Distal renal tubular acidosis with intact capacity to lower urinary pH

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.

Phenotypic variability in distal acidification defects associated with WDR72 mutations

BACKGROUND

Distal renal tubular acidosis (RTA) is typically caused by defects in ATP6V0A4, ATP6V1B1, and SLC4A1, accounting for 60-80% of patients. Genes recently implicated include FOXI1, ATP6V1C2, and WDR72, of which WDR72 is associated with dental enamel defects.

METHODS

We describe 4 patients, from three unrelated consanguineous families, with RTA and amelogenesis imperfecta. Distal tubular acidification was evaluated by furosemide-fludrocortisone test, urine-to-blood PCO2 gradient and fractional excretion of bicarbonate. Exome sequencing was performed using a panel of genes implicated in human disease.

RESULTS

Patients had polyuria, hypokalemia, hypercalciuria, and nephrocalcinosis, but metabolic acidosis varied in severity. Although all patients acidified urine to pH < 5.3 during furosemide-fludrocortisone test, urine-to-blood PCO₂ gradient was < 20 mmHg during bicarbonate loading. All patients had transient proximal tubular dysfunction with urinary losses of phosphate and beta-2-microglobulin, and generalized aminoaciduria. Homozygous pathogenic truncating variants in WDR72 was detected in all probands.

CONCLUSION

Patients with WDR72 mutations show mild rate-dependent distal RTA with variable metabolic acidosis, and intact ability to acidify the urine on provocative testing. Concomitant proximal tubular dysfunction may be present. Mutations in WDR72 should be considered in patients with suspected distal RTA, especially if associated with dental defects.

Renal Tubular Acidosis

Renal tubular acidosis (RTA) comprises a group of disorders characterized by low capacity for net acid excretion and persistent hyperchloremic metabolic acidosis, despite preserved glomerular filtration rate. RTA are classified into chiefly three types (1, 2 and 4) based on pathophysiology and clinical and laboratory characteristics. Most patients have primary RTA that presents in infancy with polyuria, growth retardation, rickets and/or hypotonia. Diagnosis requires careful evaluation, including exclusion of other entities that can cause acidosis. A variety of tests, administered stepwise, are useful for the diagnosis and characterization of RTA. A genetic or acquired basis can be determined in majority of patients through focused evaluation. Management involves correction of acidosis and dyselectrolytemia; patients with proximal RTA with Fanconi syndrome and rickets require additional supplements of phosphate and vitamin D.

Hypokalemic Distal Renal Tubular Acidosis

Distal renal tubular acidosis (DRTA) is defined as hyperchloremic, non-anion gap metabolic acidosis with impaired urinary acid excretion in the presence of a normal or moderately reduced glomerular filtration rate. Failure in urinary acid excretion results from reduced H⁺ secretion by intercalated cells in the distal nephron. This results in decreased excretion of NH₄⁺ and other acids collectively referred as titratable acids while urine pH is typically above 5.5 in the face of systemic acidosis. The clinical phenotype in patients with DRTA is characterized by stunted growth with bone abnormalities in children as well as nephrocalcinosis and nephrolithiasis that develop as the consequence of hypercalciuria, hypocitraturia, and relatively alkaline urine. Hypokalemia is a striking finding that accounts for muscle weakness and requires continued treatment together with alkali-based therapies. This review will focus on the mechanisms responsible for impaired acid excretion and urinary potassium wastage, the clinical features, and diagnostic approaches of hypokalemic DRTA, both inherited and acquired.

Renal Tubular Acidosis: H+/Base and Ammonia Transport Abnormalities and Clinical Syndromes

Renal tubular acidosis (RTA) represents a group of diseases characterized by (1) a normal anion gap metabolic acidosis; (2) abnormalities in renal HCO3- absorption or new renal HCO3- generation; (3) changes in renal NH4+, Ca2+, K+, and H2O homeostasis; and (4) extrarenal manifestations that provide etiologic diagnostic clues. The focus of this review is to give a general overview of the pathogenesis of the various clinical syndromes causing RTA with a particular emphasis on type I (hypokalemic distal RTA) and type II (proximal) RTA while reviewing their pathogenesis from a physiological "bottom-up" approach. In addition, the factors involved in the generation of metabolic acidosis in both type I and II RTA are reviewed highlighting the importance of altered renal ammonia production/partitioning and new HCO3- generation. Our understanding of the underlying tubular transport and extrarenal abnormalities has significantly improved since the first recognition of RTA as a clinical entity because of significant advances in clinical acid-base chemistry, whole tubule and single-cell H+/base transport, and the molecular characterization of the various transporters and channels that are functionally affected in patients with RTA. Despite these advances, additional studies are needed to address the underlying mechanisms involved in hypokalemia, altered ammonia production/partitioning, hypercalciuria, nephrocalcinosis, cystic abnormalities, and CKD progression in these patients.

Clinical Approach to Proximal Renal Tubular Acidosis in Children

Proximal renal tubular acidosis (pRTA) is an inherited or acquired clinical syndrome in which there is a decreased bicarbonate reclamation in the proximal tubule resulting in normal anion gap hyperchloremic metabolic acidosis. In children, pRTA may be isolated but is often associated with a general proximal tubular dysfunction known as Fanconi syndrome which frequently heralds an underlying systemic disorder from which it arises. When accompanied by Fanconi syndrome, pRTA is characterized by additional renal wasting of phosphate, glucose, uric acid, and amino acids. The most common cause of inherited Fanconi syndrome in the pediatric age group is cystinosis, a disease with therapeutic implications. In this article, we summarize the clinical presentation and differential diagnosis of pRTA and Fanconi syndrome and provide a practical approach to their evaluation in children.

