Robert F. Pitts

NEUTRALIZATION OF INFUSED ACID BY NEPHRECTOMIZED DOGS

From 8 to 15 mMof strong mineral acid per kilogram of body weight may be infused intravenously in the dog over a period of a few hours to produce a severe but non-fatal acidosis. Van Slyke and Cullen (1) showed that only one-sixth of the infused acid is neutralized by blood buffers, five-sixths presumably being neutralized by bicarbonate in interstitial fluid and lymph and by intracellular phosphate and proteinate buffers. Recently, Bergstrom and Wallace(2) have indicated that sodium and potassium on the surface of bone crystals are also available for neutralization of acid. How tissue buffers participate in the neutralization of acid has been investigated by measuring changes in total quantity of extracellular sodium, potassium, bicarbonate, and chloride ions following infusion of hydrochloric acid into nephrectomized dogs. Our preliminary reports (3, 4) were based on experiments in which extracellular fluid volume was approximated by inulin distribution. The observation of a considerable increase in the inulin volume of distribution following infusion of acid led us to compare volumes of distribution of several substances proposed by others as approximations of extracellular fluid volume. This comparison indicated that radiosulfate distribution more closely measures extracellular fluid volume than does inulin in the whole animal (5). Therefore, the experiments on the effects of hydrochloric acid infusion have been repeated using radiosulfate distribution to measure changes in extracellular fluid volume. No definitive publication of the original experiments is contemplated. The results of our recent experiments indicate that when 10 mMof hydrochloric acid per kilogram of body weight are infused into a nephrectomized dog over a period of two hours, 43 per cent of the infused acid is neutralized by bicarbonate

THE RENAL REGULATION OF ACID-BASE BALANCE IN MAN. III. THE REABSORPTION AND EXCRETION OF BICARBONATE

In the normal individual the concentration of bicarbonate in the extracellular fluid is maintained within limits of 24 and 28 mMper liter despite wide variations in the intake of acid and base forming foodstuffs. Stabilization of level depends on continuously operative renal mechanisms, for although carbonic acid is continously available, no large reserve depots of base exist upon which the body may draw when circulating stores are depleted, or to which the body may consign an excess in times of abundance. The renal problem of regulating the concentration of bicarbonate may be outlined in the following terms: 1, conservation of those stores which normally enter the renal tubules in the glomerular filtrate; 2, excretion of any excess present in the body; and 3, conversion of the salts of fixed acids into bicarbonate to replenish depleted body stores. These aspects of regulation have been studied in normal human subjects at plasma bicarbonate levels ranging from 13 to 39 mMper liter. It has been observed at plasma concentrations below 24 mMper liter that essentially all of the filtered bicarbonate is reabsorbed; negligible quantities are lost in the urine. Above 28 mMper liter a relatively constant quantity is reabsorbed, amounting on an average to 2.8 mMper 100 ml. of glomerular filtrate; any excess present in the filtrate is excreted. These properties of the reabsorptive mechanism would in themselves assure stabilization of plasma level were there no continuous drain on bicarbonate stores for purposes of neutralizing fixed metabolic acid. Because such a drain exists in the individual maintained on the usual acid ash diet, the renal tubules must split neutral salts of the glomerular filtrate, restoring the fixed base to the body as bicarbonate, and eliminating the unwanted anions either as free titratable acid or in combination with ammonia.

Renal production and excretion of ammonia

Abstract The secretion of ammonia, once thought to be limited to distal tubules and collecting ducts, has been shown to occur throughout the length of the nephron of the rat. As much as half or more of the ammonia excreted in the urine may be added to tubular fluid in the proximal segment. Additional evidence has been adduced that the passive diffusion of free-base ammonia (NH 3 ) from its site of production in tubular cells into acid tubular urine is a major mechanism of ammonia secretion. Passive diffusion from cells into peritubular blood adequately accounts for the observed rate of addition of ammonia to renal venous blood. Thus ammonia has been shown to diffuse rapidly and readily in both directions across the renal tubular epithelium of the dog and man along a gradient of hydrogen ion concentration. Ammonia in cells of the renal cortex is in diffusion equilibrium with blood in peritubular capillaries and probably with fluid in proximal and distal convoluted tubules. The partial pressure of ammonia in renal venous blood is a fair approximation of that of renal cortical cells. Although this evidence strongly supports passive nonionic diffusion, it does not exclude the possibility that a fraction of the ammonia may be secreted by the active exchange of ammonium ions for sodium ions. In both the dog and man, glutamine is the major precursor of urinary ammonia. Since the kidney extracts glutamine from renal arterial blood, yet excretes no glutamic acid in the urine and adds little to renal venous blood, it is evident that the amino as well as the amide nitrogens of glutamine must be potential sources of ammonia. Studies with glutamine labeled with N 15 in the amide position show that 35 to 40 per cent of the nitrogen of the urinary ammonia excreted by the acidotic dog is derived from amide groups. The amino nitrogen of glutamine and of other amino acids accounts for the remainder. The glutamine pool of the kidney is small and turns over rapidly, with a half-time of less than 2.5 minutes. Many factors are involved in the control of rate of excretion of ammonia, including plasma substrate concentration, activity of the mechanisms of transport of substrate into tubular cells, activity of enzymatic production of ammonia within cells, activity of mechanisms concerned with acidification of the urine and rate of urinary flow.

