Renal bicarbonate reabsorption and hydrogen ion excretion in normal infants

A review of renal tubular acidosis

OBJECTIVE

To review the current scientific literature on renal tubular acidosis (RTA) in people and small animals, focusing on diseases in veterinary medicine that result in secondary RTA.

DATA SOURCES

Scientific reviews and original research publications on people and small animals focusing on RTA.

SUMMARY

RTA is characterized by defective renal acid-base regulation that results in normal anion gap hyperchloremic metabolic acidosis. Renal acid-base regulation includes the reabsorption and regeneration of bicarbonate in the renal proximal tubule and collecting ducts and the process of ammoniagenesis. RTA occurs as a primary genetic disorder or secondary to disease conditions. Based on pathophysiology, RTA is classified as distal or type 1 RTA, proximal or type 2 RTA, type 3 RTA or carbonic anhydrase II mutation, and type 4 or hyperkalemic RTA. Fanconi syndrome comprises proximal RTA with additional defects in proximal tubular function. Extensive research elucidating the genetic basis of RTA in people exists. RTA is a genetic disorder in the Basenji breed of dogs, where the mutation is known. Secondary RTA in human and veterinary medicine is the sequela of diseases that include immune-mediated, toxic, and infectious causes. Diagnosis and characterization of RTA include the measurement of urine pH and the evaluation of renal handling of substances that should affect acid or bicarbonate excretion.

CONCLUSIONS

Commonality exists between human and veterinary medicine among the types of RTA. Many genetic defects causing primary RTA are identified in people, but those in companion animals other than in the Basenji are unknown. Critically ill veterinary patients are often admitted to the ICU for diseases associated with secondary RTA, or they may develop RTA while hospitalized. Recognition and treatment of RTA may reverse tubular dysfunction and promote recovery by correcting metabolic acidosis.

Evaluation of urinary acidification in children: Clinical utility

The kidney plays a fundamental role in acid-base homeostasis by reabsorbing the filtered bicarbonate and by generating new bicarbonate, to replace that consumed in the buffering of non-volatile acids, a process that leads to the acidification of urine and the excretion of ammonium (NH₄ ⁺). Therefore, urine pH (UpH) and urinary NH₄ ⁺ (UNH₄ ⁺) are valuable parameters to assess urinary acidification. The adaptation of automated plasma NH₄ ⁺ quantification methods to measure UNH₄ ⁺ has proven to be an accurate and feasible technique, with diverse potential indications in clinical practice. Recently, reference values for spot urine NH₄ ⁺/creatinine ratio in children have been published. UpH and UNH₄ ⁺, aside from their classical application in the study of metabolic acidosis, have shown to be useful in the identification of incomplete distal renal tubular acidosis (dRTA), an acidification disorder, without overt metabolic acidosis, extensively described in adults, and barely known in children, in whom it has been found to be associated to hypocitraturia, congenital kidney abnormalities and growth impairment. In addition, a low UNH₄ ⁺ in chronic kidney disease (CKD) is a risk factor for glomerular filtration decay and mortality in adults, even in the absence of overt metabolic acidosis. We here emphasize on the need of measuring UpH and UNH₄ ⁺ in pediatric population, establishing reference values, as well as exploring their application in metabolic acidosis, CKD and disorders associated with incomplete dRTA, including growth retardation of unknown cause.

Incomplete distal renal tubular acidosis in children

AIM

To describe incomplete distal renal tubular acidosis (iDRTA) in paediatric patients, a term used for the diagnosis of patients who do not develop spontaneous overt metabolic acidosis but are unable to acidify the urine in response to an ammonium chloride load.

METHODS

Tests used to explore urinary acidification were revised. In addition, publications in English extracted from 161 entries yielded by a PubMed database search, using 'incomplete distal renal tubular acidosis' as keyword, were reviewed.

RESULTS

Incomplete distal renal tubular acidosis has mostly been identified in adults with autoimmune diseases, nephrolithiasis, nephrocalcinosis and/or osteopenia. iDRTA has been reported in few paediatric patients with rickets, congenital abnormalities of kidney and urological tract and/or growth failure. The pathophysiological mechanisms potentially responsible for the defect of urinary acidification are discussed as well as the clinical and biochemical findings of iDRTA described in children.

CONCLUSION

The presentation of iDRTA in children differs from adults. The clinical and biochemical features of iDRTA are not well characterised in paediatric patients. The detection of iDRTA in groups of population such as heterozygous carriers of primary DRTA gene mutations and children with hypocitraturia or hypercalciuria might be of clinical interest to better know the pathophysiology and natural history of iDRTA.

Potassium citrate and metabolic acidosis in children with epilepsy on the ketogenic diet: a prospective controlled study

AIM

To investigate if potassium citrate, a mild alkaline compound, can prevent metabolic acidosis in children with epilepsy treated with the ketogenic diet without reducing antiepileptic efficacy.

