Stimulatory and inhibitory effects of a phorbol ester on chloride secretion by rat epididymal epithelium

The effects of a protein kinase C (PKC) activator, phorbol 12-myristate 13-acetate (PMA), on Cl- secretion by rat cauda epididymal epithelium were studied through use of the short-circuit current (Isc) technique. PMA alone could stimulate the Isc in a dose-dependent manner. The PMA-induced Isc was blocked by the Cl channel blocker, diphenylamine-2-carboxylate, but not by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid. PMA also exerted an inhibitory effect on the subsequent Ca(2+)-activated Isc. ATP or ionomycin-induced Isc was significantly reduced by treatment with PMA (5-10 min). Both stimulatory and inhibitory effects of PMA could be mimicked by a diacylglycerol analog, 1,2-dioctanoyl-sn-glycerol, but not an inactive analog of PMA, 4 alpha-phorbol 12,13-didecanote (4 alpha D). Down-regulation of protein PKC by prolonged treatment of epididymal cells with PMA (12 h) diminished both stimulatory and inhibitory effects of PMA on Isc. These results suggest that the dual effect of PMA on Isc was mediated by PKC. However, the PKC inhibitor, calphostin C, could block the inhibitory effect of PMA on ATP-induced Isc but not the stimulatory effect of PMA alone on Isc. The stimulatory effect of PMA was apparent only when PMA was applied to the apical aspect; in contrast, the inhibitory effect of PMA on ATP-induced Isc was readily seen with application of PMA to either side of the epithelium.(ABSTRACT TRUNCATED AT 250 WORDS)