Acidification of the urine and increased ammonium excretion without change in acid-base equilibrium: sodium reabsorption as a stimulus to the acidifying process

Primary Distal Renal Tubular Acidosis: Toward an Optimal Correction of Metabolic Acidosis

The term classic, type 1 renal tubular acidosis or primary distal renal tubular acidosis is used to designate patients with impaired ability to excrete acid normally in the urine as a result of tubular transport defects involving type A intercalated cells in the collecting duct. The clinical phenotype is largely characterized by the complications of chronic metabolic acidosis (MA): stunted growth, bone abnormalities, and nephrocalcinosis and nephrolithiasis that develop as the consequence of hypercalciuria and hypocitraturia. All these manifestations are preventable with early and sustained correction of MA with alkali therapy. The optimal target for plasma bicarbonate should be as close as possible to the range considered normal by current standards (between 23 and 28 mEq/L.). Most of the benefits of alkali therapy are tangible early in the course of the disease in childhood, but life-long treatment is required to prevent the vast array of complications attributable to chronic MA.

Disorders of Acid-Base Balance: New Perspectives

BACKGROUND

Disorders of acid-base involve the complex interplay of many organ systems including brain, lungs, kidney, and liver. Compensations for acid-base disturbances within the brain are more complete, while limitations of compensations are more apparent for most systemic disorders. However, some of the limitations on compensations are necessary to survival, in that preservation of oxygenation, energy balance, cognition, electrolyte, and fluid balance are connected mechanistically.

SUMMARY

This review aims to give new and comprehensive perspective on understanding acid-base balance and identifying associated disorders. All metabolic acid-base disorders can be approached in the context of the relative losses or gains of electrolytes or a change in the anion gap in body fluids. Acid-base and electrolyte balance are connected not only at the cellular level but also in daily clinical practice. Urine chemistry is essential to understanding electrolyte excretion and renal compensations.

KEY MESSAGES

Many constructs are helpful to understand acid-base, but these models are not mutually exclusive. Electroneutrality and the close interconnection between electrolyte and acid-base balance are important concepts to apply in acid-base diagnoses. All models have complexity and shortcuts that can help in practice. There is no reason to dismiss any of the present constructs, and there is benefit in a combined approach.

Urinary acidification assessed by simultaneous furosemide and fludrocortisone treatment: an alternative to ammonium chloride

Distal renal tubular acidosis (RTA) can lead to rickets in children or osteomalacia in adults if undetected. This disorder is normally diagnosed by means of an oral ammonium chloride-loading test; however, the procedure often leads to vomiting and abandonment of the test. In this study, we assess an alternative, more palatable approach to test urinary acidification. This was achieved by the simultaneous oral administration of the diuretic furosemide and the mineralocorticoid fludrocortisone to increase distal tubular sodium delivery, principal cell sodium reabsorption, and alpha-intercalated cell proton secretion. We evaluated 11 control subjects and 10 patients with known distal RTA by giving oral ammonium chloride or furosemide/fludrocortisone in random order on separate days. One control and two patients were unable to complete the study owing to vomiting after NH4Cl; however, there were no adverse effects with the furosemide/fludrocortisone treatment. The urine pH decreased to less than 5.3 in the controls with both tests, whereas none of the patients was able to lower the urine pH below 5.3 with either test. We conclude that the simultaneous administration of furosemide and fludrocortisone provides an easy, effective, and well-tolerated alternative to the standard ammonium chloride urinary acidification test for the diagnosis of distal RTA.

Acid-base disorders in medicine

The practice of internal medicine involves daily exposure to abnormalities of acid-base balance. A wide variety of disease states either predispose patients to develop these conditions or lead to the use of medications that alter renal, gastrointestinal, or pulmonary function and secondarily alter acid-base balance. In addition, primary acid-base disease follows specific forms of renal tubular dysfunction (renal tubular acidosis). We review the acid-base physiologic functions of the kidney and gastrointestinal tract and the current understanding of acid-base pathophysiologic conditions. This includes a review of whole animal and renal tubular physiologic characteristics and a discussion of the current knowledge of the molecular biology of acid-base transport. We stress an approach to diagnosis that relies on knowledge of acid-base physiologic function, and we include discussion of the appropriate treatment of each disorder considered. Finally, we include a discussion of the effects of acidosis and alkalosis on human physiologic functions.

