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Abstract: TH-OR005

Mg2+ Restriction Downregulates NCC Through NEDD4-2 and Prevents Its Activation by Hypokalemia

Session Information

Category: Fluid and Electrolytes

  • 901 Fluid and Electrolytes: Basic

Authors

  • McCormick, James A., Oregon Health and Science University, Portland, Oregon, United States
  • Ferdaus, Mohammed Zubaerul, Oregon Health and Science University, Portland, Oregon, United States
  • Blatt, Philip James, Oregon Health and Science University, Portland, Oregon, United States
  • Nelson, Jonathan W., Oregon Health and Science University, Portland, Oregon, United States
  • Terker, Andrew, Vanderbilt University, Nashvile, Tennessee, United States
  • Staub, Olivier, University of Lausanne, Lausanne, Switzerland
  • Lin, Daohong, New York Medical College, Valhalla, New York, United States
Background

Hypomagnesemia is associated with lower kidney function and life-threatening complications, and sustains hypokalemia. The distal convoluted tubule (DCT) determines final urinary Mg2+ excretion, and via activity of the Na+-Cl- cotransporter (NCC) plays a key role in K+ homeostasis by metering Na+ delivery to distal segments. We previously showed that short-term (3 days) or long-term (14 days) Mg2+ restriction lowered abundances of total NCC (tNCC) and the active phosphorylated NCC (pNCC), but this did not involve the NCC-activating WNK-SPAK pathway.

Methods

We set out to further explore the mechanisms involved and determine interactions with K+ restriction, a strong activator of NCC, by performing dietary manipulations in mice then Western blotting.

Results

We confirmed a previous report that long-term Mg2+ restriction does not alter NCC mRNA abundance, and found the same with short-term Mg2+ restriction. The E3 ubiquitin-protein ligase neural precursor cell expressed developmentally downregulated gene 4-2 (NEDD4-2) is known to target NCC for proteasomal degradation. We found that short-term Mg2+ restriction did not lower tNCC abundance in inducible nephron-specific NEDD4-2 knockout mice. We next examined interactions with K+ restriction. tNCC and pNCC abundances were similar after short- or long-term Mg2+ or combined Mg2+-K+ restriction, but were dramatically lower compared with low K+ diet, suggesting that Mg2+ restriction overrides the effects of K+ restriction on NCC. After combined Mg2+-K+ restriction, adding back K+ alone to the diet had no effect on tNCC abundance, but adding back Mg2+ either at the same time or after K+ replenishment increased tNCC abundance. NEDD4-2 mediates degradation of the epithelial sodium channel (ENaC) during dietary K+ restriction so we next examined the effect of Mg2+ restriction on ENaC by performing amiloride response tests. Compared with normal diet the natriuretic effect of amiloride was strongly blunted after K+ restriction but not after Mg2+ restriction.

Conclusion

Together, these data suggest that NEDD4-2 mediates proteasomal degradation of NCC during Mg2+ restriction, Mg2+ restriction exerts differential effects on NCC and ENaC, and sustained NCC downregulation may enhance distal Na+ delivery during states of hypomagnesemia, maintaining hypokalemia.

Funding

  • NIDDK Support