Inhibition and down-regulation of protein kinase C in cultured atrial myocytes: effects on distribution of specific granules and secretion of atrial natriuretic peptide.

Primary cultures of neonatal rat atrial myocytes were subjected to down-regulation of protein kinase C (PKC) and to inhibition of PKC activity. The effects on secretion of atrial natriuretic peptide (ANP), and the intracellular distribution of ANP-containing specific granules and their association to microtubules, were investigated. Treating the cultures with inhibitors of PKC, staurosporine or H7, a translocation of ANP-containing specific granules from the perinuclear sarcoplasm to the periphery of the myocytes was observed, and furthermore, secretion of ANP was significantly decreased. The microtubule network were not structurally affected by the PKC inhibitors. Down-regulation of PKC by 12-O-tetradecanoylphorbol 13-acetate (TPA) for 12 h was not followed by any alteration of localization of specific granules, and the amount of secreted ANP was still considerable. Treating the down-regulated cultures with staurosporine, secretion of ANP was still significantly reduced. The present results suggest that the decreased ANP secretion and the translocation of ANP-containing specific granules in the atrial myocytes following treatment with staurosporine or H7, is mediated through mechanisms not, or only partly, requiring PKC.