Chronic neutral phosphate supplementation induces sustained, renal metabolic alkalosis

The aim of the present study was to test whether intravenous neutral phosphate supplementation, recently shown in our laboratory to acutely stimulate proton secretion in the distal nephron, was able to induce a sustained metabolic alkalosis. Neutral Na and K phosphate supplementation for seven days, with equivalent reduction in chloride supply and unchanged intake of sodium and potassium, in ADX rats receiving fixed physiological doses of aldosterone and dexamethasone (group 1, N = 7), was responsible for a severe metabolic alkalosis (MA; delta [HCO3] 11 +/- 1.3 mM, and delta pH 0.11 +/- 0.06 unit). Metabolic alkalosis was at least in part of renal origin, since net acid excretion (NAE) transiently increased, principally due to an increment in titratable acid excretion rate. Balances were equilibrated for sodium and negative for chloride and potassium, which may have contributed to the severity of the MA. Chronic i.v. neutral Na phosphate, without change in potassium and chloride supply, in ADX rats receiving the same doses of steroids (group 2, N = 5), was responsible for a less severe MA (delta [HCO3] 7.5 +/- 0.9 mM, and delta pH 0.07 +/- 0.01 unit), also of renal origin. In this group, balances were positive for chloride and sodium and equilibrated for potassium. Finally, neutral Na and K phosphate supplementation with reduction in chloride supply in intact rats (group 3, N = 4) was also able to induce a MA (delta [HCO3] 5.5 +/- 1.8 mM, and delta pH 0.06 +/- 0.01 unit) of renal origin, with balances negative for chloride and equilibrated for potassium and sodium. In all groups, the generation and maintenance of MA probably resulted from stimulated proton secretion in the distal nephron, as suggested by the observed increase of PCO2 over HCO3 concentration ratio in the urine and a fall in urine pH despite augmented urinary buffer content throughout the phosphate infusion period. Glomerular filtration rate did not significantly vary in any group. In conclusion, chronic supplementation of neutral phosphate appears to stimulate per se proton secretion in the distal nephron, independently of sodium, chloride, and potassium balances, and adrenal steroid secretion. Thus neutral phosphate supplementation should be added to the previously known factors able to induce MA.

Distal renal tubular acidosis: the value of urinary pH, PCO2 and NH4+ measurements

Distal renal tubular acidosis (dRTA) is not a single disease. The experimental forms of the syndrome are unsatisfactory as models of the naturally occurring disease, not least because they are seldom complicated by nephrocalcinosis, which is present in the majority of patients with spontaneous disease and contributes to the renal tubular defects found in the syndrome. Impairment of minimal urine pH, reduced urine carbon dioxide tension (PCO2) during passage of alkaline urine, and reduced urinary ammonium (NH4+) excretion, have all been advocated as essential criteria for the diagnosis of dRTA. Minimal urine pH, measured during metabolic acidosis, sulphate infusion, or after oral frusemide, is the yardstick against which other criteria should be assessed. A reduced urinary PCO2 is commonly found in dRTA but is not specific for the syndrome and may be accounted for by tubular defects other than those involving reduced distal hydrogen ion secretion. NH4+ excretion is reduced in most patients with renal acidosis whatever the nature of the underlying renal disease; this function is closely related to nephron mass, and is not specifically impaired in renal tubular disease.

Transient renal acidification defect during acute infantile diarrhea: the role of urinary sodium

We studied urinary acidification daily during the hospital course of 16 infants with acute gastroenteritis and metabolic acidosis. Urine pH value on admission was higher than 5.5 in 14 (87%) patients. We hypothesized that inappropriate urinary acidification was due to sodium deficiency and inadequate sodium delivery to the distal nephron. Forty-one urinary samples were collected during metabolic acidosis. The mean pH of 24 urine samples with sodium concentration less than 10 mmol/L was significantly higher than the pH of 17 samples with sodium concentration greater than 10 mmol/L (6.04 +/- 0.06 vs 5.19 +/- 0.1; p less than 0.001). The urine ratios of titratable acid to creatinine and of total acidity to creatinine were significantly higher in urine samples containing more sodium (p less than 0.02), whereas the ammonium/creatinine ratio was not. After administration of furosemide or correction of the sodium deficit, appropriate acidification was observed. We conclude that impaired urinary acidification is frequently found during metabolic acidosis in infants with acute gastroenteritis and results from a sodium deficit rather than from transient distal renal tubular acidosis.

