Acute metabolic acidosis was induced by an i.v. administration of hydrochloric acid to dogs and rats to decrease the plasma bicarbonate concentration from 22 to 12 mM in dogs and from 26 to 10 mM in rats. Chronic metabolic acidosis was also induced in dogs by ammonium chloride feeding for 5 days. Rats also were given ammonium chloride for 24 hours. The renal metabolite profile was determined on the freeze-clamped renal tissue before and after 100 min (dogs) or 30 to 240 min (rats) of acsute acidosis. Measurements on chronically acidotic dogs and rats with 24-hour acidosis were obtained also for comparison with acute acidosis. In both species, kidney glutamine, glutamate, and alpha-ketokglutarate concentrations decreased drastically following induction of acute or chronic acidosis, In the dog, or in the rat during the first 2 hours of acidosis, malate concentration was unchanged. Malate concentration fell significantly in the rat kidney only after 2 hours of acidosis without change in phosphoenolpyruvate (PEP) concentration. In chronically acidotic dogs, malate and oxaloacetate rose fivefold with no change in PEP concentration. Phosphoenolpyruvate carboxykinase (PEPCK) activity was not stimulated by chronic metabolic acidosis in the dog in contrast to the rat. Acute acidosis by hydrochloric acid increased net renal glutamine extraction in the rat but not in the dog. These data suggest that an increased metabolic flux occurs between alpha-ketoglutarate and malate in both rat and dog kidney during acute metabolic acidosis. In the rat, however, after 2 hours, PEPCK activation modifies the kidney metabolite profile. Intrarenal glutamine transport seems to be a rate-limiting factor for adaptation to acute acidosis in the dog but not in the rat kidney.

Metabolism of glutamine results in the net production of ATP; however, cells cannot sustain an ATP production rate greater than their rate of ATP utilization. The purpose of these studies was to determine whether the rate of ATP turnover in the kidney could set an upper limit on renal glutamine metabolism and thereby renal ammoniagenesis. The acidotic dog kidneys extracted glutamine, lactate, citrate, and oxygen from the arterial blood and added ammonium and alanine to the venous blood. Renal glutamine metabolism was responsible for almost all the ammonium production. Renal ATP production was estimated from the rate of oxygen consumption and appeared to be derived roughly equally from the oxidation of glutamine and lactate. There was no apparent renal glucose production from ATP balance calculations and this impression was supported when the inhibitor of gluconeogenesis, 3-mercaptopicolinate, did not inhibit ammoniagenesis. Approximately 90% of the ATP synthesized was utilized to reabsorb sodium. When the amount of ATP utilized for sodium reabsorption in the proximal convoluted tubule (assumed to be 60% of filtered sodium) was compared with the amount of ATP produced from glutamine metabolism, the values were similar despite the fact that the glomerular filtration rate in individual dogs varied more than fourfold. When the quantity of ATP expended for sodium reabsorption was decreased by the infusion of ouabain or by the constriction of one renal artery without reducing glutamine delivery, the kidney lowered its rate of ammoniagenesis to a quantitatively predictable amount.(ABSTRACT TRUNCATED AT 250 WORDS)

Many theories have been proposed to explain the regulation of renal ammoniagenesis during chronic metabolic acidosis but none of these is entirely satisfactory. Since the activity of each of the enzymes in this pathway greatly exceeds the maximum rate of ammonium production in vivo, even when physiological substrate concentrations are used in this calculation, it follows that ammoniagenesis must be inhibited in the intact animal. We shall present a novel hypothesis for the regulation of the maximum rate of ammoniagenesis which emphasizes the fact that ATP is a product of this pathway and that a limited rate of ATP utilization could control its maximum velocity during chronic metabolic acidosis. To test the validity of our hypothesis, a quantitative analysis of the pathways of ATP production and utilization in the kidney will be reviewed. This approach is similar to one already proposed for the regulation of the maximum rate of ketogenesis in the liver.

To investigate the mechanisms of the antiammoniagenic effect of ketone bodies, acidotic dogs (NH4Cl) were infused with either beta-hydroxybutyrate or acetoacetate. Total blood ketones ranged from 2 to 4 mM. Renal ammoniagenesis fell by a mean of 53%, with a proportional decrease in glutamine extraction. Glutamate release in the renal vein rose, renal extraction of lactate fell, and aspartate and alanine production decreased. Study of the metabolite profile of the renal cortex by the freeze-clamp technique before and after ketone infusion showed that tissue glutamine concentration was unchanged, whereas glutamate, alpha-ketoglutarate, malate, and citrate rose. The intermediates of the gluconeogenic pathway, such as phosphoenolypyruvate, 2-phosphoglycerate, 3-phosphoglycerate, and glucose-6-phosphate, fell significantly. The redox state as calculated from the free NAD+/NADH ratios in the cytosolic (lactate dehydrogenase) and the mitochondrial (glutamate dehydrogenase and beta-hydroxybutyrate dehydrogenase) compartments was reduced. The present study suggests that ketone bodies inhibit renal ammoniagenesis through increased generation of alpha-ketoglutarate (metabolic or bicarbonate effect) and a decrease in the mitochondrial and cytosolic redox potentials in the kidney.

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