Regulation of rat Na+-K+-ATPase activity by PKC is modulated by state of phosphorylation of Ser-943 by PKA

We have previously shown that the rat Na(+)-K(+)-ATPase alpha 1-isoform is phosphorylated at Ser-943 by protein kinase A (PKA) and at Ser-23 by protein kinase C (PKC), which in both cases results in inhibition of enzyme activity. We now present evidence that suggests that the phosphorylation of Ser-943 by PKA modulates the response of Na(+)-K(+)-ATPase to PKC. Rat Na(+)-K(+)-ATPase alpha 1 or a mutant in which Ser-943 was changed to Ala-943 was stably expressed in COS cells. The inhibition of enzyme activity measured in response to treatment with the phorbol ester, phorbol 12,13-dibutyrate (PDBu; 10(-6) M), was significantly reduced in the cells expressing the Ala-943 mutant compared with that observed in cells expressing wild-type enzyme. In contrast, for cells expressing Na(+)-K(+)-ATPase alpha 1 in which Ser-943 was mutated to Asp-943, the effect of PDBu was slightly enhanced. The PDBu-induced inhibition was not mediated by activation of the adenosine 3',5'-cyclic monophosphate/PKA system and was not achieved via direct phosphorylation of Ser-943. Sp-5,6-DCI-cBIMPS, a specific PKA activator, increased the phosphorylation of Ser-943, and this was associated with an enhanced response to PDBu. Thus the effect of PKC on rat Na(+)-K(+)-ATPase alpha 1 is determined not only by the activity of PKC but also by the state of phosphorylation of Ser-943.