The Renal Response to Acute Respiratory Acidosis

It is well known that patients suffering from acute and chronic ventilatory insufficiency or in whomthere exists an impaired diffusion of carbon dioxide across the alveolo-capillary membrane may develop an acid-base disorder (respiratory acidosis) characterized by decrease in arterial pH and elevation of the total CO2 content of the arterial blood (1-5). This syndrome of carbon dioxide retention has been noted clinically in patients with pulmonary emphysema, tracheal or laryngeal obstruction, during the coma of severe barbiturate and morphine poisoning,and in both acute paralytic and convalescent phases of poliomyelitis, where the commonfactor is failure of CO2elimination by the lungs. Previous studies (6, 7) indicate that carbon dioxide is buffered by body buffers, principally hemoglobin of whole blood, and that the increase in plasma bicarbonate concentration characterizing respiratory acidosis occurs more or less in proportion to the degree of CO, retention. This increase is compensatory, minimizing the fall in blood pH which would otherwise ensue. The inhalation of carbon dioxide mixtures has been shown to produce a diuresis with decrease in urine osmolarity, and increase in excretion of ammonia and titratable acid (8, 9). Since the plasma bicarbonate concentration is elevated in consequence of the buffering of retained carbon dioxide, one would anticipate from a priori considerations that a part of the renal compensation in acute respiratory acidosis would consist in an enhancement of the tubular reabsorption of bicarbonate bound base so as to maintain plasma bicarbonate concentration at supranormal levels. The studies reported below indicate that acute respiratory acidosis induced by respiring carbon dioxide-oxygen mixtures does in fact result in enhancement of the renal reabsorption of bicarbonate bound base, and that the elevation of carbon

Some reflections on mechanisms of action of diuretics

IURETICS are commonly defined as agents which increase the volume flow of urine. Such a definition is inadequate both for the clinician concerned with their use and for the physiologist concerned with their mode of action. A more useful and mechanistically precise definition is that diuretics are agents which promote the urinary excretion of sodium and either chloride or bicarbonate, i.e., those ions which are largely restricted to and which constitute the major ionic components of the extracellular fluid. Secondarily, water is excreted in proportional amounts, reducing extracellular fluid volume and body weight. This definition properly emphasizes the facts that the excretion of ions is primary, that these ions are drawn from extracellular rather than cellular stores and that increased urine volume and loss of body weight are proportional to and the osmotic consequences of loss of ions. Diuretics, as we know them today, are useful only in the treatment of edema and ascites. They have no place in the treatment of acute renal failure or of the uremia

