Abstract: TH-OR073

Potassium Acts through mTORC2 to Regulate SGK1 and ENaC Directly in Collecting Duct Cells

Session Information

Category: Fluid, Electrolytes, and Acid-Base

  • 703 Na+, K+, Cl- Basic

Authors

  • Saha, Bidisha, University of California San Francisco, San Francisco, California, United States
  • Allu, Prasanna K.r., University of California San Francisco, San Francisco, California, United States
  • Wu, Peng, New York Medical College , Valhalla, New York, United States
  • Wang, WenHui, New York Medical College , Valhalla, New York, United States
  • Pearce, David, University of California San Francisco, San Francisco, California, United States
Background

The control of K+ excretion is critical to the maintenance of blood [K+]. Aldosterone controls renal tubule K+ secretion substantially by increasing SGK1 expression, which in turn stimulates ENaC. In addition to expression, SGK1 must be activated by the kinase, mTORC2. The key physiological inputs that control this phosphorylation are poorly characterized. We postulated that [K+] itself might directly regulate SGK1 activity through effects on mTORC2-dependent phosphorylation.

Methods

mpkCCD cortical collecting duct (CCD) cells were grown on Transwell filters, and treated for 4 h with aldosterone to stimulate SGK1 expression and adapted to either 1 mM or 5 mM [K+]. At t = 0, basolateral [K+] was changed from 1 to 5 mM or from 5 mM to 1 mM, followed by a return to 5 mM in the presence or absence of either an SGK1 or mTOR inhibitor. Additional experiments were performed using patch clamp to detect ENaC currents in mice subjected to low sodium diet for 7 days, followed by acute change in bath [K+] from 5 mM to 1 mM.

Results

In mpkCCD cells, raising basolateral [K+] from 1 to 5 mM increased SGK1 phosphorylation approximately 3-fold, while lowering [K+] from 5 mM to 1 mM decreased SGK1 phosphorylation approximately 4.5-fold (p < 0.01). The stimulatory effects were dependent on mTORC2 in that they were inhibited by a global mTOR inhibitor (AZD 8055) but not by the mTORC1-specific inhibitor, rapamycin. Changing apical [K+] had no significant effect. Further, shifting basolateral [K+] from 5 mM to 1 mM markedly reduced ENaC-dependent Na+ current, and returning [K+] to 5 mM induced a rapid increase in Na+ current. This latter effect was blocked by SGK1 or mTOR inhibitor. Finally, in mice subjected to patch clamp, ENaC currents were significantly greater in the presence of 5 mM (330 +/-25 pA, N=5) than in the presence of 1 mM (250 +/- 20 pA, N=5) [K+] (p < 0.05).

Conclusion

Changes in extracellular [K+] rapidly modulate mTORC2-dependent SGK1 phosphorylation resulting in altered ENaC-mediated Na+ transport. In light of recent evidence that Na-Cl cotransporter (NCC) phosphorylation and activity are regulated in distal convoluted cells directly by extracellular fluid [K+], these data support a new model of coordinated regulation of Na+ transport between distal convoluted tubule and CCD, which is directly modulated by local renal [K+].

Funding

  • NIDDK Support