Ca2+-independent form of protein kinase C may regulate Na+ transport across frog skin

Summary Activators of protein kinase C (PKC) stimulate Na^– transport (J _Na) across frog skin. We have examined the effect of Ca²⁺ on PKC stimulation ofJ _Na. Both the phorbol ester 12-O-tetradecanoylglycerol (DiC₈) were used as PKC activators. Blocking Ca²⁺ entry into the cytosol (either from external or internal stores) reduced the subsequent natriferic effect of the PKC activators. This negative interaction did not simply reflect saturation of activation of the apical Na⁺ channels, since the stimulations produced by blocking Ca²⁺ entry and adding cyclic AMP were simply additive.The Ca²⁺ dependence of the natriferic effect could have reflected either a direct action of cytosolic Ca²⁺ on PKC or an indirect action on the final receptor site (the Na⁺ channel). To distinguish between these possibilities, the TPA- and phospholipid-dependent kinase activity of broken-cell preparations was assayed. The kinase activity was not stimulated by physiological levels of Ca²⁺, and in fact was inhibited at millimolar concentrations of Ca²⁺.We conclude that the effects of Ca²⁺ on the natriferic response to PKC activators are indirect. Reducing cytosolic uptake of Ca²⁺ may have stimulated Na⁺ transport by a chemical modification of the apical channels observed in other tight epithelia. The usual stimulation of Na⁺ transport produced by PKC activators in frog skin may reflect the operation of a nonconventional form of PKC. This enzyme is Ca²⁺ independent and seems related to thenPKC or PKC observed in other systems.