Acid-base regulation by the kidneys

Abstract The kidney participates in the regulation of body neutrality by stabilizing the plasma concentration of bicarbonate-bound base at a level of 25 to 27 mEq. /L. The respiratory system participates by stabilizing the plasma carbonic acid level at 1.25 to 1.35 mEq. /L. Together the concentrations of these two components determine the reaction of the blood plasma and interstitial fluid which, under normal conditions, is maintained remarkably constant at pH 7.4. The problem of the renal stabilization of the concentration of bicarbonate is a dual one, involving both salvage of the filtered bicarbonate and restoration to the body of the base utilized in neutralizing metabolic acids. In a quantitative sense, salvage of bicarbonate from the glomerular filtrate is the more significant for each day more than a pound of the sodium salt is absorbed by the renal tubules. The efficiency of the absorptive mechanisms is such that under normal conditions less than 0.10 per cent of that filtered is wasted in the urine. Nevertheless, following the ingestion of bicarbonate large quantities can be eliminated with only a slight increase in plasma level. Somewhat more interesting from the point of view of the physiologist and the clinician as well are the tubular mechanisms which substitute hydrogen or ammonium ions for sodium ions in the tubular urine. By virtue of these substitutions, metabolic acids may be excreted in free titratable form or in combination with ammonia, without sacrifice of the limited stores of body base. Both substitutions are carried out by the distal segments of the renal tubules, and certain enzymes, namely, carbonic anhydrase, glutaminase and a group of amino acid oxidases, have been assigned specific functions. The acidosis of diabetic ketosis is primarily a consequence of flooding the renal tubules with such a load of metabolic acid that the exchange capacities of the tubules are overwhelmed. On the other hand, the acidosis of chronic renal disease is primarily a consequence of the inability of damaged renal tubules to exchange hydrogen ions and ammonia for base at a rate sufficient to compensate for a normal metabolic acid load. In either circumstance body base is drained away in neutralizing urinary acid. In chronic renal disease there is the additional factor of a reduced capacity to excrete phosphate and sulfate, both of which are filtered through the glomeruli and partially reabsorbed by the renal tubules. A reduction in the filtering bed accounts for a piling up of sulfate and phosphate in the body fluids with consequent displacement of bicarbonate. Given a period of a few days in which to effect adjustments, normal renal tubules can increase their capacity to excrete acid more than tenfold. A part of this adjustment may be brought about by increased production of adrenal cortical hormones.

STUDIES ON STRUCTURE DIURETIC ACTIVITY RELATIONSHIPS OF ORGANIC COMPOUNDS OF MERCURY

According to Friedman ( 1 ), the nature of the X substituent (usually halogen, theophylline or thioglycolate) has no effect on diuretic potency if the compound is given intravenously, but does influence both hyperacute (cardiac and respiratory arrest) and acute (7 to 14-day) renal toxicity. The nature of the OYgroup is determined by the solvent in which the mercuration is carried out and is an hydroxyl group, if the solvent is water. or a methoxy or ethoxy group, if the solvent is the corresponding alcohol. Within these limits, i.e., OH, OCH3, or OC.,H5, the nature of the OY group is without appreciable effect on either diuretic potency or toxicity. In contrast, the nature of the R group, which is commonly rather complex, has a very great effect on both toxicity and diuretic activity. In the diuretic mersalyl, R is o-carbamylphenoxyacetic acid; in mercaptomerin, R is camphoramic acid; in meralluride, R is succinyl-urea; in chlormerodrin, R is urea; and in mercumatilin, R is coumarin. Much of the interest in the structure-activity relationships of mercurial compounds has centered around the effects of modification of R and relatively less attention has been paid to simple organo-mercurial compounds. A wide variety of substituted allyl compounds

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.

Measurement of Extracellular Fluid Volume in Nephrectomized Dogs1

The volumes of distribution of a number of substances of widely varying molecular size and chemical properties, including sulfate, thiosulfate, mannitol, sucrose, and inulin, have been found to represent 15 to 25 per cent of body weight of man, dog, and other mammals. The apparently equal volumes of distribution of these dissimilar substances has suggested that the distribution of each of these substances measures the same portion of body fluid and has led to efforts to identify this volume of body fluid with the extracellular fluid volume. In a series of individuals, however, there is a considerable range in the fraction of body weight which the volume of distribution of any one of these substances represents. In only a few instances have the distributions of two of these substances been compared simultaneously in the same individual. Schwartz (1) reported a ratio of thiosulfate volume to simultaneously measured mannitol volume of 0.90 (0.87 to 0.96) in four normal dogs, a ratio of 1.02 and 0.98 in two normal human subjects, and a ratio of inulin to simultaneously measured mannitol volume of 0.97 (0.93 to 1.02) in six normal human subjects (2). Walser, Seldin, and Grollman (3) reported a ratio of radiosulfate volume to simultaneously measured inulin volume of 0.95 (S.D. + 0.11) in nine normal human subjects. Deane, Schreiner, and Robertson (4) measured volumes of sucrose and inulin in each of four normal human subjects on separate occasions and found an average ratio of sucrose volume to inulin volume of 0.97 (0.94 to 1.03). Most of these comparisons depend on quantitative recovery in the urine of infused inulin. Kruhaffers (5) data indicate a larger volume of distribution for sucrose than inulin in nephrectomized rabbits. Raisz, Young, and Stinson (6) have reported a ratio of inulin volume to thio-

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.