MST3 Involvement in Na(+) and K(+) Homeostasis with Increasing Dietary Potassium Intake

K(+) loading inhibits NKCC2 (Na-K-Cl cotransporter) and NCC (Na-Cl cotransporter) in the early distal tubules, resulting in Na(+) delivery to the late distal convoluted tubules (DCTs). In the DCTs, Na(+) entry through ENaC (epithelial Na channel) drives K(+) secretion through ROMK (renal outer medullary potassium channel). WNK4 (with-no-lysine 4) regulates the NCC/NKCC2 through SAPK (Ste20-related proline-alanine-rich kinase)/OSR1 (oxidative stress responsive). K(+) loading increases intracellular Cl(−), which binds to the WNK4, thereby inhibiting autophosphorylation and downstream signals. Acute K(+) loading-deactivated NCC was not observed in Cl(−)-insensitive WNK4 mice, indicating that WNK4 was involved in K(+) loading-inhibited NCC activity. However, chronic K(+) loading deactivated NCC in Cl(−)-insensitive WNK4 mice, indicating that other mechanisms may be involved. We previously reported that mammalian Ste20-like protein kinase 3 (MST3/STK24) was expressed mainly in the medullary TAL (thick ascending tubule) and at lower levels in the DCTs. MST3(−/−) mice exhibited higher ENaC activity, causing hypernatremia and hypertension. To investigate MST3 function in maintaining Na(+)/K(+) homeostasis in kidneys, mice were fed diets containing various concentrations of Na(+) and K(+). The 2% KCl diets induced less MST3 expression in MST3(−/−) mice than that in wild-type (WT) mice. The MST3(−/−) mice had higher WNK4, NKCC2-S130 phosphorylation, and ENaC expression, resulting in lower urinary Na(+) and K(+) excretion than those of WT mice. Lower urinary Na(+) excretion was associated with elevated plasma [Na(+)] and hypertension. These results suggest that MST3 maintains Na(+)/K(+) homeostasis in response to K(+) loading by regulation of WNK4 expression and NKCC2 and ENaC activity.