Importance of medullary events in ammonium excretion: studies in acute respiratory and acute metabolic acidosis.

The renal medulla can play an important role in acid excretion by modulating both hydrogen ion secretion in the medullary collecting duct and the medullary PNH3. The purpose of these experiments was to characterize the intrarenal events associated with ammonium excretion in acute acidosis. Cortical events were monitored in two ways: first, the rates of glutamine extraction and ammoniagenesis were assessed by measuring arteriovenous differences and the rate of renal blood flow; second, the biochemical response of the ammoniagenesis pathway was examined by measuring glutamate and 2-oxoglutarate, key renal cortical metabolites in this pathway. There were no significant differences noted in any of these cortical parameters between acute respiratory and metabolic acidosis. Despite a comparable twofold rise in ammonium excretion in both cases, the urine pH, PNH3, and the urine minus blood PCO2 difference (U-B PCO2) were lower during acute hypercapnia. In these experiments, the urine PCO2 was 34 mmHg (1 mmHg = 133.322 Pa) lower than that of the blood during acute respiratory acidosis while the U-B PCO2 was 5 +/- 3 mmHg in acute metabolic acidosis. Thus there were significant differences in medullary events during these two conditions. Although the urine pH is critical in determining ammonium excretion in certain circumstances, these results suggest that regional variations in the medullary PNH3 can modify this relationship.