New Findings on the Pathogenesis of Distal Renal Tubular Acidosis

Background

Distal renal tubular acidosis (dRTA) is characterized by an impairment of the urinary acidification process in the distal nephron. Complete or incomplete metabolic acidosis coupled with inappropriately alkaline urine are the hallmarks of this condition. Genetic forms of dRTA are caused by loss of function mutations of either SLC4A1, encoding the AE1 anion exchanger, or ATP6V1B1 and ATP6V0A4, encoding for the B1 and a4 subunits of the vH⁺ATPase, respectively. These genes are crucial for the function of A-type intercalated cells (A-IC) of the distal nephron.

Summary

Alterations of acid-base homeostasis are variably associated with hypokalemia, hypercalciuria, nephrocalcinosis or nephrolithiasis, and a salt-losing phenotype. Here we report the diagnostic test and the underlying physiopathological mechanisms. The molecular mechanisms identified so far can explain the defect in acid secretion, but do not explain all clinical features. We review the latest experimental findings on the pathogenesis of dRTA, reporting mechanisms that are instrumental for the clinician and potentially inspiring a novel therapeutic strategy.

Key Message

Primary dRTA is usually intended as a single-cell disease because the A-IC are mainly affected. However, novel evidence shows that different cell types of the nephron may contribute to the signs and symptoms, moving the focus from a single-cell towards a renal disease.

Primary sclerosing cholangitis: a new cause of distal renal tubular acidosis

We describe the first case of distal renal tubular acidosis (dRTA) associated with primary sclerosing cholangitis. A 26-year-old Lao-Thai male patient presented with severe jaundice, metabolic acidosis and hypokalaemia. He was diagnosed of dRTA. Liver transplantation resulted in correction of electrolyte disturbances and hyperbilirubinaemia. A fludrocortisone-furosemide test revealed normal urinary acidification, demonstrating no residual dRTA. This observation suggests that dRTA may be an early manifestation of bilirubin-associated nephropathy or the consequence of an immune mechanism.

Unique chloride-sensing properties of WNK4 permit the distal nephron to modulate potassium homeostasis

Dietary potassium deficiency activates thiazide-sensitive sodium chloride cotransport along the distal nephron. This may explain, in part, the hypertension and cardiovascular mortality observed in individuals who consume a low-potassium diet. Recent data suggest that plasma potassium affects the distal nephron directly by influencing intracellular chloride, an inhibitor of the with-no-lysine kinase (WNK)-Ste20p-related proline- and alanine-rich kinase (SPAK) pathway. As previous studies used extreme dietary manipulations, we sought to determine whether the relationship between potassium and NaCl cotransporter (NCC) is physiologically relevant and clarify the mechanisms involved. We report that modest changes in both dietary and plasma potassium affect NCC in vivo. Kinase assay studies showed that chloride inhibits WNK4 kinase activity at lower concentrations than it inhibits activity of WNK1 or WNK3. Also, chloride inhibited WNK4 within the range of distal cell chloride concentration. Mutation of a previously identified WNK chloride-binding motif converted WNK4 effects on SPAK from inhibitory to stimulatory in mammalian cells. Disruption of this motif in WNKs 1, 3, and 4 had different effects on NCC, consistent with the three WNKs having different chloride sensitivities. Thus, potassium effects on NCC are graded within the physiological range, which explains how unique chloride-sensing properties of WNK4 enable it to mediate effects of potassium on NCC in vivo.

Beyond the definitions of the phenotypic complications of sickle cell disease: an update on management

The sickle hemoglobin is an abnormal hemoglobin due to point mutation (GAG → GTG) in exon 1 of the β globin gene resulting in the substitution of glutamic acid by valine at position 6 of the β globin polypeptide chain. Although the molecular lesion is a single-point mutation, the sickle gene is pleiotropic in nature causing multiple phenotypic expressions that constitute the various complications of sickle cell disease in general and sickle cell anemia in particular. The disease itself is chronic in nature but many of its complications are acute such as the recurrent acute painful crises (its hallmark), acute chest syndrome, and priapism. These complications vary considerably among patients, in the same patient with time, among countries and with age and sex. To date, there is no well-established consensus among providers on the management of the complications of sickle cell disease due in part to lack of evidence and in part to differences in the experience of providers. It is the aim of this paper to review available current approaches to manage the major complications of sickle cell disease. We hope that this will establish another preliminary forum among providers that may eventually lead the way to better outcomes.

Complete renal tubular acidosis late after kidney transplantation

BACKGROUND

Neither the prevalence nor the associated risk factors of late post-transplant renal tubular acidosis (RTA) are known.

METHODS

We conducted a cross-sectional study with 576 patients for more than 12 months after kidney transplantation, and a glomerular filtration rate (GFR) >40 ml/min. RTA was diagnosed by measurement of the urine anionic gap, urine pH and plasma potassium during acidosis, and fractional bicarbonate excretion after bicarbonate loading. Uni- and multi-variable analysis were used to isolate factors associated with post-transplant RTA, and with the different RTA subtypes.

RESULTS

All patients (n = 76) had distal post-transplant RTA. A significant association with the presence of RTA was found for the intake of tacrolimus or renin-angiotensin-aldosterone blockers, the Parathyroid hormone level and the GFR. Type Ia (classic, distal), type Ib (hyperkalaemic, voltage-dependent), rate-limited and type IV RTA were present in 37, 14, 21 and 28% of the patients. Acute transplant rejection was the only significant different parameter between the RTA subtypes and more often present in patients with type Ia or Ib RTA.

CONCLUSIONS

We conclude that a significant fraction of stable long-term renal transplant recipients with adequate graft function develop post-transplant RTA, with a preponderance for type Ia and type IV, and absence of type II. In addition, acute transplant rejection seems to have an influence on the subtype of RTA present post-transplantation.

