Does the early aldosterone-induced SGK1 play a role in early Kaliuresis?

Urinary K potassium excretion rapidly increases after a potassium-rich meal. The early aldosterone-induced sgk1 gene (encoding serum and glucocorticoid-induced kinase 1), activates potassium clearance, but the role of this kinase in the early activation of K secretion has not been clearly defined. Here, we challenged inducible renal-tubule-specific Sgk1  knockout mice with an acute high-potassium load (HK:5%K ) and compared the physiological and molecular responses to control mice. We observe that urinary excretion after a K load over the first 3 h is not dependent on SGK1 but is coincident with the rapid dephosphorylation of the Na ,Cl -cotransporter (NCC) to increase distal salt delivery. Molecular analyses indicate that whereas SGK1-mediated phosphorylation of the ubiquitin-protein ligase NEDD4-2 begins to increase by 3h, SGK1-dependent proteolytic activation of ENaC only becomes detectable after 6 h of HK intake. Consistent with SGK1-dependent ENaC activation via inhibition of NEDD4-2-mediated ubiquitylation, Sgk1  mice are unable to efficiently inhibit NEDD4-2 or increase ENaC cleavage after 6 h of HK. Nevertheless, no defect in acute K balance was detected in the mutant mice after 6 h of HK. Moreover, we found that Sgk1  mice reduce NCC phosphorylation and NCC-mediated salt absorption to a greater extent than control mice after a K load, promoting increased amiloride-sensitive Na -reabsorption via ENaC to maintain adequate kaliuresis. Together, these data indicate that: (a) during the early 3 h of HK intake, K excretion is SGK1-independent even under an extreme K challenge, (b) shortly after, SGK1 inhibits NEDD4-2 and activates ENaC to stimulate K -secretion, (c) SGK1-dependent phosphorylation of NCC occurs, acting more likely as a brake pedal to prevent excessive K loss.