic agonis ts, [Ca2+](i)-elevating substances, PKC inhibitors, followed by measur ement of cyclic GMP accumulation. It was found that alpha(1)-adrenergi c stimulation did not affect NO-activated cyclic GMP synthesis. Theref ore alpha(1)-mechanisms act prior to NO induction of cyclic GMP. Agent s, 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 adrenoce ptor-cyclic GMP signalling pathway. The inhibitory effect of Ca2+ was unchanged after inhibition of phosphodiesterases suggesting that it ma y interfere with cytosolic guanylyl cyclase activation. Inhibition of PKC, but not of other protein kinases, decreased NO-activated cyclic G MP formation. Therefore it appears that non-alpha(1)-adrenergic-regula ted PKC possesses a regulatory role in NO-induced cyclic GMP formation .