The urine-blood PCO gradient as a diagnostic index of H(+)-ATPase defect distal renal tubular acidosis

BACKGROUND

Urine pH during acidemia and urine PCO2 upon alkalization both may be useful to indicate H+ secretion from collecting ducts. The urine anion gap has been used to detect urinary NH4+ for differential diagnosis of hyperchloremic metabolic acidosis. We have previously demonstrated that the lack of normal H(+)-ATPase may underlie secretory defect distal renal tubular acidosis (dRTA). In this study we evaluated the diagnostic value of the urine-blood (U-B) PCO2 in H(+)-ATPase defect dRTA, and compared it with that of urine pH and urine anion gap during acidemia.

METHODS

In H(+)-ATPase defect dRTA, the diagnostic values of three urinary parameters were evaluated: (1) urine pH measured after acid (NH4Cl) loading; (2) urine-to-blood carbon dioxide tension gradient (U-B PCO2) during alkali (NaHCO3) loading; and (3) urine anion gap during acidemia. Seventeen patients were diagnosed as having H(+)-ATPase defect dRTA based on reduced urinary NH4+ and an absolute decrease in H(+)-ATPase immunostaining in intercalated cells on renal biopsy. Eight patients with non-dRTA renal disease served as control patients.

RESULTS

Upon NaHCO3 loading, U-B PCO2 was < or =30 mm Hg in all 17 dRTA patients and >30 mm Hg in all 8 control patients. With NH4Cl loading, urine pH was >5.4 in 15 of 17 dRTA patients and < or =5.4 in 7 of 8 control patients, and the urine anion gap was >5 mmol/L in 13 of 17 dRTA patients and< or =5 mmol/L in 6 of 8 control patients. Therefore, the sensitivity and specificity of U-B PCO2 < or =30 mm Hg during NaHCO3 loading were both 100%, whereas those of urine pH >5.4 or urine anion gap >5 mmol/L during NH4Cl loading were below 90%. In control patients, the U-B PCO2 was found to be well correlated with the urinary NH4+ (r= 0.79, P < 0.05).

CONCLUSION

The U-B PCO2 during NaHCO3 loading is an excellent diagnostic index of H(+)-ATPase defect dRTA.

Renal tubular acidosis and vasculitis associated with IgE deposits in the kidney and small vessels

We report a woman with a history of allergies, polyuria, polydipsia, proteinuria, renal loss of electrolytes, renal tubular acidosis, nephrocalcinosis, and palpable purpura. A proximal defect was excluded by a normal bicarbonate reabsorption curve, and a distal tubular defect was shown because urine pH did not decrease to less than 6.4 despite ammonium chloride-induced systemic acidosis. Moreover, furosemide failed to improve urinary acidification. Urine-to-blood PCO(2) gradient was less than 14 mm Hg, although the urine bicarbonate level reached values as high as 89 mEq/L. Combining bicarbonate and neutral phosphate infusions increased the urine-to-blood PCO(2) gradient to only 20 mm Hg. These subnormal PCO(2) gradient values point to proton-pump dysfunction in the collecting tubule. Histological evidence of tubulointerstitial disease accompanied the tubular defects. The striking histological feature was the presence of immunoglobulin E (IgE) deposits in glomeruli, tubuli, and vessels. Concurrent with these findings, she had high serum IgE titers and CD23 levels. IgE antibodies from her serum were reactive against human renal tubuli, with binding to two regions that matched two different proteins present in cortex and medulla. One of these proteins corresponded to carbonic anhydrase II (31 kd); the second, to an unidentified protein that seems attached to cell membranes. We suggest that these IgE antibodies could have had a pathogenic role in this patient's glomerular, tubular, and small-vessel disease.

Study of urinary acidification in patients with idiopathic hypocitraturia

Hypocitraturia (HCit) is one of the most remarkable features of renal tubular acidosis, but an acidification defect is not seen in the majority of hypocitraturic patients, whose disease is denoted idiopathic hypocitraturia. In order to assess the integrity of urinary acidification mechanisms in hypocitraturic idiopathic calcium stone formers, we studied two groups of patients, hypocitraturic (HCit, N = 21, 39.5 +/- 11.5 years, 11 females and 10 males) and normocitraturic (NCit, N = 23, 40.2 +/- 11.7 years, 16 females and 7 males) subjects, during a short ammonium chloride loading test lasting 8 h. During the baseline period HCit patients showed significantly higher levels of titratable acid (TA). After the administration of ammonium chloride, mean urinary pH (3rd to 8th hour) and TA and ammonium excretion did not differ significantly between groups. Conversely, during the first hour mean urinary pH was lower and TA and ammonium excretion was higher in HCit. The enhanced TA excretion by HCit during the baseline period and during the first hour suggests that the phosphate buffer mechanism is activated. The earlier response in ammonium excretion by HCit further supports other evidence that acidification mechanisms react promptly. The present results suggest that in the course of lithiasic disease, hypocitraturia coexists with subtle changes in the excretion of hydrogen ions in basal situations.

Use of the urine-to-blood carbon dioxide tension gradient as a measurement of impaired distal tubular hydrogen ion secretion among neonates

To evaluate the utility of the urinary-minus-blood partial pressure of carbon dioxide (U-B PCO2) gradient for the diagnosis of distal renal tubular acidosis in neonates, we measured the U-B PCO2 gradient corresponding to different urinary bicarbonate concentrations in 40 neonates. The U-B PCO2 gradient in these neonates had a significant linear relationship to the urinary bicarbonate concentration. When the urinary bicarbonate concentration was > 10 mmol/L, in all the neonates the U-B PCO2 could be increased above the 20 mm Hg level. We conclude that it is appropriate to determine the U-B PCO2 gradient as an index of distal urinary acidification and that it is a necessary test for diagnosis of distal renal tubular acidosis in neonates.

