CELLULAR MECHANISMS FOR SYNTHESIS AND SECRETION OF ATRIAL-NATRIURETIC-PEPTIDE AND BRAIN NATRIURETIC PEPTIDE IN CULTURED RAT ATRIAL CELLS

To investigate the cellular mechanism for the synthesis and secretion of atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP), we examined the effects of vasoactive agents on the secretion rates and gene expression of ANP and BNP in cultured rat atrial cells. Endothelin (10(-7) M, +61%), 12-O-tetradecanoylphorbol 13-acetate (TPA, 10(-6) M, +62%), the calcium ionophore A23187 (10(-6) M, +95%), and Bay K 8644 (10(-6) M, +34%) (p<0.05 each) all increased the secretion of ANP into the culture media in a dose-dependent fashion. On the other hand, endothelin (10(-7) M, +57%) and TPA (10(-6) M, +55%) (p<0.01 each) increased the secretion of BNP in a dose-dependent manner, whereas A23187 (10(-6) M, -45%, p<0.001) suppressed the secretion of BNP in a dose-dependent manner, and Bay K 8644 caused no significant effects on BNP secretion. The molecular forms of intracellular ANP were exclusively gamma-ANP, whereas those of BNP were gamma-BNP and its carboxy terminal 45-amino-acid peptide, BNP-45. The ratio of media to cell contents was much higher in BNP than in ANP. Northern blot analysis revealed that both ANP mRNA and BNP mRNA levels were significantly increased by 10(-7) M endothelin (ANP mRNA, +52%; BNP mRNA, +36%; p<0.05 each) and 5 x 10(-5) M 1-oleoyl-2-acetylglycerol (ANP mRNA, +296%; BNP mRNA, +133%; p<0.01 each) but not by 10(-6) M A23187. Thus, the secretion of ANP is stimulated by both the elevation of [Ca2+]i and the activation of protein kinase C, whereas its synthesis is increased mainly by the activation of protein kinase C. The synthesis and secretion of BNP are augmented by the activation of protein kinase C rather than the elevation of [Ca2+]i. Furthermore, the processing and secretion of ANP and BNP may be regulated in different manners.