INHIBITORY EFFECT OF CALYCULIN-A, A SER THR PROTEIN PHOSPHATASE TYPE-I INHIBITOR, ON RENIN SECRETION/

We have recently shown that several putative selective inhibitors of C a2+-calmodulin-dependent myosin light chain kinase (MLCK), such as ML- 9 aphthalene-1-sulfonyl)1H-hexahydro-1,4-diazepine], reversibly stimul ate renin secretion [C. S. Park, S.-H. Chang, H. S. Lee, S.-H. Kim, J. W. Chang, and C. D. I-long. Am. J. Physiol. 271(Cell Physiol. 40): C2 42-C247, 1996]. We hypothesized that Ca2+ inhibits renin secretion, vi a phosphorylation of 20-kDa myosin light chain (MLC20), by activating MLCK. In the present studies, we have investigated the types of protei n phosphatase (PP) involved in the control of renin secretion through inhibition of MLC dephosphorylation using inhibitors of various types of serine/threonine-specific protein phosphatases. Cyclosporin A, a pu tative inhibitor of PP type 2 (calcineurin), was without effect. Calyc ulin A and okadaic acid, putative selective inhibitors of both PP type 1 (PP1) and type 2A (PP2A), significantly inhibited renin secretion u nder control conditions. Calyculin A had inhibitory effects at least 1 0-fold more potent than okadaic acid, suggesting that PP1, rather than PP2A, is involved in the control of renin secretion. Furthermore, cal yculin A blocked the reversal of renin secretion preinhibited by raise d intracellular Ca2+ concentrations in a concentration-dependent manne r. Calyculin A (10(-6) M) significantly inhibited renin secretion stim ulated by lowering intracellular Ca2+ concentrations and blocked the s timulatory effect of ML-9 on renin secretion. Taking all of these resu lts into consideration, we hypothesize that dephosphorylation of MLC20 by Ca2+-independent PP1 stimulates renin secretion, whereas phosphory lation of MLC20 by Ca2+-calmodulin-dependent MLCK inhibits it. This hy pothesized regulatory model of renin secretion predicts that the rate of renin secretion at a given time is determined by the ratio of phosp horylated to dephosphorylated MLC20, which is, in turn, determined by the dynamic balance between activity of MLCK and MLC phosphatase.