RELATIONSHIP OF PNH3 OF TUBULAR CELLS TO RENAL PRODUCTION OF AMMONIA

Intra- and inter-nephron heterogeneity of ammoniagenesis in rats: effects of chronic metabolic acidosis and potassium depletion

In order to determine intra- and inter-nephron heterogeneity of ammoniagenesis, ammoniagenic activity in microdissected nephron segments of control, acidotic and potassium (K)-depleted rats was examined. Intranephron distribution of ammoniagenic activity in control rats revealed the highest amount at the second segment of the proximal tubule (S2). Chronic metabolic acidosis induced ammoniagenesis markedly at the first segment of the proximal tubule (S1) by 235% and the thick ascending limb of Henle's loop by 198% and moderately at the S2 by 49%. K-depletion increased ammonia production significantly in the S1 by 298% and the S2 by 107%, which is a pattern quite similar to the result of chronic metabolic acidosis. Ammonia production in K-depletion was also increased in the cortical and medullary collecting tubule by 71% and 102%, respectively, probably due to increases in protein amounts (41% and 158%, respectively) there. To evaluate inter-nephron heterogeneity of ammoniagenesis, ammonia formation from glutamine in the S1 of superficial (SF) and juxtamedullary (JM) nephrons was examined. Although there was no difference in ammonia production between SF-S1 and JM-S1 in control rats, ammonia production in SF-S1 was significantly higher than that in JM-S1 in both metabolic acidosis and K-depletion. From these studies, we conclude: The increase of ammonia production in the proximal tubule was quite similar in both acidosis and K-depletion, suggesting that the main trigger of ammoniagenesis in both conditions might be a reduction of intracellular pH. SF-S1 was the nephron most reactive to acidosis and K-depletion. JM nephrons could be considered to be important not for ammonia production but for ammonia secretion.

Ammonia production and pathways of glutamine utilization in rat kidney slices

Ammonia production from glutamine was studied in slices from non-acidotic and acidotic rat kidneys. Slices from non-acidotic kidneys made 53% as much ammonia from D-glutamine as from L-glutamine during the initial 15 min of incubation. Thereafter the production rate from the L-isomer accelerated while that from the D-isomer remained constant. The accelerated rate of ammonia production from L-glutamine was dependent upon tissue swelling since prevention of swelling reduced the production rate. Swelling activates the mitochondrial glutaminase I pathway as evidenced by the rise in ammonia produced per glutamine utilized ratio as well as by the accelerated rate of CO2 production derived from the oxidative disposal of glutamin's carbon skeleton. Cortical slice swelling activates the mitochondrial pathway in a manner not unlike that seen in vivo during chronic acidosis and may reflect increased permeability to glutamine. Acidotic rat kidneys are not swollen in vivo while cortical slices initially produce 4-fold more ammonia than do non-acidotic slices. After 15 min, this 4-fold difference in total ammonia production drops to only a 2-fold difference due to the swelling-induced activation of the mitochondrial pathway. Consequently, slice swelling obliterates the important fact that ammonia production by the mitochondrial pathway is 15-fold greater in acidotic than in non-acidotic kidneys.

Renal hemodynamics and ammoniagenesis. Characteristics of the antiluminal site for glutamine extraction