Pathophysiology of the renal acidification defect present in the syndrome of familial hypomagnesaemia-hypercalciuria

A distal acidification defect is frequently observed in the syndrome of familial hypomagnesaemia-hypercalciuria and hence this condition can be confused with primary distal renal tubular acidosis (RTA). This study demonstrates that in four unrelated patients with familial hypomagnesaemia-hypercalciuria the acidification defect is functionally different from that present in primary distal RTA. All patients exhibited hypomagnesaemia, hypermagnesuria, hypercalciuria, hyposthenuria, nephrocalcinosis and slight reduction of glomerular filtration rate (GFR). A moderate degree of metabolic acidosis was also present and basal data showed an inappropriately high urine pH (5.7-5.9) and a positive urine anion gap (Na + K-Cl = 11-28 mmol/l). Stimulation of distal acidification induced a fall in urine pH (4.7-5.6), but ammonium excretion remained low despite factoring by GFR (26-46 mumol/min per 1.73 m2, 35-54 mumol/100 ml GF). The urine to blood PCO2 gradient also remained low after sodium bicarbonate loading (1.3-17.7 mmHg). These results are best explained by both defective ammonia transfer to the deep nephron and impaired hydrogen ion secretion at the level of the medullary collecting duct, and probably are secondary effects of the medullary interstitial nephropathy.

Renal acidification in children with idiopathic hypercalciuria

Distal renal tubular acidosis is frequently associated with hypercalciuria. To further investigate the cause-and-effect relationships between the two conditions, we examined 20 children (5 to 18 years of age) with idiopathic hypercalciuria for evidence of renal tubular acidosis. Serum electrolytes and urine citrate levels were normal in all subjects. After a single dose of furosemide, 1 of the 20 subjects did not show a decrease in urine pH < 5.5, which suggests an acidification defect in the cortical collecting duct. Three other patients failed to show an increase in urine-minus-blood partial pressure of carbon dioxide > 20 mmHg after urine alkalinization with orally administered acetazolamide, a finding compatible with a rate-dependent distal renal tubular acidosis. These four subjects underwent acute acid loading with arginine hydrochloride. In all four subjects urine pH decreased < 5.5 but urinary ammonium excretion failed to increase normally; this supports the diagnosis of a defect in distal acidification. Four of six patients with nephrolithiasis had evidence of distal renal tubular acidosis, in contrast to none of the 14 patients without stones (p = 0.003). We conclude that distal acidification abilities seem to be intact in children with hypercalciuria in the absence of nephrolithiasis. We speculate that calcium precipitation may lead to tubular damage, including distal renal tubular acidosis.

Urine-to-blood carbon dioxide tension gradient and maximal depression of urinary pH to distinguish rate-dependent from classic distal renal tubular acidosis in children

We determined the prevalence and clinical features of rate-dependent distal renal tubular acidosis (dRTA) in 31 children examined for possible renal tubular acidosis by measuring the urinary-minus-blood partial pressure of carbon dioxide (U-B PCO2) gradient, minimal urinary pH, and fractional excretion of bicarbonate. Of 20 patients with low U-B PCO2 gradients, nine could not lower urinary pH < or = 5.5, indicating classic dRTA, whereas 11 could lower urinary pH < or = 5.5, as described in rate-dependent dRTA. When patients with rate-dependent dRTA and classic (type I) dRTA were compared, there was no difference in the mean U-B PCO2 gradient or in clinical findings, including age, reason for referral, presence of nephrocalcinosis, or depression of linear growth. We conclude that children with rate-dependent dRTA are susceptible to at least some of the same sequelae as children with classic dRTA. Measurement of minimal urinary pH will not detect this subtle form of dRTA. Determination of the U-B PCO2 gradient should be considered a routine part of evaluation for suspected renal tubular acidosis in a child.

Might distal renal tubular acidosis be a proximal tubular cell disorder?

Incomplete renal tubular acidosis (RTA) and overt distal RTA may be different stages of the same underlying pathophysiology in certain individuals. The rationale that draws these conditions together is the relatively alkaline pH of the urine, hypocitraturia, and a possible familial association. The rate of excretion of ammonium (NH4+), on the other hand, suggests that these conditions stem from fundamentally different lesions. To explain this difference, we suggest that two possible disorders may result in the evolution from incomplete RTA to overt distal RTA. One subgroup could have gradient-limited distal RTA, while the other subgroup may have a lower pH of the intracellular fluid of the proximal convoluted tubular epithelium. Indices of proximal intracellular pH (rates of excretion of NH4+, NH3, and citrate) were culled from the literature spanning the years 1959 to 1991 on patients with incomplete RTA and overt distal RTA. Three points emerge: (1) the rate of excretion of NH4+ was lower in patients with overt distal RTA than in normals following an acute acid load (23 +/- 1 v 49 +/- 3 mumol/min); (2) the concentration of NH3 in the urine was almost 25-fold higher in incomplete RTA than in normals (69 +/- 14 v 3 +/- 0.4 nmol/min); and (3) in incomplete RTA, the pH of the urine fell to very low values (4.9 +/- 0.1) when high urine flows were induced with furosemide. The low pH of the urine would therefore suggest that many of these patients do not gradient-limited distal RTA, but more likely have proximal renal epithelial cell acidosis. We hypothesize that this high rate of excretion of NH4+ and low rate of excretion of citrate in the absence of acidosis or hypokalemia is consistent with proximal cell acidosis. To explain a transition from incomplete RTA to overt distal RTA, we speculate that toxicity of high concentrations of NH3 in the medullary interstitium as well as nephrolithiasis and nephrocalcinosis due to low urinary citrate and possibly an alkaline medullary interstitium may lead to damage of structures in this region.