Coordinately increased lysozymuria and lysosomal enzymuria induced by maleic acid

During the acute renal tubular dysfunction of Fanconi syndrome and type 2 renal tubular acidosis (FS/RTA2) induced by maleic acid in the unanesthetized dog, we observed: 30 minutes after the onset of FS/RTA2, the urinary excretion of lysosomal enzymes, N-acetyl-beta-glucosaminidase (NAG), beta-glucuronidase (beta-gluc) and beta-galactosidase (beta-galac), increased simultaneously with the anticipated increase in renal clearance of lysozyme; the severities of all these hyperenzymurias increased rapidly, progressively, and in parallel, all reaching a peak some 60 to 80 minutes after their onset; thereafter, while the FS/RTA2 continued undiminished in severity, the severity of the hyperenzymurias decreased rapidly, greatly, progressively, and in parallel; and sodium phosphate loading strikingly attenuated the FS/RTA2 and the hyperenzymurias. Thus, the maleic acid-induced FS/RTA2 is attended by an acute reversible-complex derangement in the renal tubular processing of proteins that: affects not only lysozyme which is normally filtered, but also NAG and other lysosomal enzymes, which are not; and is to some extent functionally separable from that of FS/RTA2. The findings suggest that the derangements in renal processing of lysozyme and lysosomal enzymes are linked, and that a phosphate-dependent metabolic abnormality in the proximal tubule can participate in the pathogenesis of both these derangements and the FS/RTA2.

The regulation of renal acid secretion: new observations from studies of distal nephron segments

In this review we have attempted to present for the general reader the new information on renal acidification that has emerged from the study of discrete segments of the distal nephron. We have structured our presentation in the context of the cation exchange hypothesis which has strongly influenced modern thinking of acid-base regulation. We have shown that distal nephron acidification is active and can proceed even in the absence of sodium. We have also shown beyond doubt, that pH or the determinants of pH can influence the rate of proton secretion in probably all of the distal nephron segments. We have drawn attention to an exciting new means by which chloride (or its substitution) could alter the rate of net bicarbonate transport. A possible role for bicarbonate secretory activity in the mammalian distal nephron has been discussed as has the influence of mineralocorticoids on acid secretion. There is no question that all of this new information has created the need for a reassessment of the validity of the cation exchange hypothesis. After all, this is a view which specifically denies that renal acid excretion is modulated by pH of the blood, and affirms that it is intrarenal sodium handling that is the "driving force", so to speak, behind acidification responses. However, it seems inappropriate at this time to insist that current data do not allow for a component of sodium transport by the distal nephron to modulate the rate of acid secretion. It is also possible, as we have suggested, that an important effect of chloride gradients, independent of blood pH, could alter bicarbonate retrieval. Most importantly, we wish to stress that much of the in vitro perfusion data does not derive from animals subjected to the chronic acid-base derangements which were precisely those situations to which the cation exchange hypothesis was directed. Simply put, the whole animal studies of Schwartz and his colleagues provided no experimental observations on intrarenal sodium handling or acidification mechanisms, just as the microperfusion studies, both in vivo and in vitro, provide insufficient data that can be applied to whole animals subjected to chronic disturbances.(ABSTRACT TRUNCATED AT 400 WORDS)

Hyperkalemic distal renal tubular acidosis in salt-losing congenital adrenal hyperplasia

Functional indices of distal urinary acidification were assessed in two male infants, aged 1 and 3 months, with salt-losing congenital adrenal hyperplasia. In both cases the diagnosis was sustained by the presence of elevated plasma levels of 17-hydroxyprogesterone, hyponatremia, hyperkalemia, metabolic acidosis and increased plasma renin activity. Both patients were unable to lower urinary pH below 5.9 either during acute ammonium chloride-induced acidosis or after i.v. administration of furosemide. One patient also failed to decrease urine pH below 5.5 and to increase urinary potassium excretion during sodium sulfate infusion. Oral sodium bicarbonate loading was given to both patients but failed to induce a significant increase in the urine minus blood PCO2 gradient. This gradient remained low also after neutral phosphate administration. Repeated studies after acute administration of fludrocortisone in one case or after prolonged administration of hydrocortisone in the other resulted in complete normalization of all functional studies. We conclude that salt-losing congenital adrenal hyperplasia can lead to hyperkalemic distal renal tubular acidosis in early infancy. The defective renal secretion of hydrogen ion and potassium is probably related to the abolishment of the negative potential difference in the cortical collecting tubule induced by the impaired reabsorption of sodium.

