1. An isolated perfused rat kidney preparation which responds to acidification of the perfusion medium with the production of an acid urine and increased ammonia production was used to study the metabolic regulation of ammonia production from glutamine.

2. An inhibitor of gluconeogenesis at phosphoenolpyruvate carboxykinase(GTP), mercaptopicolinate, completely prevented the increase in ammoniagenesis, without preventing acidification of the urine.

3. Acidification of the perfusion medium from pH 7·4 to 7·0 reduced the renal concentrations of malate and 2-oxoglutarate.

4. Malate concentration was restored by inhibition of phosphoenolpyruvate carboxykinase(GTP), but 2-oxoglutarate content remained low. This indicates that accelerated gluconeogenesis in acute acidosis cannot be the explanation for the fall in 2-oxoglutarate concentration.

5. The fall in 2-oxoglutarate content is taken to indicate an important fall in tissue pH or in the redox ratio (NAD+/NADH) or both during acute metabolic acidosis.

6. From these studies with lowered bicarbonate two separate stimuli to ammoniagenesis in acute metabolic acidosis are postulated: urinary trapping of ammonia and increased disposal of glutamine carbon atoms via the pathway of glucose synthesis.

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