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Abstract: TH-PO497

NBCe1A Regulates Citrate Excretion During Hypokalemia Through NaDC1 Expression-Dependent and Independent Mechanisms

Session Information

Category: Fluid and Electrolytes

  • 901 Fluid and Electrolytes: Basic

Authors

  • Webster, Kierstin L., University of Florida, Gainesville, Florida, United States
  • Lee, Hyun-Wook, University of Florida, Gainesville, Florida, United States
  • Harris, Autumn N., University of Florida, Gainesville, Florida, United States
  • Romero, Michael F., Mayo Clinic College of Medicine, Rochester, Minnesota, United States
  • Verlander, Jill W., University of Florida, Gainesville, Florida, United States
  • Weiner, I. David, University of Florida, Gainesville, Florida, United States
Background

Urinary citrate is an alkali equivalent and chelates calcium, thus is critical to acid-base homeostasis and prevention of calcium nephrolithiasis. Hypokalemia causes hypocitraturia; proximal tubule NaDC1 mediates citrate reabsorption and is thought to be the major regulator of citrate excretion. However, whether NaDC1 protein expression changes in hypokalemia and the signaling pathways involved are unknown.

Methods

Mice were fed either a K-control diet for 2 days or K-control diet for 2 days then K-free diet for 4 days. Urinary citrate was measured using 1H-NMR. We determined NaDC1 expression in the proximal convoluted tubule (PCT), proximal straight tubule in the medullary ray (PST-MR) and outer medulla (PST-OM) using quantitative immunohistochemistry. To determine the role of NBCe1-A in the signaling pathway regulating NaDC1 expression, we compared mice with NBCe1-A deletion (KO) with wild-type (WT) littermates.

Results

While on K-control diet, urinary citrate excretion did not differ between WT and NBCe1-A KO mice. However, KO mice had significantly less NaDC1 immunolabel than WT mice in the PCT and PST-MR. A K-free diet for 4 days decreased citrate excretion significantly in both genotypes, but the decrease was significantly blunted in KO mice compared to WT mice (WT, 95±3% decrease; KO, 83±6% decrease; P<0.05). In WT mice, K-free diet increased PCT and PST-MR NaDC1 immunolabel significantly compared to K-control. In contrast, in NBCe1-A KO mice NaDC1 expression did not differ significantly between K-control and K-free mice in any proximal tubule segment, PCT, PST-MR or PST-OM.

Conclusion

(1) K-free diet decreases citrate excretion by increasing NaDC1 expression in the PCT and PST-MR through a mechanism that involves signaling through the basolateral membrane protein NBCe1-A; (2) In NBCe1-A KO mice fed K-control diet, either alternative citrate transporters or NaDC1 regulation through a mechanism other than steady-state protein expression enables similar rates of citrate reabsorption despite less total NaDC1 expression than WT; and, (3) In the absence of NBCe1-A a K-free diet acts through NBCe1-A-independent signaling pathways to increase citrate reabsorption through a mechanism that does not involve altered NaDC1 protein expression.

Funding

  • NIDDK Support