Hypophosphaturia impairs the renal defense against metabolic acidosis

It is known that Pi normally provides the major source of non-NH3 urinary buffer and that Pi-buffered renal H+ excretion (titratable acidity, TA) accounts for a large fraction of daily renal net acid excretion (NAE). Whether the presence of luminal non-NH3 buffers is a prerequisite to normal renal regulation of systemic acid-base equilibrium under any conditions has not been investigated. Accordingly, I investigated whether chronic renal regulation of plasma (p) [HCO3] might be impaired under conditions of normophosphatemic hypophosphaturia (NHP) produced by short-term dietary Pi restriction. During a steady-state of HCl-induced acidosis in NaCl-replete NHP dogs (group 1A, N = 6), [HCO3-]p averaged 14.1 +/- 0.6 mEq/liter and arterial (a) [H+] averaged 54 +/- 2 nEq/liter. Substitution K+ 2.5 mEq/kg as neutral Pi for equivalent dietary KCl for 7 to 8 days resulted in significant amelioration of acidosis (delta [HCO3-]p + 2.2 +/- 0.5 mEq/liter, P less than 0.01; delta [H+]a -6 +/- 2 nEq/liter, P less than 0.01) in association with a cumulative increment (sigma delta) in TA excretion (+ 103 mEq, P less than 0.001) and NAE (+ 22 mEq). To investigate whether Pi-induced amelioration of acidosis was related to enhanced urinary buffer capacity, an additional group (group 1B, N = 5) with NHP and chronic HCl acidosis was administered the non-Pi buffer, neutral creatinine (5.0 mmoles/kg daily). As with Pi, acidosis was ameliorated by creatinine administration and sigma delta NAE increased.(ABSTRACT TRUNCATED AT 250 WORDS)

Renal tubular acidosis induced by dietary chloride

Previous studies have demonstrated that dietary intake of anions with high renal reabsorbability (Cl- greater than SO4=) can result in either exacerbation of chronic metabolic acidosis or correction of chronic metabolic alkalosis. These results, however, fail to predict the renal acid-base response to Cl- administration when systemic acid-base composition is initially normal, but accompanied by an extracellular fluid (ECF) volume-mediated renal avidity for Cl- reabsorption; that is, the renal options include HCl retention, KCl retention, and phosphaturia. Accordingly, the present metabolic balance studies evaluated the response to substitution of dietary Cl- (2.5 mEq/kg/day) for Pi in five dogs previously ECF-depleted with diuretics and maintained on a dietary K+ supplement, 5.0 mEq/kg daily as neutral Pi (electrolyte-free diet) during a steady-state control period. Dietary Cl- resulted in a decrease in arterial plasma [HCO3-] from 21.2 +/- 0.7 to 17.8 +/- 0.8 mEq/liter, (P less than 0.01) and increase in [H+] from 38.5 +/- 0.7 to 43.3 +/- 0.8 nEq/liter (P less than 0.001). Urine pH increased (P less than 0.01), the cumulative change in net acid excretion decreased (-79 mEq, P less than 0.05), and Cl- retention (39 mEq, P less than 0.05) occurred. No change in Na+, K+, or Pi excretion occurred. The renal acidosis was fully corrected when SO4= was substituted for dietary Cl- and redeveloped when Cl- was resubstituted . Superimposition of a large oral buffer load (creatinine) did not ameliorate Cl- -induced renal acidosis. The results indicate that dietary reabsorbable anions can result in renal acidosis when Cl- reabsorption is stimulated and suggest that anion reabsorbability characteristics and not anion buffer properties are responsible.

Characterization of acidification in the cortical and medullary collecting tubule of the rabbit

