Effects of fluorocitrate on renal ammoniagenesis and glutamine metabolism in the intact dog kidney

Insights into the biochemical mechanism of maleic acid-induced Fanconi syndrome

Maleic acid administration is known to produce the Fanconi syndrome, although the biochemical mechanism is incompletely understood. In this study the effect of a single injection of maleic acid (50 mg/kg body wt, i.v.) on the rat renal ATPases was examined. Maleic acid rapidly caused bicarbonaturia, natriuresis, and kaliuresis. When nephron segments were microdissected, there was an 81 +/- 2% reduction in proximal convoluted tubule (PCT) Na-K-ATPase activity (P < 0.005) and a 48 +/- 4% reduction in PCT H-ATPase activity (P < 0.01). Enzyme activity (Na-K-ATPase, H-ATPase, H-K-ATPase) in the medullary thick ascending limb of Henle's loop and distal nephron segments was normal. In vitro, maleic acid (1 and 10 mM) inhibited Na-K-ATPase in PCT, but it had no effect on H-ATPase in PCT. Prior phosphate infusion to maleic acid-treated rats attenuated urinary bicarbonate wastage by 50% (P < 0.05); activity of proximal tubule Na-K-ATPase and H-ATPase activities were partially protected as compared to the animals given maleic acid alone (P < 0.05). Renal cortical ATP levels were not altered at the concentration of maleic acid used in this study (that is, 50 mg/kg body wt), but higher doses of maleic acid (that is, 500 and 1000 mg/kg body wt) caused ATP levels to fall. Maleic acid did not affect cortical medullary total phosphate concentration, however, P32 turnover (1 and 24 hr) was altered by prior phosphate infusion. A protective effect of prior phosphate loading on the membrane bound Pi pool (insoluble) was seen while the cytosolic Pi pool (soluble) was not different from control. Thus, maleic acid-induced "Fanconi" syndrome likely results from both direct inhibition of proximal tubule Na-K-ATPase activity and membrane-bound phosphorus depletion. The former mechanism would reduce activity of the sodium-dependent transporters (that is, Na/H antiporter), while the latter would inhibit the electrogenic proton pump (H-ATPase). The combination of reduced proximal tubule Na-H exchange and H-ATPase activities would markedly inhibit bicarbonate reabsorption and result in the metabolic acidosis universally seen in the Fanconi syndrome.

Ammonia and acid-base homeostasis

The metabolic pathways involved in renal ammonia production have been considered and potential sites of regulatory control have been delineated. New information that acute acidosis stimulates renal ammonia production and that chronic respiratory acidosis does not result in an adaptive increase in the renal capacity to produce ammonia has been emphasized. The effect of potassium on renal ammonia production and the physiologic and pathophysiologic implication of this relationship have been detailed. Finally, the mechanism of urinary ammonium excretion and the impact of altered ammoniagenesis on urinary acidification, and the interpretation of clinical acidification tests have been discussed.

Influence of pyruvate on ammonia metabolism by renal cortical mitochondria

The effect of the tricarboxylic acid (TCA) cycle precursor, pyruvate, on glutamine metabolism by isolated renal cortical mitochondria was assessed by quantitating its key nitrogen and carbon metabolites. When mitochondria from normal rats were incubated at pH 7.4, pyruvate (2 mM) inhibited ammonia production by almost completely erradicating glutamate deamination and by diminishing glutamine deamidation but to a lesser extent. Alpha KG, citrate, and malate accumulation in the incubation medium were increased dramatically reflecting the increased flux of pyruvate through the TCA cycle; the intramitochondrial concentrations of both Alpha KG and glutamate were increased. Thus, pyruvate primarily inhibits flux through glutamate dehydrogenase as a result either of an increase in Alpha KG concentration and/or a decrease in the redox (NAD/NADH) potential secondary to enhanced flux through the TCA cycle. Glutamine deamidation is secondarily inhibited, presumably due to the increased intramitochondrial concentration of glutamate. Citrate (2 mM) produced changes comparable to those observed with pyruvate. Mitochondria from normal rats incubated at pH 7.0 as well as mitochondria from rats with chronic metabolic acidosis responded to pyruvate in a fashion qualitatively similar to normal mitochondria incubated at pH 7.4. Glutamate deamination was inhibited significantly, but a high rate persisted with chronic acidosis despite the presence of pyruvate. Nevertheless, when glutamine metabolism was contrasted with normal mitochondria incubated at pH 7.4, the response to in vitro incubation in an acid pH as well as to chronic metabolic acidosis was similar quantitatively regardless of whether glutamine alone or in combination with pyruvate was present in the incubation medium.

Evidence of activation of the renal glutamate dehydrogenase pathway in intact acidotic dogs

To determine if activity of the renal glutamate dehydrogenase (GD) pathway changes during chronic acidosis in intact dogs, we assessed the deamination of glutamate formed within renal cells during glutamine and alanine infusions. Infusing glutamine into chronically acidotic, normal and acutely alkalotic dogs enhanced renal ammonia production; more was formed as glutamine loading increased. In 4 acidotic dogs, the ratio of ammonia produced to glutamine extracted by the kidneys during exogenous glutamine loading was 1.93 compared with 0.99 for 5 alkalotic dogs and 1.23 for 2 control dogs. Little glutamate and alanine were released into the renal vein in acidotic dogs, whereas over 50% of the exogenous glutamine extracted in acutely alkalotic dogs could be accounted for as glutamate and alanine released into the renal vein. Renal glutamate concentrations were not elevated in acidosis compared with alkalosis despite greater deamidation. When glutamine infusions increased renal ammoniagenesis in acutely alkalotic and control dogs to levels seen in chronically acidotic dogs receiving no exogenous glutamine, approximately 4 to 6 times more glutamate was released from the kidneys. Infusing alanine into 7 chronically acidotic dogs enhanced ammoniagenesis significantly (p less than 0.01), but lesser augmentation was seen in 3 control dogs and no augmentation was seen in 6 acutely alkalotic dogs. The increases were secondary to enhanced glutamate deamination, not secondary to any changes in glutamine extraction and/or transaminase activity. We conclude that the glutamate dehydrogenase pathway is more active in intact acidotic dogs than it is in control and alkalotic dogs.