Stimulation of Xenopus oocyte Na+,K+ATPase by the serum and glucocorticoid-dependent kinase sgk1

The serum and glucocorticoid-dependent kinase-1 (sgk1) is expressed in a wide variety of tissues including renal epithelial cells. As it is up-regulated by aldosterone, it is considered to participate in the regulation of renal Na+ reabsorption. Indeed, co-expression of sgk1 with the renal epithelial Na+ channel (ENaC) augments Na+ channel activity. The aim of the present study was to examine possible effects of sgk1 on Na+/K+-ATPase activity. To this end dual-electrode voltage-clamp experiments were performed in Xenopus oocytes expressing the active kinase (S422D)sgk1 or the inactive mutant (K127N)sgk1. Na+/K+-ATPase activity was estimated from the hyperpolarization (DeltaV(m))and the outwardly-directed current (I-P) created by addition of extracellular K+ in the presence of K+ channel blocker Ba2+. Both DeltaV(m) and I-P were significantly larger in oocytes expressing (S422D)sgk1 than in those expressing (K127N)sgk1 or having been injected with water. I-P was fully inhibited by ouabain. Ion-selective microelectrodes showed that the stimulation of pump current was not the result of altered cytosolic Na+ activity or pH. The present results thus point to an additional action of sgk1 that may participate in the regulation of renal tubular Na+ transport. Moreover, sgk1 may be involved in the regulation of Na+/K+-ATPase in extrarenal tissues.