Na⁺-K⁺-ATPase in frog esophagus mucociliary cell membranes: Inhibition by protein kinase C activation

We examined protein kinase C (PKC)-dependent regulation of Na⁺-K⁺- ATPase in frog mucociliary cells. Activation of PKC by 12-O- tetradecanoylphorbol-13-acetate (TPA) or 1,2-dioctanoyl-sn-glycerol (diC8) either in intact cells or isolated membranes resulted in a specific inhibition of Na⁺-K⁺ATPase activity by ~25-45%. The inhibitory effects in membranes exhibited time dependence and dose dependence [half-maximal inhibition concentration (IC₅₀) = 0.5 ± 0.1 nM and 2.4 ± 0.2 μM, respectively, for TPA and diC8] and were not influenced by Ca²⁺. Analysis of the ouabain inhibition pattern revealed the presence of two Na⁺-K⁺- ATPase isoforms with IC₅₀ values for cardiac glycoside of 2.6 ± 0.8 nM and 409 ± 65 nM, respectively. Most importantly, the isoform possessing a higher affinity for ouabain was almost completely inhibited by TPA, whereas its counterpart was hardly sensitive to the PKC activator. The results suggest that, in frog mucociliary cells, PKC regulates Na⁺-K⁺-ATPase and that this action is related to the specific Na⁺-K⁺-ATPase isoform.