Urinary acidification in renal stone patients from northeastern Thailand

Hypokalemia, hypokaliuria and hypocitraturia are common findings in patients with renal stone disease in Northeastern Thailand. However, hyperchloremic metabolic acidosis seldom is seen. Therefore, we studied renal acidification in 29 renal stone disease patients who were living in rural Northeast Thailand. Baseline blood and average 24-hour urine biochemical parameters were measured. Hypokalemia, hypokaliuria and hypocitraturia were found in 10%, 83% and 93% of the patients, respectively. By multiple regression, urinary citrate excretion correlated positively with serum potassium and urinary potassium excretion, and negatively with urinary ammonium (r = 0.640, p = 0.005). An abnormal response to acid loading was found in only 1 patient. Thus, hypokaliuria and hypocitraturia in our renal stone disease subjects were infrequently due to distal renal tubular acidosis. Perhaps potassium depletion might be a contributing factor in these metabolic abnormalities.

Should the urine PCO2 or the rate of excretion of ammonium be the gold standard to diagnose distal renal tubular acidosis?

A high rate of excretion of ammonium (NH4+) during chronic metabolic acidosis should rule out the diagnosis of distal renal tubular acidosis (RTA). Bearing this in mind, the purpose of this report is to demonstrate that a low urine minus blood PCO2 difference in alkaline urine (U-B PCO2) is a less reliable indicator of the diagnosis of distal RTA. The patient who is the subject of this report sniffs glue on a chronic, but intermittent basis. He presented with metabolic acidosis (pH 7.20; bicarbonate, 10 mmol/L) and an anion gap in plasma of 20 mEq/L. The urine anion gap (-14 mEq/L) and osmolal gap (185 mmol/L [mOsm/kg] H2O) suggested that there was a high, rather than a low, rate of excretion of NH4+. This was confirmed by direct measurement of NH4+ in the urine (101 mumol/min). The high rate of excretion of NH4+ suggested that the metabolic acidosis was due, in large part, to an abnormally high rate of production of acid (hippuric acid, because the rate of excretion of hippurate was 76 mumol/min). The U-B PCO2 was low (10 mm Hg) on the second hospital day, after the acidosis was corrected. Potential reasons for the discrepancy between the high rate of excretion of NH4+ and the low U-B PCO2 are discussed.

Oral acetazolamide in the assessment of (urine-blood) PCO2

Urine-blood (U-B)Pco2 difference in children is usually assessed following urine alkalinization with oral sodium bicarbonate (NaHCO3). Since oral NaHCO3 is often poorly tolerated by children, we compared oral acetazolamide with oral NaHCO3 in a study of (U-B)Pco2. In the first phase of the study 14 children and adolescents aged 11.1 +/- 3.7 years (mean +/- SD) were studied. Eight participants had normal kidney function and 6 had disturbed distal acidification capacity. Each child was studied twice, once with oral NaHCO3 (2.5 mEq/kg) and once with acetazolamide (17 +/- 2 mg/kg). All studies were performed according to the standard protocol. Acetazolamide administration resulted in a lower blood pH than NaHCO3 (7.30 +/- 0.03 vs 7.38 +/- 0.06, P less than 0.001) and a lower serum bicarbonate (HCO3-) concentration (25.1 +/- 2.2 mEq/l vs 27.5 +/- 2.1 mEq/l, P less than 0.025). Acetazolamide also resulted in a higher urine Pco2 (81.9 +/- 26.2 mm Hg vs 71.6 +/- 18.2 mm Hg) than NaHCO3 (P less than 0.025). No significant differences between acetazolamide and NaHCO3 were observed with respect to their effects on urinary pH and HCO3- concentration, plasma Pco2 and (U-B)Pco2. Good linear correlations were found between the effects of acetazolamide and NaHCO3 on urine Pco2 (r = 0.878, P less than 0.001), and on (U-B)Pco2 (r = 0.795, P less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Type 4 renal tubular acidosis (subtype 2) in a patient with methylmalonic acidaemia

A 10-month-old male infant with vitamin B12 non-responsive methylmalonic acidaemia is reported. Laboratory results revealed hyperkalaemic, hyperchloraemic, metabolic acidosis with slight azotaemia. The urinary pH decreased (below 5.5) to compensate for acidaemia. Levels of plasma renin activity and plasma aldosterone concentration were low. The renal biopsy showed tubulo-interstitial nephritis. We suggested the diagnosis of type 4 renal tubular acidosis, subtype 2, i.e. hyporeninaemic hypoaldosteronism. We suggest that chronic renal disease may be a common complication of methylmalonic acidaemia.

Selective effect of mineralocorticoid replacement therapy on renal acid excretion in congenital adrenal hyperplasia

The renal acid excretion of eight children with salt-losing congenital adrenal hyperplasia, was studied in three different situations: before treatment (period I), under glucocorticoid therapy (period II) and when both glucocorticoid and mineralocorticoid were given as replacement treatment (period III). Although administration of glucocorticoid therapy alone allowed the correction of acidemia, normalization of urinary net acid excretion was achieved only after mineralocorticoid was added to the treatment.

Renal tubular acidosis

The term renal tubular acidosis (RTA) is applied to a group of transport defects in the reabsorption of bicarbonate (HCO3-), the excretion of hydrogen ions, or both. On clinical and pathophysiological grounds, RTA can be separated into three main types: distal RTA (type 1), proximal RTA (type 2) and hyperkalaemic RTA (type 4). Some patients present combined types of proximal and distal RTA or of hyperkalaemic and distal RTA. Diagnosis of RTA should be suspected when a patient presents a normal plasma anion gap, and hyperchloraemic metabolic acidosis. A normal plasma anion gap (Na(+)-[Cl- + HCO3-] = 8-16 mEq/l) reflects loss of HCO3- from the extracellular fluid via the gastro-intestinal tract or the kidney, dilution of extracellular buffer or administration of hydrochloric acid (HCl) or its precursors. Distinction of RTA from other disorders is greatly facilitated by the study of the urine anion gap (Na+ + K+ - Cl-). This index estimates the urinary concentration of ammonium in a patient with hyperchloraemic metabolic acidosis. A negative urine anion gap (Cl- much greater than Na+ + K+) suggests the presence of gastro-intestinal or renal loss of HCO3-, while a positive urine anion gap (Cl- less than Na+ + K+) is indicative of a distal acidification defect. Determination of plasma potassium, of urine pH at low plasma HCO3- concentration, and of urine PCO2 and fractional excretion of HCO3- at normal plasma HCO3- concentration permits the differentiation between the various types of RTA.