METHOD

In this prospective controlled study, we investigated the frequency of initial uncompensated metabolic acidosis in 51 participants. There were 22 participants with and 29 without potassium citrate supplementation. The ketogenic diet was used as add-on treatment to children with drug resistant epilepsy. We also estimated the proportion of participants with a greater than 50% seizure reduction after 7 months.

RESULTS

None of the 22 participants (15 males, seven females; median age 1y 7mo, interquartile range [IQR] 3y 3mo) with, and 10 of 29 (12 males, 17 females; median age 6y 1mo, IQR 4y 8mo) without potassium citrate developed metabolic acidosis (odds ratio=0.04, 95% CI 0.00-0.75 [p<0.01]); median pH 7.32 vs 7.24; [p<0.001]), and median bicarbonate 19.7mmol/L vs 14.0mmol/L (p<0.001). The number of seizures was reduced by more than 50% in 9 of 22 with potassium citrate and 8 of 29 participants without potassium citrate, 7 months after introducing a ketogenic diet (p=0.4).

INTERPRETATION

In the ketogenic diet, potassium citrate supplementation can prevent metabolic acidosis, without reducing antiepileptic efficacy.

WHAT THIS PAPER ADDS

Citrate supplementation prevents metabolic acidosis in children treated with a ketogenic diet. Efficacy of the ketogenic diet is not affected by supplementation with citrate. Citrate supplementation does not affect beta-hydroxybuturate concentration. Potassium citrate reduces the time needed to reach an optimal ketogenic ratio. This article is commented on by Schoeler on page 8 of this issue.

Renal Tubular Acidosis

Renal tubular acidosis should be suspected in poorly thriving young children with hyperchloremic and hypokalemic normal anion gap metabolic acidosis, with/without syndromic features. Further workup is needed to determine the type of renal tubular acidosis and the presumed etiopathogenesis, which informs treatment choices and prognosis. The risk of nephrolithiasis and calcinosis is linked to the presence (proximal renal tubular acidosis, negligible stone risk) or absence (distal renal tubular acidosis, high stone risk) of urine citrate excretion. New formulations of slow-release alkali and potassium combination supplements are being tested that are expected to simplify treatment and lead to sustained acidosis correction.

Uses and misuses of sodium bicarbonate in the neonatal intensive care unit

Over the past several decades, bicarbonate therapy continues to be used routinely in the treatment of acute metabolic acidosis in critically ill neonates despite the lack of evidence for its effectiveness in the treatment of acid-base imbalance, and evidence indicating that it may be detrimental. Clinicians often feel compelled to use bicarbonate since acidosis implies a need for such therapy and thus the justification for its use is based on hearsay rather than science. This review summarizes the evidence and refutes the clinical practice of administering sodium bicarbonate to treat metabolic acidosis associated with several specific clinical syndromes in neonates.

Clinical and laboratory approaches in the diagnosis of renal tubular acidosis

In the absence of a gastrointestinal origin, a maintained hyperchloremic metabolic acidosis must raise the diagnostic suspicion of renal tubular acidosis (RTA). Unlike adults, in whom RTA is usually secondary to acquired causes, children most often have primary forms of RTA resulting from an inherited genetic defect in the tubular proteins involved in the renal regulation of acid-base homeostasis. According to their pathophysiological basis, four types of RTA are distinguished. Distal type 1 RTA, proximal type 2 RTA, mixed-type 3 RTA, and type 4 RTA can be differentiated based on the family history, the presenting manifestations, the biochemical profile, and the radiological findings. Functional tests to explore the proximal wasting of bicarbonate and the urinary acidification capacity are also useful diagnostic tools. Although currently the molecular basis of the disease can frequently be discovered by gene analysis, patients with RTA must undergo a detailed clinical study and laboratory work-up in order to understand the pathophysiology of the disease and to warrant a correct and accurate diagnosis.

Proximal renal tubular acidosis: a not so rare disorder of multiple etiologies

Proximal renal tubular acidosis (RTA) (Type II RTA) is characterized by a defect in the ability to reabsorb HCO₃ in the proximal tubule. This is usually manifested as bicarbonate wastage in the urine reflecting that the defect in proximal tubular transport is severe enough that the capacity for bicarbonate reabsorption in the thick ascending limb of Henle's loop and more distal nephron segments is overwhelmed. More subtle defects in proximal bicarbonate transport likely go clinically unrecognized owing to compensatory reabsorption of bicarbonate distally. Inherited proximal RTA is more commonly autosomal recessive and has been associated with mutations in the basolateral sodium-bicarbonate cotransporter (NBCe1). Mutations in this transporter lead to reduced activity and/or trafficking, thus disrupting the normal bicarbonate reabsorption process of the proximal tubules. As an isolated defect for bicarbonate transport, proximal RTA is rare and is more often associated with the Fanconi syndrome characterized by urinary wastage of solutes like phosphate, uric acid, glucose, amino acids, low-molecular-weight proteins as well as bicarbonate. A vast array of rare tubular disorders may cause proximal RTA but most commonly it is induced by drugs. With the exception of carbonic anhydrase inhibitors which cause isolated proximal RTA, drug-induced proximal RTA is associated with Fanconi syndrome. Drugs that have been recently recognized to cause severe proximal RTA with Fanconi syndrome include ifosfamide, valproic acid and various antiretrovirals such as Tenofovir particularly when given to human immunodeficiency virus patients receiving concomitantly protease inhibitors such as ritonavir or reverse transcriptase inhibitors such as didanosine.