Renal production of ammonia by the left kidney was studied in 31 acidotic dogs (NH(4)Cl) after acute constriction of the renal artery. Renal ammoniagenesis fell in direct proportion with the reduction in glomerular filtration rate and renal plasma flow. The renal extraction of glutamine by the experimental kidney fell in direct proportion with the reduction in renal hemodynamics. Extracted glutamine remained greater than filtered glutamine indicating that both the luminal and antiluminal transport sites were operative. The relationship between renal extraction of glutamine and ammoniagenesis observed during control was maintained after renal artery constriction (1.7 mumol NH(3) produced for each mumol of glutamine extracted). Systemic venous or renal intra-arterial infusion of glutamine during arterial constriction increased renal production of ammonia to or above control values. These observations indicate that the mechanisms responsible for glutamine extraction and ammonia production were operating normally despite reduced hemodynamics. When measured immediately after arterial clamping, the renal venous pNH(3) was found to rise significantly decreasing progressively thereafter towards control values. The extracted fraction of total glutamine delivered to the kidney (31%) did not change after acute reduction of the glutamine load. Thus, the antiluminal extraction site was incapable of lowering renal venous plasma glutamine concentration below 0.33 muM/ml. In a second series of experiments, the properties of the antiluminal site of transport for glutamine were studied after complete occlusion of the left ureter in acidotic and nonacidotic animals. Under these circumstances, it was demonstrated that the antiluminal site is capable of extracting sufficient glutamine to maintain total ammonia production at 60% or more of control. In acidotic animals, changes in cellular pNH(3) appeared to play a key role on the antiluminal extraction of glutamine since the significant rise in renal blood flow often observed after ureteral occlusion prevented the rise in pNH(3) noted when blood flow remained constant. Thus, when renal blood flow rose glutamine extraction and ammonia production were maintained at control values. In these acidotic animals, glutamine infusion failed to influence ammonia production until luminal transport was restored by release of ureteral clamp and resumption of glomerular filtration. The latter observation establishes that reabsorbed glutamine is utilized at least in part for ammonia production.

Metabolism of glutamine by the intact functioning kidney of the dog. Studies in metabolic acidosis and alkalosis

The renal conversion of glutamine to glucose and its oxidation to CO(2) were compared in dogs in chronic metabolic acidosis and alkalosis. These studies were performed at normal endogenous levels of glutamine utilizing glutamine-(34)C (uniformly labeled) as a tracer. It was observed in five experiments in acidosis that mean renal extraction of glutamine by one kidney amounted to 27.7 mumoles/min. Of this quantity, 5.34 mumoles/min was converted to glucose, and 17.5 mumoles/min was oxidized to CO(2). Acidotic animals excreted an average of 41 mumoles/min of ammonia in the urine formed by one kidney. In contrast, in five experiments in alkalosis, mean renal extraction of glutamine amounted to 8.04 mumoles/min. Of this quantity, 0.92 mumole/min was converted to glucose, and 4.99 mumoles/min was oxidized to CO(2). Alkalotic animals excreted an average of 3.23 mumoles/min of ammonia in the urine. We conclude that renal gluconeogenesis is not rate limiting for the production and excretion of ammonia in either acidosis or alkalosis. Since 40% of total CO(2) production is derived from oxidation of glutamine by the acidotic kidney and 14% by the alkalotic kidney, it is apparent that renal energy sources change with acid-base state and that glutamine constitutes a major metabolic fuel in acidosis.

Pathways of ammonia metabolism in the intact functioning kidney of the dog

Studies in which (15)N-labeled precursors of urinary ammonia were infused into the artery of an intact functioning kidney of an acidotic dog have led to the following conclusions: Preformed ammonia and ammonia derived from the amide nitrogen of plasma glutamine are added directly to urine without significant incorporation into amino acid intermediates of renal tissue. Thus, reductive amination of alpha-ketoglutarate to form glutamate does not occur to an appreciable extent nor is there significant transfer of the amide nitrogen of glutamine to the corresponding keto acids to form glutamate, aspartate, alanine, or glycine. The enzyme system "glutaminase II" may participate to a significant extent in the metabolism of glutamine by forming aspartate and alanine by direct transamination of oxalacetate and pyruvate and liberating the amide nitrogen as ammonia. Renal alanine exists as a well mixed pool derived in roughly equal amounts from filtered and reabsorbed plasma alanine and newly synthesized alanine. The alanine pool of tubular cells does not equilibrate with the alanine of peritubular capillary blood. Transfer of the nitrogen of alanine to alpha-ketoglutarate and subsequent oxidative demination of the resulting glutamate can account for the ammonia formed from alanine. Glycine is not an important intermediate in renal nitrogen metabolism.