As part of the homeostatic adaptation to metabolic acidosis, the kidney exhibits an increased production and excretion of ammonium ions. Increased ammoniagenesis provides an expendable cation which facilitates the excretion of anions and titratable acids. The ammonium ions are derived from the renal metabolism of plasma glutamine that is initiated by the mitochondrial phosphate-dependent glutaminase. The adaptations that are responsible for the rat renal response to metabolic acidosis have been well characterized. Acute adaptations include an increased synthesis of muscle glutamine, prompt acidification of the urine, and an activation of yield-ketoglutarate dehydrogenase due to decreased intracellular pH. These adaptations result in an increased availability of substrate and the rapid removal of the products of the mitochondrial glutaminase and glutamate dehydrogenase reactions. Subsequent adaptations include an increase in the level of phosphoenolpyruvate carboxykinase and phosphate-dependent glutaminase within the proximal convoluted portion of the renal nephron. The latter adaptations contribute to the maintenance of increased renal ammoniagenesis during chronic acidosis. The interorgan nature of the response and the cell specific induction of key enzymes suggest that ammoniagenesis is regulated by specific hormones. Our primary objective is to further characterize the regulation of the adaptations that occur in chronic acidosis.
The specific aims i nclude: the characterization of the biosynthesis and processing of the mitochondrial phosphate-dependent glutaminase; the utilization of primary cultures of rat renal proximal convoluted tubule cells to characterize the induction of phosphate-dependent glutaminase and of phosphoenolpyruvate carboxykinase; the use of a cloned cDNA specific for phosphoenolpyruvate carboxykinase to identify the hormone(s) or conditions responsible for the regulation of its mRNA synthesis; the isolation and characterization of a cloned cDNA specific for the phosphate-dependent glutaminase; and the use of the glutaminase cDNA to further characterize the mechanism responsible for its increased synthesis during acidosis.
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