Ouabain and lithium decrease acidification in open-circuited bladders by eliminating the electrical gradient favoring acidification. The effect of ouabain and lithium on acidification in cortical and medullary collecting tubules derived from starved New Zealand white rabbits was studied by using the techniques of isolated nephron microperfusion and microcalorimetric determination of total CO2 flux. Bath and perfusion solutions were symmetric throughout all studies, and solutions contained 25 meq of bicarbonate and were bubbled with 93.3% O2/6.7% CO2 gas mixtures. In cortical collecting tubules, ouabain (10(-8) M) addition to bath resulted in a decrease in both potential difference (PD), from -16.4 to -2.2 mV (P less than 0.001), and total CO2 flux (JTCO2), from +6.0 to 1.5 pmol/mm per min (P less than 0.005). In medullary collecting tubules neither PD nor JTCO2 changed with the addition of ouabain in either 10(-8) or 10(-4) M concentration. Replacement of 40 mM NaCl with 40 mM LiCl in both perfusate and bath in cortical collecting tubules resulted in decreases in both PD, from -11.6 to 0.4 mV (P less than 0.005), and JTCO2, from +10.8 to +4.2 pmol/mm per min (P less than 0.025). This substitution had no effect on medullary collecting tubules. When control flux rates were plotted against animal bladder urine pH, both medullary and cortical tubules showed good inverse correlation between these variables, with higher values of flux rate for the medullary tubules. The data support a role for transepithelial PD in acidification in the cortical collecting tubule and also suggest that both cortical and medullary segments of the collecting tubule participate when urinary acidification is increased during starvation in the rabbit.

Mineralo- and glucocorticoid effects on renal excretion of electrolytes

The acute effects of mineralo- and glucocorticoids on urinary electrolyte excretion were studied in the conscious, acutely potassium deprived, adrenalectomized rat. Sodium, potassium, and creatinine were measured in the urine excreted from 2.5 to 5.5 h after injection of one or more of the following steroids: aldosterone (Aldo), 9-alpha fluorocortisol (FC), deoxycorticosterone (DOC), dexamethasone (Dex), and spironolactone (Spiro). The hierarchy (a) for increasing creatinine excretion was Dex greater than FC greater than Aldo greater than DOC greater than Spiro greater than none, a hierarchy consistent with glucocorticoid potency; and (b) for producing anti-natriuresis was Aldo greater than DOC greater than or equal to FC greater than or equal to none = Spiro greater than Dex, a hierarchy consistent with mineralocorticoid potency. In contrast, the kaliuresis produced by mineralo- and glucocorticoids appears different. A "mineralocorticoid" kaliuresis is 1) elicited by anti-natriuretic doses of Aldo and FC, 2) approximately twice control UKV, 3) unrelated to changes in glomerular filtration rate (GFR), and 4) inhibited by Spiro. A "glucocorticoid" kaliuresis is 1) elicited by Dex and high doses of Aldo and FC, 2) about seven to twenty-fold greater than control UKV, 3) possibly dependent, in part, on changes in GFR, and, 4) not inhibited by Spiro. DOC was not kaliuretic at anti-natriuretic doses. The urinary Na/K ratio was an unreliable index of mineralocorticoid action.

The acidosis of chronic renal failure

The acidosis of chronic renal failure is not due to bicarbonate wastage per se; rather, bicarbonate reabsorption per nephron is markedly enhanced. The ability to lower the urine pH is preserved. While overall ammonium production may be decreased in chronic renal failure, both ammonium production and excretion are markedly increased when expressed per remaining nephron. Titratable acid excretion in chronic renal failure is essentially maximal, owing to the effect of parathyroid hormone on phosphate excretion by the kidney. Thus, it appears that the acidosis of chronic renal failure is solely the consequence of the reduction in functional renal mass. Extrarenal buffering may contribute substantially to the maintenance of a near normal acid-base status in patients with marked reduction in glomerular filtration rate. That homeostasis is so well preserved until glomerular filtration rate falls to approximately 10 per cent of normal is remarkable; the price, however, may be considerable. Prolonged acidosis may magnify the tendency of renal failure to cause osteodystrophy. An obvious treatment for the acidosis of renal failure is exogenous alkali therapy. Most clinicians withhold alkali therapy until the bicarbonate concentration falls below 20 mEq per L. If the acidosis cannot be safely corrected with exogenous therapy, dialysis should be initiated.

Distal renal tubular acidosis: pathogenesis and classification

Distal renal tubular acidosis results from ineffective addition of hydrogen ions to the lumen of the distal nephron. The syndrome is manifested by hyperchloremic metabolic acidosis often associated with hypokalemia. More recently, it has been recognized that hyperkalemia rather than hypokalemia can be a dominant feature of some cases of distal renal tubular acidosis. It has been generally accepted that all cases of this syndrome ultimately resulted from a similar mechanism. The prevailing view was that the abnormality underlying distal renal tubular acidosis was that of inability to either generate or maintain a steep pH gradient across the distal nephron. Recent advances in our understanding of the process of distal acidification have provided evidence that different mechanisms can alter distal hydrogen ion secretion. In this article, the significance of the various indices of urinary acidification and their use in the characterization of the mechanism underlying distal renal tubular acidosis are revised. A classification of distal renal tubular acidosis on the basis of mechanism is presented. The importance of plasma potassium and renal potassium excretion in the evaluation of patients with distal renal tubular acidosis is emphasized.

