Endothelin-induced atrial natriuretic peptide release from cultured neonatal cardiac myocytes: the role of extracellular calcium and protein kinase-C.

Regulation of atrial natriuretic peptide (ANP) secretion from neonatal rat myocytes cultured on microcarriers was studied using endothelin-1 (ET-1) as a secretagogue. Myocytes were cultured for 3 days on microcarriers, packed in a chromatography column, and perifused with Krebs-Henseleit bicarbonate buffer. ANP secretion was measured by RIA, and the cytosolic free calcium concentration (Ca2+f) was measured continuously during secretion by the fluorescent calcium indicator fura-2. In perifused atrial and ventricular cells, basal values for Ca2+f were 146 and 167 nM, and immunoreactive ANP (IR-ANP) secretion rates were 61 and 65 pg/min.mg protein, respectively. ET-1 at concentrations of 1, 10, and 100 nM caused a concentration-dependent increases in Ca2+f and IR-ANP secretion in atrial myocytes. The maximal increases in Ca2+f and IR-ANP secretion were 30% and 100%, respectively. Diltiazem (1 microM), an inhibitor of voltage-sensitive Ca2+ channels, inhibited Ca2+f increments, but had no effect on ET-induced IR-ANP secretion. Staurosporine (10 nM), a protein kinase-C inhibitor, augmented Ca2+f changes, but inhibited the sustained phase of ET-induced IR-ANP secretion (P less than 0.05). Diltiazem abolished the stimulatory effect of staurosporine on Ca2+f and its inhibitory effect on IR-ANP secretion. ET-1 caused increases in Ca2+f and IR-ANP secretion in ventricular myocytes similar to those in atrial myocytes. Peptides corresponding in size to pro-ANP and ANP-(1-28) were detected in the original cell culture medium and perifusion effluent, and ET-1 did not change their concentration ratio in the eluate. Lactate dehydrogenase was not detected in the effluents before or during ET infusion, showing that the increase in IR-ANP secretion was not due to cell damage. This study shows that ET stimulates atrial and ventricular ANP secretion. The results also suggest that sustained ET-induced atrial ANP secretion is dependent on protein kinase-C, but does not require the influx of extracellular calcium.