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 i s 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 phosph orylation 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 chang ed to Ala-943 was stably expressed in COS cells. The inhibition of enz yme activity measured in response to treatment with the phorbol ester, phorbol 12,l3-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 expressi ng 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 monophosphat e/PKA system and was not achieved via direct phosphorylation of Ser-94 3. Sp-5,6-DCl-cBIMPS, a specific PKA activator, increased the phosphor ylation 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 deter mined not only by the activity of PKC but also by the state of phospho rylation of Ser-943.