Uncoupling Proximal Tubule Ammoniagenesis From Bicarbonate Generation by PCK1 Deletion

Area of Science:

• Renal physiology
• Acid-base homeostasis
• Metabolic regulation

Background:

• Ammonium excretion is a long-recognized factor in maintaining acid-base balance.
• The precise role of ammonium excretion—whether direct acid excretion or a marker of bicarbonate generation—remains unclear.
• Proximal tubule (PT) glutamine metabolism is central to renal bicarbonate production.

Purpose of the Study:

• To determine if ammonium excretion directly contributes to net acid excretion.
• To investigate whether ammonium excretion serves as a marker for proximal tubule glutamine-derived bicarbonate generation.
• To elucidate the role of phosphoenolpyruvate carboxykinase (PEPCK) in PT bicarbonate production and ammonium handling.

Main Methods:

• Utilized mice with a proximal tubule-specific deletion of phosphoenolpyruvate carboxykinase (PT-PEPCK-KO).
• Analyzed ammonium excretion, urine acidification, and serum bicarbonate levels under control and acid-loading conditions.
• Assessed the expression of key enzymes involved in glutamine metabolism, including phosphate-dependent glutaminase and glutamine synthetase.

Main Results:

• PT-PEPCK-KO mice showed a fourfold increase in urine ammonia excretion and increased urine acidification, yet had lower serum bicarbonate levels.
• Acid-loading exacerbated these effects in KO mice, with serum bicarbonate decreasing significantly compared to wild-type (WT) mice.
• Phosphate-dependent glutaminase expression was upregulated, while glutamine synthetase expression was decreased in PT-PEPCK-KO mice.

Conclusions:

• PEPCK deletion in the proximal tubule blocks glutamine-derived bicarbonate generation but not ammonium generation and excretion.
• Ammonium generation and excretion do not significantly contribute to acid-base homeostasis.
• Ammonium excretion primarily functions as a marker of proximal tubule-derived bicarbonate generation.