Phorbol myristate acetate inhibits acidification by turtle urinary bladder

Abstract

The effects of phorbol myristate acetate (PMA) on acid secretion by the turtle urinary bladder were examined to evaluate the importance of protein phosphorylation in modulating the distal acidification system. In HCO3-free PO4 buffer 0.2 mM mucosal PMA inhibited reverse short-circuit current (RSCC) by 42%. The inhibition of RSCC was dose dependent, and RSCC approached zero at high concentrations of PMA. PMA also inhibited acid secretion measured titrimetrically but had no effect on RSCC from bladders in which proton secretion was selectively inhibited by an adverse pH gradient or serosal 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid. The inhibition by PMA was duplicated by 1-oleoyl-2-acetyl-rac-glycerol, but not by an inactive phorbol ester. The PMA inhibition was much more potent with mucosal compared with serosal application. The PMA inhibition caused a significant 0.15 pH unit reduction of carbonic anhydrase (CA) cell cytoplasmic pH and a change in the CA cell morphology. Unlike the inhibition of proton transport induced by acetazolamide, the PMA inhibition was neither reversed nor prevented by sodium azide. We conclude that PMA decreases basal acid secretion and blocks the stimulatory effects of CO2 by an azide-insensitive mechanism distinct from that of acetazolamide. In view of recent findings that PMA stimulates HCO3 secretion by the turtle bladder, these results suggest that kinase C-mediated protein phosphorylation may be a central event in the transition from the secretion of acid to the secretion of base.