Kidney Week

Abstract: TH-PO0024

Proximal Tubule (PT) Bicarbonate Generation, Not Ammonium Generation, Is Necessary for Acid-Base Homeostasis

Session Information

• Fluid, Electrolyte, and Acid-Base Disorders: Basic Research
  October 22, 2026 | Location: Exhibit Hall A, Convention Center
  Abstract Time: 10:00 AM - 12:00 PM

Category: Fluid, Electrolytes, and Acid-Base Disorders

• 1101 Fluid, Electrolyte, and Acid-Base Disorders: Basic

Authors

• Lee, Hyun-Wook, University of Florida, Gainesville, Florida, United States

Hyun-Wook Lee, PhD

• Weiner, I. David, University of Florida, Gainesville, Florida, United States

I. David Weiner, MD, FASN

Background

Urine ammonium is considered a critical component of net acid excretion, but whether it contributes directly to acid excretion is untested. Phosphoenolpyruvate carboxykinase (PEPCK) is expressed exclusively in the PT, where it is downstream of glutamine-derived ammonium-generating steps, but upstream of critical bicarbonate-generating reactions. This study used PT PEPCK deletion (PT-PEPCK-KO) to differentiate the roles of PT ammonium and bicarbonate generation in acid-base homeostasis.

Methods

PT-PEPCK-KO mice were PEPCK^fl/fl, Pax8-Cre-positive; wild-type (WT) mice were PEPCK^fl/fl, Cre-negative. We studied mice either on a control diet or after one week of acid-loading. Acid-loading involved adding 0.1 M HCl (1 ml/gm) to their chow; this is 75% less than our usual acidosis protocol.

Results

Immunoblot analysis and immunohistochemistry confirmed PT PEPCK deletion. On the control diet, neither serum Na⁺ nor K⁺ differed significantly between WT and KO mice. However, serum bicarbonate was significantly lower in KO mice than in WT mice. Urinary ammonium was greater in KO than in WT mice (WT, 67±27; KO, 377±51 μmol/d, P<0.001), and urine pH was more acidic in KO mice than in WT mice (WT, 6.29±0.06; KO, 5.73±0.06, P<0.001). KO mice had increased PT expression of phosphate-dependent glutaminase (PDG), a key ammonia-generating enzyme, and decreased expression of glutamine synthetase (GS), a key ammonia-recycling enzyme. Acid-loading was fully compensated in WT mice, with no significant change in serum bicarbonate, but it significantly decreased serum bicarbonate in KO mice. Urine ammonium excretion increased to a greater extent in KO than in WT mice, and urine pH remained more acidic in KO than in WT mice. Acid-loading increased PDG and decreased GS expression to similar extents in both genotypes.

Conclusion

1) PT-PEPCK-KO caused metabolic acidosis under basal conditions and blocked the ability to maintain acid-base homeostasis in response to a minimal acid load despite increased ammonium generation and excretion. 2) Because PT-PEPCK-KO blocks glutamine-derived bicarbonate generation, we conclude that glutamine-derived bicarbonate generation, not ammonium generation and excretion, is critical to basal and acidosis-stimulated acid-base homeostasis. 3) In the absence of PEPCK deletion, urinary ammonium is only a marker of PT bicarbonate generation and not a direct method of acid excretion.

Funding

• NIDDK Support