Uncoupling proximal tubule ammoniagenesis from bicarbonate generation by PCK1 deletion

Ammonium excretion has been recognized for more than 100 yr 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-knockout (PT-PEPCK-KO) to address this question. Although on a control diet, PT-PEPCK-KO mice exhibited an approximately four times 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 knockout (KO) mice, but to a greater extent in KO mice. Despite the increased ammonium excretion, serum bicarbonate decreased in KO mice compared with mice on a control diet, whereas WT mice exhibited no significant change. Phosphate-dependent glutaminase expression was greater in KO mice than 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. As 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.NEW & NOTEWORTHY These studies show that renal ammonium generation and excretion are not a direct method of acid-base homeostasis. Instead, they show that urinary ammonium excretion functions in acid-base homeostasis as a marker of proximal tubule new bicarbonate generation.