Role of mTORC2 in Aldosterone-Independent Potassium Regulation: Resolution of the Aldosterone Paradox

Background: ENaC activity and potassium (K+) secretion in the DCT2 and early CNT (eCNT) are not regulated by aldosterone due to constitutive activation of MR by cortisol. Under low aldosterone conditions, ENaC and ROMK activity are higher in the DCT2 and eCNT than in the late CNT and CD. Acute high K+ (HK) loads stimulate ENaC and K+ secretion via mTORC2 independently of aldosterone. Prolonged HK conditions involve aldosterone-dependent activation of ENaC and ROMK for maximal K+ secretion. This study examined mTORC2’s role in K+ secretion across distal nephron segments, focusing on the aldosterone-independent DCT2/eCNT and its role in the aldosterone paradox. Methods: Rictor, a key mTORC2 component, was selectively knocked out (KO) in DCT2, CNT and CD using Calbindin as Cre-driver (CRKO) or in late CNT and CD using AQP2 as the Cre-driver (ARKO). Both wild-type (WT) and KO mice were subjected to a HK diet for 48 hours. Parameters assessed included urinary and blood electrolyte levels, plasma membrane expression and intracellular localization of renal transporters, and phosphorylation of mTORC2 targets Results: On a normal K+ diet, both CRKO and ARKO mice maintained Na+ and K+ balance with normal plasma [K+] levels, but CRKO mice had elevated plasma aldosterone. On a HK diet, CRKO mice showed lower K+ excretion, hyperkalemia, and elevated aldosterone. Apical αENaC was reduced in DCT2 and eCNT, while late CNT and CCD were mostly unaffected. Phosphorylation of mTORC2 targets like SGK1 and its target, Nedd4-2 was reduced in CRKO mice. In contrast, ARKO mice maintained normal Na+ and K+ levels, elevated aldosterone, and preserved ENaC apical localization in the late CNT and CCD. Conclusion: The data suggest that the DCT2 and eCNT are crucial for ENaC regulation supporting K+ secretion under HK conditions with mTORC2 playing a key role. This regulation is aldosterone-independent; so we propose the term “Aldosterone Independent Distal Nephron” (AIDN). High aldosterone action in the late CNT and CCD can partly compensate for mTORC2 deficiency to maintain ENaC activity but, ENaC activity in the more distal segments cannot fully compensate, disrupting K+ balance. Thus, local control of K+ secretion along the distal nephron ensures potassium homeostasis across various aldosterone levels and highlights the importance of AIDN in resolving the aldosterone paradox. Funding: NIDDK Support, Private Foundation Support