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

Effect of NBCe1 deletion on renal citrate and 2-oxoglutarate handling

The bicarbonate transporter, NBCe1 (SLC4A4), is necessary for at least two components of the proximal tubule contribution to acid-base homeostasis, filtered bicarbonate reabsorption, and ammonia metabolism. This study's purpose was to determine NBCe1's role in a third component of acid-base homeostasis, organic anion metabolism, by studying mice with NBCe1 deletion. Because NBCe1 deletion causes metabolic acidosis, we also examined acid-loaded wild-type adult mice to determine if the effects of NBCe1 deletion were specific to NBCe1 deletion or were a non-specific effect of the associated metabolic acidosis. Both NBCe1 KO and acid-loading decreased citrate excretion, but in contrast to metabolic acidosis alone, NBCe1 KO decreased expression of the apical citrate transporter, NaDC-1. Thus, NBCe1 expression is necessary for normal NaDC-1 expression, and NBCe1 deletion induces a novel citrate reabsorptive pathway. Second, NBCe1 KO increased 2-oxoglutarate excretion. This could not be attributed to the metabolic acidosis as experimental acidosis decreased excretion. Increased 2-oxoglutarate excretion could not be explained by changes in plasma 2-oxoglutarate levels, the glutaminase I or the glutaminase II generation pathways, 2-oxoglutarate metabolism, its putative apical 2-oxoglutarate transporter, OAT10, or its basolateral transporter, NaDC-3.

IN SUMMARY

(1) NBCe1 is necessary for normal proximal tubule NaDC-1 expression; (2) NBCe1 deletion results in stimulation of a novel citrate reabsorptive pathway; and (3) NBCe1 is necessary for normal 2-oxoglutarate metabolism through mechanisms independent of expression of known transport and metabolic pathways.

Regulation of glutamine metabolism in dog kidney in vivo

In summary, we propose: that renal ammoniagenesis is regulated both by factors dependent and independent of the acid-base status, the net effect of the ammoniagenic process on the proton balance being directly related to the rate of urinary ammonium excretion; that the renal metabolism of glutamine should not be examined independently of the metabolism of other substrate physiologically taken up by the kidney; that different pathways for glutamine metabolism will change during acid-base disorders of organic or nonorganic origin; that, among the main glutamine utilizing pathways, only the GLDH pathway is influenced directly by the acid-base status; the ammoniagenic transamination pathways is regulated by substrate availability in the kidney; that the lowest ammoniagenic flux in the kidney coincides with the rate of alanine production since alanine appears to derive directly from glutamine. When this pathway is stimulated without concomitant acidosis, most of the ammonia produced is not excreted in urine but released in the renal venous blood: thus, no significant effect on the acid-base balance is produced; that glutamine is metabolized by proximal kidney tubules of acidotic dogs probably through net oxidation; that the quantitative analysis of the metabolic consequence of this process indicates that the rate of ATP turnover at this site may effectively place an upper limit to the rate of glutamine oxidation, and ammonia production by the kidney, and that this limit is nearly reached in chronically acidotic animals.