In rat pinealocytes the cyclic GMP response to NO is regulated by Ca 2+ and protein kinase C

Abstract

There is ample evidence that β-adrenergic stimulation of cyclic GMP formation is potentiated by α 1-adrenergic mechanisms, the latter leading to elevation of intracellular Ca 2+ concentration ([Ca 2+] 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 α 1-adrenergic mechanisms, Ca 2+ 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 α 1-adrenergic agonists, [Ca 2+] i-elevating substances, PKC inhibitors, followed by measurement of cyclic GMP accumulation. It was found that α 1-adrenergic stimulation did not affect NO-activated cyclic GMP synthesis. Therefore α 1-mechanisms act prior to NO induction of cyclic GMP. Agents, which elevate [Ca 2+] i depressed NO-induced cyclic GMP formation. Since literature data show that Ca 2+ stimulates pineal NO formation it is apparent that Ca 2+ has antagonistic effects in the pineal adrenoceptor-cyclic GMP signalling pathway. The inhibitory effect of Ca 2+ 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, decreases NO-activated cyclic GMP formation. Therefore it appears that non- α 1-adrenergic-regulated PKC possesses a regulatory role in NO-induced cyclic GMP formation.