Distal renal tubular acidosis with intact capacity to lower urinary pH

The sine qua non for the diagnosis of distal renal tubular acidosis requires that the urinary pH cannot decrease maximally during systemic acidosis. A defect in distal acidification however, could also result from a decrease in the capacity (or rate) of distal hydrogen ion secretion. In this type of defect, the ability to lower the urinary pH during acidemia could be preserved as long as a certain capacity for hydrogen ion secretion remained. In this report, we describe four patients with deranged distal urinary acidification, in whom urinary pH was able to decrease (4.99 +/- 0.11) during acidemia. One of the patients had hyperchloremic metabolic acidosis whereas the remaining three were not spontaneously acidotic. In these patients, the defect for distal urinary acidification was disclosed by the inability of the urine-blood pCO2 gradient to increase normally (i.e., above 30 mm Hg) during bicarbonate loading. In contrast, a normal increase in the urine-blood pCO2 gradient was observed in each patient in response to neutral sodium phosphate infusion. The reabsorptive capacity of bicarbonate was not depressed in these patients, which indicated that the acidification process in the proximal nephron was intact. We propose that our four patients had a defect in distal urinary acidification caused by a reduction in the rate of distal hydrogen ion secretion rather than an inability to generate a steep pH gradient across the distal nephron. Our data also suggest that the inability to raise urinary pCO2 normally during sodium bicarbonate loading may be the most sensitive index of decreased distal urinary acidification available.

Hyperkalemic hyperchloremic metabolic acidosis in sickle cell hemoglobinopathies

This report describes the occurrence of hyperkalemic hyperchloremic metabolic acidosis in six patients with sickle cell hemoglobinopathies. Three patients had sickle cell anemia, two had sickle cell trait and one had S-C disease. In all patients, decreased renal potassium excretion was demonstrated by the finding of a fractional potassium excretion lower than that of control subjects with comparable glomerular filtration rates. Two patterns of impaired urinary acidification were discerned. Four patients had a urinary pH above 5.5 in the presence of systemic acidosis and, thus, were classified a having distal renal tubular acidosis. The remaining two patients had very low rates of ammonium excretion despite intact capacity to lower urinary pH below 5.5 during systemic acidosis; this pattern was ascribed to selective aldosterone deficiency. Sickle cell hemoglobinopathies should be included in the differential diagnosis of hyperkalemic hyperchloremic metabolic acidosis.

Studies on the regulation of hydrogen ion secretion in the collecting duct in vivo: evaluation of factors that influence the urine minus blood PCO2 difference

The purpose of these studies was to clarify the basis of the relationship between the urine bicarbonate concentration and the urine minus blood PCO2 difference in alkaline urine (U-B PCO2) and hence shed light on factors that influence hydrogen ion secretion in the collecting duct in vivo. The U-B PCO2 was used to monitor this latter parameter. In dogs with a normal extracellular fluid (ECF) volume, the U-B PCO2 was not primarily influenced by the urine bicarbonate concentration but rather it was related to the rate of sodium excretion. The U-B PCO2 could be abolished by amiloride when the urine bicarbonate concentration was less than 60 mm. At higher urine bicarbonate concentrations, there was a linear correlation between the U-B PCO2 and the urine bicarbonate concentration in normovolemic dogs given amiloride, but the absolute values were lower than they were in normovolemic animals not treated with amiloride. In the dogs with an expanded ECF volume, the U-B PCO2 was lower than it was in the normovolemic animals, and the U-B PCO2 was nor directly related to the urine bicarbonate concentration and not influenced by the rate of sodium excretion. Amiloride had little influence on the U-B PCO2 under these conditions. These results are interpreted to suggest that the magnitude of collecting duct hydrogen ion secretion is determined primarily by the electrical gradient generated by sodium reabsorption in normovolemic dogs and by the intracellular and lumenal hydrogen ion concentrations when the ECF volume is expanded or when active sodium reabsorption is inhibited by amiloride.