A patient with hyperkalemia and metabolic acidosis

Uptake of potassium by extrarenal tissues, primarily muscle and liver, represents a major defense mechanism in the maintenance of normokalemia following an acute elevation in the serum potassium concentration. Insulin, epinephrine, and aldosterone all play major roles in maintaining the normal distribution of potassium between the intracellular and extracellular environment. In addition to hormonal regulation, changes in blood pH and tonicity also exert a strong influence on extrarenal potassium metabolism. Last, the serum potassium concentration per se directly influences its own cellular uptake and this transport mechanism appears to be inhibited by uremia.

Two case studies from a family with primary Fanconi syndrome

We report two cases from a family with primary Fanconi syndrome. A 39-year-old white woman with a history of frequent bone fractures developed hypophosphatemia, hypouricemia, hypokalemia, metabolic acidosis, and renal glycosuria. Her 15-year-old son had renal glycosuria without metabolic acidosis. Both had mildly to moderately impaired renal function. Determination of amino acids in 24-hour urine specimens confirmed the generalized nature of aminoaciduria. Acid-loading, bicarbonate-loading, and phosphate-loading tests revealed that the mother had proximal (type II) renal tubular acidosis and excessive renal loss of phosphate for her level of renal function. These tests for the son were normal or within normal limits of his renal function. Known causes of Fanconi syndrome, such as cystinosis and Wilson's disease, were excluded by slit-lamp eye examination and leukocyte cystine level determination. One unexpected finding in the son was the preseNce of nephrocalcinosis on x-rays; a percutaneous needle biopsy of the kidney showed tubular atrophy, interstitial fibrosis, and calcium oxalate crystal deposits. The two cases presented here represent a familial variety of the primary Fanconi syndrome, a rare entity with a limited number of cases reported in the literature.

The acidification response of normal subjects to ammonium chloride using a 3-day loading test

The acidification response to NH4 Cl loading (0.1 g/kg bw/day) was tested in 16 normal healthy subjects in the basal fasting state on Day 4, the subjects having taken the salt daily for the 3 previous days. The response was measured in terms of blood pH and in urine, creatinine, phosphate, pH, titratable acidity, ammonium, net acidity and creatinine clearance. To minimise inter-subject variation the urine values were adjusted to a standard body surface area of 1.73 m2. A normal range for the blood pH of the mean value +/- 2 SD, encompassed the observed range of values. However, to fit the observed range of acid-base values in urine into the 2 SD range required a logarithmic transformation of the data. Statistical analysis confirmed a significant correlation between blood [H+] net acid secretion, urine titratable acidity and ammonium. Urine net acid secretion was positively correlated with urinary phosphate, titratable acidity and ammonium.

The use of the urinary anion gap in the diagnosis of hyperchloremic metabolic acidosis

We evaluated the use of the urinary anion gap (sodium plus potassium minus chloride) in assessing hyperchloremic metabolic acidosis in 38 patients with altered distal urinary acidification and in 8 patients with diarrhea. In seven normal subjects given ammonium chloride for three days, the anion gap was negative (-27 +/- 9.8 mmol per liter) and the urinary pH under 5.3 (4.9 +/- 0.03). In the eight patients with diarrhea the anion gap was also negative (-20 +/- 5.7 mmol per liter), even though the urinary pH was above 5.3 (5.64 +/- 0.14). In contrast, the anion gap was positive in all patients with altered urinary acidification, who were classified as having classic renal tubular acidosis (23 +/- 4.1 mmol per liter, 11 patients), hyperkalemic distal renal tubular acidosis (30 +/- 4.2, 12 patients), or selective aldosterone deficiency (39 +/- 4.2, 15 patients). When the data on all subjects studied were pooled, a negative correlation was found between the urinary ammonium level and the urinary anion gap. We conclude that the use of the urinary anion gap, as a rough index of urinary ammonium, may be helpful in the initial evaluation of hyperchloremic metabolic acidosis. A negative anion gap suggests gastrointestinal loss of bicarbonate, whereas a positive anion gap suggests the presence of altered distal urinary acidification.

Prevalence, pathogenesis, and treatment of renal dysfunction associated with chronic lithium therapy

From the analysis of several studies published from 1979 to 1986 comprising 1,172 patients, we estimated that glomerular filtration rate (GFR) was normal in 85% of unselected patients on chronic lithium therapy. The remaining 15% of patients displayed only mild reduction in GFR, clustering at approximately 60 mL/min. Thus, the data available to date do not support earlier concerns that long-term lithium therapy could eventuate into renal insufficiency. The most prevalent renal effect of lithium is impairment of concentrating ability, which we estimated to be present in at least 54% of 1,105 unselected patients on chronic lithium therapy. This defect translated into overt polyuria in only 19% of unselected cases. A renal lesion confined to the collecting tubule has been described in humans who have taken lithium for short periods of time. This lesion may represent the collecting tubule's response to the intracellular accumulation of lithium, which interferes with cAMP formation and results in an early and probably reversible inhibition of antidiuretic hormone (ADH)-mediated water transport. However, long-term lithium therapy may induce a progressive and partly irreversible defect in concentrating ability. The potential risk for dehydration associated with lithium-induced polyuria, as well as the discomfort inherent to this side effect, deserves evaluation and consideration for therapeutic intervention. Amiloride has additional advantages over conventional treatment of nephrogenic diabetes insipidus using thiazide diuretics. The action of amiloride on ADH-mediated water transport seems specific in as much as it is capable of preventing the uptake of lithium in high resistance epithelia and thereby prevents the inhibitory effect of intracellular lithium on water transport. Unlike thiazides, amiloride has a weak natriuretic effect and is less likely to increase plasma lithium levels by causing volume contraction. In addition, amiloride, by conserving potassium, obviates the need for potassium supplementation that is usually required to prevent hypokalemia when thiazides are used to treat lithium-induced polyuria. Since amiloride may prevent chronic intracellular lithium accumulation in the collecting tubule, future studies should elucidate whether amiloride also has a role in preventing lithium-induced chronic tubulo-interstitial damage.

