Uncoupling Proximal Tubule Ammoniagenesis From Bicarbonate Generation by PCK1 Deletion.

Ammonium excretion has been recognized for more than 100 years as an integral component of renal maintenance of acid-base homeostasis. However, whether ammonium excretion directly contributes to net acid excretion or whether it is a marker of proximal tubule glutamine-derived bicarbonate generation has not been directly determined. The current studies use mice with a proximal tubule (PT)-specific deletion of phosphoenolpyruvate carboxykinase (PT-PEPCK-KO) to address this question. While on a control diet, PT-PEPCK-KO mice exhibited a ~4x increase in urine ammonia excretion, in association with increased urine acidification, yet had significantly lower serum bicarbonate levels. Acid-loading, with a protocol providing only 25% of our standard acid-load, increased ammonium excretion in both wild-type (WT) and knock-out (KO) mice, but to a greater extent in KO mice. Despite the increased ammonium excretion, serum bicarbonate decreased in KO mice compared to mice on a control diet, whereas WT mice exhibited no significant change. Phosphate-dependent glutaminase expression was greater in KO mice than in WT mice, both on a control diet and after acid-loading. Expression of glutamine synthetase, a PT ammonium-recycling enzyme, was decreased in PT-PEPCK-KO mice on both control and acid-loading diets. Because PEPCK deletion blocks glutamine-derived bicarbonate generation, but not ammonium generation and excretion, we conclude that ammonium generation and excretion do not significantly contribute to acid-base homeostasis, and instead serve primarily as a marker of PT-derived bicarbonate generation.