Renal net acid excretion in the adrenalectomized rat

Although adrenalectomy is usually associated with an impairment of ammonium and/or titratable acid excretion by the kidney, it is uncertain whether rates of renal net acid excretion are also reduced. Further, it is unclear whether the absence of the adrenal gland itself or other factors of adrenal insufficiency mediate such changes in renal acidification parameters. For example, dramatic increases in ammonium excretion can accompany correction of the hyperkalemia seen in adrenal insufficiency. There is also evidence that reduced rates of acid excretion can result from changes in food intake, urine flow rate, urine pH or distal sodium delivery rates. With these considerations in mind, we undertook studies to isolate the chronic effects of adrenalectomy on renal net acid excretion rates in the unanaesthetized rat. To avoid supranormal potassium stores, we gave the adrenalectomized animals potassium-restricted diets. In balance studies, urine flow rates, urine pH, food intake, and distal sodium delivery rates were all successfully controlled for 13 days by pair feeding and by appropriately changing the sodium and potassium contents of diets. Adrenalectomized rats excreted less net acid than did control animals with or without ammonium chloride loading. Further, the severe metabolic acidosis associated with ammonium chloride loading was clearly mitigated by steroid replacement. Accordingly, we conclude that the adrenal gland is essential for normal renal net acid excretion.

Hyperkalemic distal renal tubular acidosis associated with obstructive uropathy

We studied renal function in 13 patients with obstructive uropathy and hyperkalemic metabolic acidosis to characterize the pathogenesis of this disorder. Base-line fractional potassium excretion was lower in all patients than in controls with similar glomerular filtration rates. Acetazolamide was given to 11 patients but failed to increase fractional potassium excretion to normal. In five patients, impaired potassium excretion was associated with decreased ammonium excretion, a urinary pH below 5.5 (5.18 +/- 0.07, mean +/- S.E.M.), and aldosterone deficiency. In the remaining eight patients, the urinary pH did not fall below 5.5 (6.4 +/- 0.2) with acidosis, and we failed to lower the urinary pH and increase fractional potassium excretion to normal by administering a mineralocorticoid and sodium sulfate. A syndrome of hyperkalemic distal renal tubular acidosis may occur in patients with obstructive uropathy. In some patients, this syndrome results from a defect in hydrogen and potassium secretion in the distal nephron rather than from aldosterone deficiency. Obstructive uropathy should be included in the differential diagnosis of hyperkalemic acidosis and renal insufficiency.

Effect of mineralocorticoid replacement therapy on renal acid-base homeostasis in adrenalectomized patients

Chronic balance studies were performed in six adrenalectomized patients to investigate the renal and systemic acid-base consequences of mineralocorticoid deficiency in the absence of either glucocorticoid deficiency or parenchymal renal disease. Constant glucocorticoid replacement was provided with dexamethasone, 750 to 875 micrograms/day, administered orally. Creatinine clearance averaged 98 +/- 8 ml/min/1.73 m2. Following a control period, mineralocorticoid replacement with fludrocortisone (100 to 200 micrograms/day) was either discontinued (N = 3) or initiated (N = 2). In an additional patient, mineralocorticoid replacement was initiated and sustained (5 days) by continuous i.v. infusion of aldosterone, at a dose approximating the normal secretion rate (120 micrograms/day). Net acid excretion (NAE) and plasma total carbon dioxide decreased in each patient in whom mineralocorticoid was discontinued and increased in each patient in whom mineralocorticoid was initiated. The cumulative change in NAE (sigma delta NAE) independent of direction averaged 66 +/- 20 mEq (P less than 0.05) by the fifth experimental day in the six patients, and the corresponding change in plasma total CO2 averaged 1.2 +/- 0.3 mmoles/liter (P less than 0.02). The magnitude of sigma delta NAE correlated with the basal rate of NAE (r = 0.87, P less than 0.05), which averaged 0.9 +/- 0.1 mEq/kg body wt per day. The change in plasma total CO2 correlated with sigma delta NAE (r = 0.83, P less than 0.05). The changes in NAE correlated positively with the corresponding changes in sodium balance and negatively with the corresponding changes in potassium balance. These findings provide the first evidence that renal acidification is under tonic stimulation by mineralocorticoid at levels not exceeding those in normal subjects ingesting acid-producing diets of normal sodium and potassium content. The extent to which the tonic stimulation of renal acidification is mediated by a direct effect of mineralocorticoid on renal hydrogen ion transport or by an indirect effect dependent on altered renal sodium and/or potassium transport requires further investigation. The findings implicate mineralocorticoid deficiency as a significant renal acidosis-producing condition not dependent on the presence of renal disease or glucocorticoid deficiency, and potentially amplified when endogenous acid production is increased by diet or disease.