Type 4 renal tubular acidosis (sub-type 2) associated with idiopathic interstitial nephritis

An 18-month-old girl presenting with anorexia and failure to thrive, was referred for adenoidectomy. Arterial hypertension was discovered on physical examination. Laboratory results revealed hyperkalaemic, hyperchloraemic, metabolic acidosis, with slight azotemia. Urinary aldosterone excretion and plasma renin were decreased. Renal biopsy showed idiopathic interstitial nephritis. The diagnosis of type 4 renal tubular acidosis, sub-type 2, i.e. primary hyporeninaemic secondary hypoaldosteronism was proposed. According to our knowledge, this disease has not previously been reported in young children, but is well known in azotaemic adults. We therefore propose the inclusion of this uncommon renal disease in the differential diagnosis of failure to thrive in childhood.

Segmental characterization of defects in collecting tubule acidification

The aim of this study was to investigate cortical collecting tubule (CCT) function in normal individuals and in patients with distal renal tubular acidosis (DRTA) using furosemide (80 mg orally) as a tool to stimulate H+ and K+ secretion by enhancing Na delivery and transport in this nephron segment. In ten normal subjects, furosemide resulted in a fall in urine pH below 5.5 and an increase in net acid and K+ excretion. These effects were obliterated by amiloride, a drug which decreases transtubular epithelial voltage (lumen-negative) in the CCT by blocking Na reabsorption. In 13 patients with DRTA, defined by failure to lower urine pH below 5.5 during acidemia, three distinctive responses to furosemide were found. In six patients with the hyperkalemic variety, furosemide failed to lower urine pH below 5.5 and resulted in a blunted increase in K+ excretion, thereby suggesting that a normal transtubular voltage in the CCT could not be generated in such patients. In five patients with classic RTA, furosemide failed to lower urine pH below 5.5, but K+ excretion increased normally. The increase in K+ excretion indicated that a normal transtubular voltage in the CCT could be generated, while the inability to lower urine pH denotes the presence of a proton pump defect involving the CCT. In two patients with classic RTA, furosemide resulted in both a normal fall in urine pH and an increase in K+ excretion, thereby indicating that the CCT was normal in regards to both proton pump function and in its ability to generate a normal transtubular voltage.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship of urinary and blood carbon dioxide tension during hypercapnia in the rat. Its significance in the evaluation of collecting duct hydrogen ion secretion

This study was designed to establish the relationship between urinary pCO2 and systemic blood pCO2 during acute hypercapnia and to investigate the significance of this relationship to collecting duct hydrogen ion (H+) secretion when the urine is acid and when it is highly alkaline. In rats excreting a highly alkaline urine, an acute increase in blood pCO2 (from 42 +/- 0.8 to 87 +/- 0.8 mmHg) resulted in a significant fall in urine minus blood (U-B) pCO2 (from 31 +/- 2.0 to 16 +/- 4.2 mmHg, P less than 0.005), a finding which could be interpreted to indicate inhibition of collecting duct H+ secretion by hypercapnia. The urinary pCO2 of rats with hypercapnia, unlike that of normocapnic controls, was significantly lower than that of blood when the urine was acid (58 +/- 6.3 and 86 +/- 1.7 mmHg, P less than 0.001) and when it was alkalinized in the face of accelerated carbonic acid dehydration by infusion of carbonic anhydrase (78 +/- 2.7 and 87 +/- 1.8 mmHg, P less than 0.02). The finding of a urinary pCO2 lower than systemic blood pCO2 during hypercapnia suggested that the urine pCO2 prevailing before bicarbonate loading should be known and the blood pCO2 kept constant to evaluate collecting duct H+ secretion using the urinary pCO2 technique. In experiments performed under these conditions, sodium bicarbonate infusion resulted in an increment in urinary pCO2 (i.e., a delta pCO2) which was comparable in hypercapnic and normocapnic rats (40 +/- 7.2 and 42 +/- 4.6 mmHg, respectively) that were alkalemic (blood pH 7.53 +/- 0.02 and 7.69 +/- 0.01, respectively). The U-B pCO2, however, was again lower in hypercapnic than in normocapnic rats (15 +/- 4.0 and 39 +/- 2.5 mmHg, respectively, P less than 0.001). In hypercapnic rats in which blood pH during bicarbonate infusion was not allowed to become alkalemic (7.38 +/- 0.01), the delta pCO2 was higher than that of normocapnic rats which were alkalemic (70 +/- 5.6 and 42 +/- 4.6 mmHg, respectively, P less than 0.005) while the U-B pCO2 was about the same (39 +/- 3.7 and 39 +/- 2.5 mmHg). We further examined urine pCO2 generation by measuring the difference between the urine pCO2 of a highly alkaline urine not containing carbonic anhydrase and that of an equally alkaline urine containing this enzyme. Carbonic anhydrase infusion to hypercapnic rats that were not alkalemic resulted in a fall in urine pCO(2) (from 122+/-5.7 to 77+/-2.2 mmHg) which was greater (P <0.02) than that seen in alkalemic normocapnic controls (from 73+/- 1.9 to 43+/-1.3 mmHg) with a comparable urine bicarbonate concentration and urine nonbicarbonate buffer capacity. CO(2) generation, therefore, from collecting dust H(+) secretion and titration of bicarbonate, was higher in hypercapnic rats that in normocapnic controls. We conclude that in rats with actue hypercapnia, the U-B p(CO(2)) achieved during bicarbonate loading greatly underestimates collecting duct H(+) secretion because it is artificially influenced by systemic blood pCO(2). the deltapCO(2) is a better qualitative index of collecting duct H+ secretion that the U-B pCO(2), because it is not artificially influenced by systemic blood pCO(2) and it takes into account the urine PCO(2) prevailing before bicarbonate loading.

