Effect of metabolic acidosis on renal gluconeogenesis in vivo

A B S T R A C T The purpose of this study was to clarify the mechanism (s) responsible for regulation of ammonia production and excretion in the rabbit. The normally low ammonia excretion rate during acute metabolic acidosis was stimulated acutely and increased approximately ninefold after infusion of sodium phosphate, but remained low if sodium sulphate or Tris was substituted for phosphate. Ammonia production was increased significantly by phosphate in rabbit renal cortex slices and in isolated renal cortex mitochondria.In isolated mitochotidria, mersalyl, an inhibitor of both the phosphate/hydroxyl and phosphate/dicarboxylate mitochondrial carriers, inhibited the phosphateinduced stimulation, indicating that phosphate must enter the mitochondrion for stimulation. A malate/phosphate exchange seemed to be involved since N-ethylmaleimide, an inhibitor of the phosphate/hydroxyl exchange, did not inhibit phosphate-stimulated ammonia production, whereas there was inhibition by 2-n-butylmalonate, a competitive inhibitor of the dicarboxylate carrier. Phosphate itself was not essential since malonate stimulated ammoniagenesis in the absence of added phosphate. Similarly, citrate stimulated ammoniagenesis in isolated mitochondria in the absence of inorganic phosphate provided that it induced L-malate exit on the citrate transporter associated with inhibition of citrate oxidation by fluoroacetate. Similar results were also seen in mitochondria from rat renal cortex.Dr. Yu and Dr. Giammarco are postdoctoral Fellows, Medical Research Council of Canada.Received for putblication 11 December 1975 and in revised form 19 April 1976. A fall in mitochondrial a-ketoglutarate level resulted in an increase in ammonia production. This could be achieved directly with malonate or indirectly via L-malate exit. Simultaneous measurements of glutamate showed that the rate of ammonia production was reciprocally related to the glutamate content. We conclude that phosphate-induced stimulation of ammoniagenesis in the rabbit kidney is mediated by removal of glutamate, the feedback inhibitor of phosphate-dependent glutaminase. Glutamate removal is linked to phosphateinduced dicarboxylate exit across the mitochondrial membrane. INTRODUCTIONThe regulatory mechanisms involved in increased renal glutamine extraction, deamidation, and deamination during metabolic acidosis play a major role in acid-base homeostasis by permitting increased urine excretion of hydrogen ions in the form of ammonium. Urine ammonia excretion increases during metabolic acidosis in man, rat, and dog. However, despite many attempts to gain an understanding of this phenomenon, confusion still exists as to the role various mechanisms play (1-3).Previous studies have shown that ammonia excretion varies markedly from species to species, carnivores having a high rate of excretion (1-3) and herbivores a low rate of excretion (4). Several factors may be involved. Rats and dogs tend to have a neutral or acidic urine while rabbits, which are low ammonia excretors, have an alkali...

Section: Discussionmentioning

confidence: 99%

Stimulation of ammonia production and excretion in the rabbit by inorganic phosphate. Study of control mechanisms.

Yu¹,

Giammarco²,

Goldstein³

et al. 1976

“…These data have raised the question of whether the kidney releases enough glucose to contribute to maintenance of homeostasis of the blood glucose level. There have been many attempts to answer this question by measuring the renal venoarterial (RV-A) difference of the glucose concentration, but the very small RV-A difference, as a result of the rapid renal blood flow, has made it difficult to obtain a definite answer (7)(8)(9)(10)(11). Bergman and Drury (12) and others (13)(14)(15) showed that the decrease in the blood glucose concentration in eviscerated animals with intact kidneys was less than that in completely eviscerated animals, suggesting net glucose release from the kidney.…”

Section: Introductionmentioning

confidence: 99%

Renal Net Glucose Release In Vivo and Its Contribution to Blood Glucose in Rats

Kida¹,

Nakajo²,

Kamiya³

et al. 1978

104

A B S T RThe results indicate that the net glucose release by the kidney in vivo in normal fed rats was 0.75+0.13 mg/dl per min, and that its contribution to blood glucose was 25.9±5.0%. When unilateral nephrectomy was performed, under the same conditions, renal net glucose release was one-half of that in rats with two intact kidneys, which indicates the quantitative accuracy ofthe isotope-dilution method employed in this study.In rats starved for 24 h, the renal net glucose release increased to 0.99+0.08 mg/dl per min. Diabetic rats showed a remarkably higher renal net glucose of 2.28 +0.33 mg/dl per min, which was 360% of the normal level. Treatment of diabetic rats with insulin, restored the renal net glucose release to the normal level. In

“…The relative magnitude of C02 and glucose formation from any of the substrates tested indicates that glucose formation is a minor pathway of metabolism from these substrates since the quantity of carbon atoms converted to C02 greatly exceeds the amount incorporated into glucose. The results also indicate that the primary site at which chronic acid-base changes act to 27 mEq/liter and the minimum was 23 mEq/liter. Tissue was prepared for incubation as previously described (8).…”

Section: Introductionmentioning

confidence: 69%

“…While there is considerable evidence which makes this action-at-a-distance concept at least a plausible basis for the increase in glutamine utilization in acidosis, some recent results suggest the need for investigation of other alternatives. For example, increased net renal glucose production either does not occur in the intact kidney of the acidotic dog or occurs only to a very limited extent (23)(24)(25)(26). Species differences in the metabolic response to acidosis do exist, as noted in the preceding paragraph, and it is possible that the dog is unique in its response to acidosis.…”

Section: Resultsmentioning

confidence: 99%

Pathways of glutamine and organic acid metabolism in renal cortex in chronic metabolic acidosis

Simpson¹

1972

A B S T R A C T The metabolism of labeled glutamine and of several labeled organic acid anions was compared in tissue slices of renal cortex from chronically acidotic and alkalotic littermate dogs. 'NHa formation and 'N-amideglutamine utilization were significantly increased by slices from acidotic animals providing further evidence for the similarity of the metabolic responses seen in the tissue slice system and the physiologic effects produced by chronic metabolic acidosis on renal metabolism in the intact animal. Slices from acidotic dogs formed more "CO2 and glucose-"C than did slices from alkalotic animals when labeled glutamine, citrate, or malate was used as substrate but 'COa production from pyruvate-1-l4C was slightly reduced in acidotic tissue. With most of the substrates used glucose-"'C formation was small compared with 14CO2 formation. Using the amount of glucose-'4C formed, the expected 14CO2 production was calculated based on the hypothesis that the primary site of action of metabolic acidosis is on a cytoplasmic step in gluconeogenesis. The actual difference in 1'CO2 production between slices from acidotic and alkalotic animals always greatly exceeded this predicted amount, indicating that stimulation of gluconeogenesis represents a minor metabolic response to chronic metabolic acidosis. Evidence from experiments with citrate labeled in various positions showed that metabolic acidosis has its principal effect on an early step in substrate metabolism which must be intramitochondrial in location.