Ammoniagenesis in the rat kidney during recovery from metabolic acidosis

Parry, David Morris (1980) Ammoniagenesis in the rat kidney during recovery from metabolic acidosis. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The regulation of renal glutamine metabolism in relation to acid-base status was investigated in rats. The model of NH₄Cl-induced acidosis was used and two new experimental situations introduced. Recovery from acidosis was investigated in rats allowed to drink H₂O in lieu of NH₄Cl and the process of recovery was accelerated by administration of NaHCO₃ (1.5 mmol/lOOg body wt). Animals allowed to recover from acidosis were again challenged with various acid loads and the responses of these rechallenged rats compared to the responses of naive-challenged rats. -- Urinary ammonia excretion, total renal ammonia production and glutamine extraction returned to normal by about 24h in animals which were allowed to recover from metabolic acidosis by drinking H₂O. In comparison, in animals administered NaHCO₃ at the start of recovery, these parameters were back to normal by 8h. Rats which were permitted three days to recover from metabolic acidosis were able to excrete more acid as ammonium salt than animals challenged with NH₄Cl for the first time, when challenged with high acid loads. There was no difference in response to lower acid loads. -- Decreases in the renal contents of glutamine, α-ketoglutarate and malate were found during metabolic acidosis and increases in giutamine, glutamate, α-ketoglutarate, malate, citrate, lactate, phosphoenolpyruvate and 3-phosphoglycerate occurred during recovery. No significant difference in the renal content of metabolites was observed between rats administered NaHCO₃ and rats administered the same amount of NaCl during recovery. -- The renal activities of phosphoenolpyruvate carboxykinase (PEPCK), glutaminase (PDG) and glutamate dehydrogenase (GDH) in these animals were also investigated. PEPCK activity increased identically in naive-challenged and rechallenged rats. During recovery PEPCK activity returned to normal by about 16h and no significant difference in activity could be discerned between animals intubated with NaHCO₃ and animals intubated with NaCl. Immunotitration of PEPCK revealed that the increased activity during acidosis and the decreased activity during recovery are due to changes in the content of this enzyme. The activities of PDG and GDH increased during acidosis and remained elevated long after renal glutamine metabolism returned to normal, in vivo. -- Ammonia formation by isolated mitochondria and the fluxes through PDG and GDH increased during metabolic acidosis. However, normal rates of ammonia production in vivo were attained 12-24h after the induction of acidosis while no acceleration of GDH flux in isolated mitochondria was evident at this time. In the most physiological medium used (1 mM glutamine, 3 mM glutamate, 4 mM phosphate) ammonia formation and GDH flux decreased coincidentally during recovery and preceeded the fall in PDG flux. The differences in renal ammoniagenesis between naive-challenged rats and rechallenged rats and between rats intubated with NaCl and NaHCO₃ during recovery could not be accounted for by metabolic differences in isolated mitochondria. In the most unphysiological medium used (1 mM glutamine, 20 mM phosphate) the mitochondrial capacity to metabolize glutamine remained elevated for at least 15 days of recovery. -- These observations suggest that renal glutamine metabolism is not controlled simply by changes in the contents of PDG or PEPCK or by the mitochondrial events which are responsible for the changes in the fluxes through PDG or GDH in isolated mitochondria. The changes observed in αketoglutarate are consistent with its purported regulatory role.

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