Renal metabolism of alanine.

In the acidotic dog, alanine is extracted from plasma and utilized as a precursor of ammonia. Simultaneously, it is formed de novo within tubular cells and added to renal venous blood. When plasma concentration is within a normal range, production of alanine greatly exceeds utilization. Increasing the plasma concentration reduces production and increases utilization of plasma alanine. The infusion of glutamine increases the renal production of alanine without appreciable change in utilization of plasma alanine. These results are consonant with the view that alanine is metabolized by transamination with alpha-ketoglutarate to form glutamate, which is subsequently deaminated oxidatively to liberate ammonia. Conversely, alanine is formed by transamination of pyruvate with either glutamate or glutamine and is added to renal venous blood. The balance between production and utilization is dependent, at least in part, on the concentrations of the reactants.

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.

Diffusion equilibrium for ammonia in the kidney of the acidotic dog.

Inflow of preformed ammonia in arterial blood, renal production of ammonia, outflow of ammonia in renal venous blood, and urinary excretion of ammonia were measured during the infusion of (15)NH(4)Cl into one renal artery of dogs with chronic metabolic acidosis. Our results show that the specific activity of ammonia measured in the urine and that calculated in the renal pool agree within 95%. Pool specific activity is obtained by dividing the rate of infusion of isotope by the pool turnover rate, i.e., the sum of the rate of ammonia output in the urine and that in renal venous blood. An average of 35% of urinary ammonia is derived from arterial ammonia in these experiments. We conclude that ammonia is distributed evenly throughout all phases of the kidney within a period less than the transit time of blood through the kidney. Furthermore, from the proportion of urinary ammonia we found to be derived from preformed arterial ammonia (35%), and from our previous demonstration that 73% of urinary ammonia derives from deamidation and/or deamination of plasma glutamine, alanine, glycine, and glutamate, we can account for all of the ammonia that leaves the kidney in renal venous blood and in urine.

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.

Metabolism of blood glucose by the intact functioning kidney of the dog.

The CO2 produced in the metabolism of blood glucose by the kidney has been measured by the i.v. infusion 14C-UL-D-glucose as a tracer. A small fraction of circulating 14C-glucose is converted to labeled lactate in extrarenal tissues, returned to, extracted by and metabolized to CO2 in the kidney. Correcting the apparent 14CO2 produced in the kidney from glucose for the 14CO2 produced from lactate yields that produced directly from glucose. Differences in production of CO2 from glucose in chronic metabolic acidosis and alkalosis are small and probably are not significant. If so, somewhat less than one-quarter of the total metabolism of the intact functioning kidney is supported by blood glucose. These measurements have been made at normal endogenous blood concentrations of glucose.

Action of phlorizin on luminal and antiluminal membranes of proximal cells of kidney.

In accord with results of others, we have observed that the infusion of phlorizin at low rates (2.16-117 microgram-kg-1-min-1) progressively increases fractional excretion of glucose from 0.47 to 0.85. Further increasing the rate of infusion to 2.19 mg-kg-1-min-1 increases fractional excretion to 1.0. The relationship appears to describe a single function having characteristics of an adsorption isotherm. The metabolism of the kidney, expressed as rate of total CO2 production from all substrates, is modestly and variably reduced by infusion of phlorizin at both high and low rates. The metabolism of glucose, expressed as rate of CO2 production, is variably but not consistently altered by infusion of phlorizin. The oxidation of [14C]lactate derived from [14C]glucose is negligible and introduces no significant error into measurement of 14CO2 from [14C]glucose. The percent of total CO2 derived from glucose does not differe significantly in control periods from the mean of all periods following phlorizin. Accordingly interaction of phlorizin with peritubular membranes at high rates of infusion, in the sense of blocking penetration of glucose, does not occur. Our methods do not rule out nor do they prove adsorption of phlorizin to these membranes.

Glutamine synthetase in kidneys of monkey and man.

1. The synthesis of gamma-glutamylhydroxamate from glutamate and hydroxylamine has been utilized as an approximation of glutamine synthetase activity in kidneys of rabbit, rat, dog, monkey and man. 2. Kidneys of rabbit contain glutamine synthetase in high activity; those of rat, in intermediate activity; and those of dog, monkey and man, in negligible activity. 3. No more enzyme is present in kidneys of the latter two species than in those of the dog, in which the enzyme is generally considered to be absent.

Renal CO2 production from glutamine and lactate as a function of arterial perfusion pressure in dog.

The energy cost of renal function in the intact kidney of the dog was assessed at a series of arterial perfusion pressures. Pressure was varied by partially inflating a balloon at the tip of a catheter positioned in the aorta above the origins of the renal arteries. Either L-[U-14C]-lactate was infused intravenously in tracer amounts throughout each experiment. Total renal CO2 production and 14CO2 production from each isotope permitted assessment of total renal oxidative metabolism and the proportions derived from the two major substrates of the kidney. Stepwise inflation of the aortic balloon progressively lowered glomerular filtration rate, renal blood inflow, filtered and consequently reabsorbed Na+, total renal CO2 production, and 14CO2 derived from glutamine and lactate. The percent of total CO2 derived from lactate decreased more or less in proportion to the decrease in percent of total CO2 produced. Results were consistent with the view that reabsorption of sodium is the major energy sink of the kidney. They suggest that the oxidation of glutamine supplies energy for tubular transport and basal demands such as synthesis of hormones and maintenance of structure, whereas the oxidation of lactate supplies energy mainly for transport activities.