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-myristat e 13-acetate (PMA), on Cl- secretion by rat cauda epididymal epitheliu m were studied through use of the short-circuit current (I-SC) techniq ue. PMA alone could stimulate the I-SC in a dose-dependent manner. The PMA-induced I-SC 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 Ca2+-ac tivated I-SC. ATP or ionomycin-induced I-SC was significantly reduced by treatment with PMA (5-10 min). Both stimulatory and inhibitory effe cts of PMA could be mimicked by a diacylglycerol analog, 1,2-dioctanoy l-sn-glycerol, but not an inactive analog of PMA, 4 alpha-phorbol 12,1 3-didecanote (4 alpha D). Down-regulation of protein PKC by prolonged treatment df epididymal cells with PMA (12 h) diminished both stimulat ory and inhibitory effects of PMA on I-SC. These results suggest that the dual effect of PMA on I-SC was mediated by PKC. However, the PKC i nhibitor, calphostin C, could block the inhibitory effect of PMA on AT P-induced I-SC but not the stimulatory effect of PMA alone on I-SC. Th e stimulatory effect of PMA was apparent only when PMA was applied to the apical aspect; in contrast, the inhibitory effect of PMA on ATP-in duced I-SC was readily seen with application of PMA to either side of the epithelium. Therefore, while results from different experimental m anipulations all suggest that the inhibitory effect of PMA on ATP-indu ced I-SC was mediated by PKC, the involvement of PKC in mediating thei nvolvement of PKC in mediating the stimulatory effect of PMA was not e ntirely substantiated. It appears that at least part of the stimulator y effect of PMA on I-SC was not mediated by PKC. The dual effect of PM A on I-SC illustrates the complexity of PKC action in epididymal secre tion and suggests an important role of PKC in the fine-tuning of the e pididymal microenvironment.