PKA-mediated phosphorylation and inhibition of Na+-K+-ATPase in response to beta-adrenergic hormone

The activity of Na+-K+-ATPase can be regulated by hormones that activate adenosine 3',5'-cyclic monophosphate-dependent protein kinase (PKA). Here, using a site-directed phosphorylation state-specific antibody, we show that hormonal regulation of Na+-K+-ATPase can occur via phosphorylation of Ser-943 on its alpha-subunit. cDNAs coding for wild-type rat Na+-K+-ATPase and Na+-K+-ATPase in which the PKA phosphorylation site Ser-943 was mutated to Ala were stably and transiently transfected into COS cells. In COS cells expressing wild-type Na+-K+-ATPase the beta-adrenergic agonist isoproterenol (1 mu M) significantly increased the level of phosphorylation of the alpha-subunit. Phosphorylation was accompanied by a significant inhibition of the enzyme activity, as reflected by a decrease in ATP hydrolysis and Rb-86(+) transport. The effect of isoproterenol was reproduced by the PKA activator forskolin used in combination with the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine and was abolished by the specific PKA inhibitor H-89. Okadaic acid, an inhibitor of protein phosphatases 1 and 2A, enhanced phosphorylation and inhibition of Na+-K+-ATPase induced by isoproterenol. The changes in activity of Na+-K+-ATPase linearly correlated with the extent of the alpha-subunit of Na+-K+-ATPase being phosphorylated. When Ser-943 was replaced by alanine, stimulation of the phosphorylation and inhibition of the activity of Na+-K+-ATPase induced by isoproterenol, alone or in combination with okadaic acid, were not observed. These results indicate that, in intact cells, modulation of the activity of Na+-K+-ATPase can be achieved by regulation of the state of phosphorylation of Ser-943. Moreover, they provide a biochemical mechanism by which beta-adrenergic agonists can regulate Na+-K+-ATPase activity.