Mechanism of the metabolic acidosis of selective mineralocorticoid deficiency

The mechanism of generation of metabolic acidosis in selective mineralocorticoid deficiency was investigated in bilaterally adrenalectomized (ADX) rats treated with dexamethasone and in sham-operated (S) rats. ADX rats had significantly lower plasma sodium and bicarbonate concentrations and significantly higher plasma potassium concentrations than S rats did. ADX rats developed negative sodium balance when fed a "zero" sodium diet. The minimum urine pH achieved during sodium sulfate infusion and during ammonium chloride administration was not significantly different between ADX and S rats. Bicarbonate reabsorption and urine minus blood PCO2 gradient were not different between ADX and S rats. For any given urine pH, absolute ammonium excretion was significantly lower in ADX than it was in S rats, both during sodium sulfate infusion and during chronic ammonium chloride administration. Glomerular filtration rate (GFR) was significantly lower in ADX than it was in S rats; ammonium excretion corrected for GFR was not different between the two groups. To determine the role of decreased distal sodium delivery (secondary to decrease in GFR and enhanced proximal sodium reabsorption which resulted from distal sodium chloride wastage) on ammonium excretion, ADX rats were fed 0.9% sodium chloride in an effort to keep body weight constant. Salt-loaded ADX rats had a plasma bicarbonate concentration higher than did S rats. Salt-loading also led to a significant increase in GFR; absolute ammonium excretion was significantly higher than that of other ADX rats with the same degree of acidosis. At comparable levels of GFR, there was no difference in ammonium excretion between ADX and S rats. Ammonium excretion was linearly related to GFR. ADX rats fed a zero potassium diet had significantly greater ammonium excretion than did all other groups of ADX or S rats receiving a normal potassium intake. These data suggest that volume contraction is a major factor responsible for the acidosis of selective mineralocorticoid deficiency.

The nature of the renal response to chronic disorders of acid-base equilibrium

The rate of acid excretion by the kidney appears to be determined by factors regulating the site and the rate of sodium reabsorption, rather than by a homeostatic mechanism that responds to systemic pH. This hypothesis, although unconventional, is supported by much experimental evidence, and it accounts for a wide variety of clinical and physiologic findings that heretofore have been difficult or impossible to explain.

Hydrogen ion metabolism

The renal and respiratory systems regulate acid base homeostasis by modifying the bicarbonate buffer pair HCO3- and CO2; other buffer systems adjust to alterations in this pair. The pH change following addition of metabolic acid, or base, is modified initially by the body's buffers. Subsequent respiratory compensation, by retention or excretion of CO2, modifies further this change before renal corrective responses finally occur. A primary respiratory acid base change is modified initially by cellular buffers, with renal compensatory mechanisms adjusting slowly to this change. However correction of the respiratory pH disorder only occurs with correction of the primary disease process. The body's capacity, therefore, to adjust to a matabolic acid base defect, appears greater than that observed with a primary respiratory pH abnormality. Treatment in all acid base disorders focuses initially upon the primary diseases process; thereafter therapeutic manipulation of the HCO3-, CO2 buffer pair, to aid the body's compensatory processes, can be considered.

Relationship of renal ammonia production and potassium homeostasis

Renal ammonia production appears to be intimately related to potassium homeostasis, and the two may comprise the components of a closed loop regulatory system. Studies with both intact organisms and in vitro systems indicate that potassium depletion stimulates and chronic potassium-loading suppresses renal ammonia production. An increase in ammoniagenesis has been shown to decrease potassium excretion. These observations suggest that changes in potassium modulate ammonia production, which in turn maintains hydrogen ion homeostasis and influences potassium excretion. Potassium depletion increases rat renal cortical ammonia production by altering metabolism in fashion identical to metabolic acidosis, but there is no convincing evidence that both processes are mediated by similar changes in either cellular hydrogen ion or potassium concentration. By contrast, potassium-loading, which depresses ammonia production, appears to affect primarily the outer medulla, a region that is not influenced by potassium depletion. Thus, potassium-loading apparently affects different portions of the renal tubule than depletion does, but the specific mechanism and physiologic significance of the different sites of action is unknown.

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.