Amelioration of polyuria by amiloride in patients receiving long-term lithium therapy

Vasopressin-resistant diabetes insipidus is a common side effect of the treatment of affective disorders with lithium. We studied the effect of amiloride on lithium-induced polyuria in nine such patients receiving maintenance lithium therapy who had a vasopressin-resistant defect in urinary concentrating ability. After a mean (+/- S.E.) of 24 +/- 6 days of amiloride administration, the urine volume fell (from 4.7 +/- 0.6 to 3.1 +/- 0.3 liters per 24 hours; P less than 0.005), and the urine osmolality increased (from 228 +/- 35 to 331 +/- 34 mOsm per kilogram of H2O; P less than 0.001). The decrease in urine output was sustained during six months of observation in the absence of any significant change in plasma levels of lithium, potassium, or bicarbonate; urinary excretion of sodium or lithium; or creatinine clearance. Amiloride administration was also associated with a significant increase in urine osmolality (from 575 +/- 54 to 699 +/- 48 mOsm per kilogram of H2O; P less than 0.005) measured after fluid deprivation and the injection of exogenous vasopressin. We conclude that amiloride mitigates lithium-induced polyuria, at least partly, by blunting the inhibitory effect of lithium on water transport in the renal collecting tubule. Thus, amiloride may provide a specific therapy for polyuria in lithium-treated patients while obviating the need for potassium supplementation in the treatment of this kind of polyuria.

Hyporeninemic hypoaldosteronism in a patient with multiple myeloma

A patient with progressive renal failure due to multiple myeloma presented with a mixed acid-base disorder (non-anion gap acidosis and respiratory alkalosis) with persistent severe hyperkalemia. Studies revealed an intact ability to lower urine pH during acid loading, markedly decreased plasma renin and aldosterone concentrations despite volume depletion, and an inappropriately low fractional excretion of potassium. Renal biopsy demonstrated plasma cell infiltration of the renal interstitium and typical proteinaceous intratubular casts. Both proximal and distal renal tubular acidification defects have been described previously in patients with multiple myeloma, but this is the first report of hyporeninemic hypoaldosteronism, hyperkalemia, and hyperchloremic metabolic acidosis in association with renal involvement in multiple myeloma.

[Primary hypoaldosteronism, pseudo-hypoaldosteronism and distal tubular acidosis]

Aldosterone deficiency is caused by various defects of aldosterone biosynthesis in the adrenal gland or hyporeninism. The most important symptoms are hyponatremia and hyperkalemia. These electrolyte disturbances are also found in pseudohypoaldosteronism. Pseudohypoaldosteronism type I is characterized by insensitivity of the distal nephron for aldosterone. Hyperabsorption of chloride in the distal nephron leads to pseudohypoaldosteronism type II, which is linked with hypertension, whereas blood pressure in the other mentioned disorders is decreased. Renal tubular acidosis, mainly type 4, with impaired production of ammonia due to hyperkalemia, is frequently observed in hypoaldosteronism and both types of pseudohypoaldosteronism as well. The therapeutic regimen is different: low doses of fludrocortisone in hypoaldosteronism, potassium restriction, sodium bicarbonate and loop diuretics in type I of pseudohypoaldosteronism, and sodium restriction and chloruretic diuretics (thiazide) in type II of pseudohypoaldosteronism. In some cases hyperkalemia requires the use of potassium-binding resins.

[Primary hypoaldosteronism and secondary pseudo-hypoaldosteronism]

We observed a 23-year-old man with pronounced hyperkalemia (max. 6.8 mmol/l) and hyponatremia (min. 112 mmol/l), which had been existent for 3 years without complaint except a transitory psychorganic syndrome due to hyponatremia. Physical examination showed no abnormality except hypotension (blood pressure 100/70 mmHg). Renal function tests were normal. Fractional clearance of sodium was significantly increased (0.8%), whereas that of potassium was decreased (2.4%). Plasma renin activity was tripled and rose after furosemide. Plasma aldosterone was lowered and showed no rise after furosemide. Suppression of plasma renin and aldosterone by saline infusion was normal. Pressor dose of angiotensin II was increased (17,9 ng AT II/kg/min). Urinary excretion of aldosterone and its conjugates was below normal, and aldosterone precursors were within normal range. The findings were interpreted as selective primary hypoaldosteronism caused by corticosterone methyl oxidase defect type II. However, neither fludrocortisone (0.5 mg/day) nor sodium chloride (200 mmol/day) led to a normalization of sodium and potassium in plasma. Additional pseudohypoaldosteronism was thus assumed. Aldosterone infusion (3 mg in 1 h) decreased renal excretion of sodium; potassium excretion failed, however, to increase in contrast to its pattern in normal man. These findings resemble additional pseudohypo-aldosteronism of type II. After 8 weeks' application of additional 80 mmol sodium (as sodium bicarbonate) plasma sodium and potassium showed normal values under combined treatment with fludrocortisone (0.1 mg/day) and sodium bicarbonate (80 mmol/day). It is to be assumed that the patient suffers from a reduced aldosterone biosynthesis in the presence of an additional transitory secondary pseudohypoaldosteronism.

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.