Effects of TPA on short-circuit current across frog skin.

TPA (12-O-tetradecanoylphorbol-13-acetate) is an effective tumor promoter that affects a variety of ion transport processes. To examine the relationship between effects on transport and growth and differentiation, the authors have been studying the actions of TPA on frog skin, a particularly well-characterized epithelium. They have reported that high concentrations of TPA stimulate base-line short-circuit current (I/sub SC/) and inhibit the subsequent natriferic action of vasopressin. The current study of 89 preparations extends those findings. The K/sub m/ of the stimulatory effect of TPA is approx. 3 nM; this high affinity indicates that the transport phenomenon does not simply reflect a nonspecific interaction of phorbol ester with the plasma membranes. TPA acts largely or entirely at the mucosal surface of both split and whole skins; thus the sidedness of the effect does not arise from adsorption onto the underlying connective tissue when TPA is applied to the serosal surface of whole skin. Amiloride, an inhibitor of apical Na entry, abolishes I/sub SC/ across frog skins pretreated with TPA. The phorbol ester also increases I/sub SC/ across split skins, preparations which do not produce net Cl transport. The present results indicate that frog skin is highly responsive to TPA at concentrations knownmore » to activate protein kinase C in broken-cell preparations. The actions on I/sub SC/ appear to reflect changes in transepithelial Na transport modulated at the apical membranes. The full biochemical events triggered by TPA remain to be clarified; in part, TPA's actions may be mediated by leukotrienes produced by activation of the lipoxygenase pathway of arachidonic acid metabolism.« less