Effect of metabolic acidosis on neonatal proximal tubule acidification

The serum bicarbonate in neonates is lower than adults due in large part to a lower rate of proximal tubule acidification. It is unclear if the neonatal proximal tubule is functioning at maximal capacity or if the proximal tubule can respond to metabolic acidosis as has been described in adult proximal tubules. We find that neonatal mouse brush-border membranes have a lower Na(+)/H(+) exchanger (NHE) 3 protein abundance (neonate 0.11 ± 0.05 vs. adult 0.64 ± 0.07; P < 0.05) and a higher NHE8 protein abundance (neonate 1.0 ± 0.01 vs. adult 0.13 ± 0.09; P < 0.001) compared with adults. To examine if neonates can adapt to acidosis, neonatal mice were gavaged with either acid or vehicle for 4 days, resulting in a drop in serum bicarbonate from 19.5 ± 1.0 to 8.9 ± 0.6 meq/l (P < 0.001). Proximal convoluted tubule Na(+)/H(+) exchanger activity (dpH(i)/dt) was 1.68 ± 0.19 pH units/min in control tubules and 2.49 ± 0.60 pH units/min in acidemic neonatal mice (P < 0.05), indicating that the neonatal proximal tubule can respond to metabolic acidosis with an increase in Na(+)/H(+) exchanger activity. Similarly, brush-border membrane vesicles from neonatal rats had an increase in Na(+)/H(+) exchanger activity with acidemia that was almost totally inhibited by 10(-6) M 5-(N-ethyl-n-isopropyl)-amiloride, a dose that has little effect on NHE3 but inhibits NHE8. There was a significant increase in both NHE3 (vehicle 0.35 ± 0.07 vs. acid 0.73 ± 0.07; P < 0.003) and NHE8 brush-border membrane protein abundance (vehicle 0.41 ± 0.05 vs. acid 0.73 ± 0.06; P < 0.001) in acidemic mouse neonates compared with controls. A comparable increase in NHE3 and NHE8 was found in neonatal rats with acidosis. In conclusion, the neonatal proximal tubule can adapt to metabolic acidosis with an increase in Na(+)/H(+) exchanger activity.

Relationship between rickets and incomplete distal renal tubular acidosis in children

Background

In the Sub Saharan Africa Rickets has now been established to be due primarily to calcium deficiency and sometimes in combination with vitamin D deficiency. The main thrust of management is calcium supplementation with or without vitamin D. An observation was made that some children with nutritional rickets do not respond to this management modality. The recently reported high prevalence of Incomplete Distal Renal Tubular Acidosis (idRTA) in adults with osteoporosis as brought to fore the possibility of this being a possible cause of calcium wastage and therefore the poor response in these group of children with rickets.

Aim

To determine the prevalence of idRTA amongst a cohort of subjects with rickets

To show a relationship between rickets and incomplete distal renal acidosis

To determine the response of children with rickets and idRTA to addition of Shohl's solution to therapy

Methodology

Two separate cohorts of children with rickets performed the ammonium chloride loading test to detect those with incomplete renal tubular acidosis. Following identification for idRTA, Shohl's solution was added to therapy of calcium and vitamin D supplementation and their response compared to those without idRTA on calcium and vitamin D supplementation solely.

Results

50 children with rickets aged from two to six years of age and composed of 29 females and 21males were investigated. Incomplete renal tubular acidosis was found in 38% of them. Prevalence of idRTA was highest amongst those aged 3-6 years of age. Those with idRTA had worse limb deformities, biochemical and radiological parameters than those who hadn't. Rate of response on those with idRTA treated with Shohl's solution was at par with those without idRTA.

Conclusion

Incomplete idRTA exist amongst children with rickets and should be looked out for in severe rickets and older children. Treatment of idRTA will lead to optimal response and healing of rickets.

Luminal Na(+)/H (+) exchange in the proximal tubule

The proximal tubule is critical for whole-organism volume and acid-base homeostasis by reabsorbing filtered water, NaCl, bicarbonate, and citrate, as well as by excreting acid in the form of hydrogen and ammonium ions and producing new bicarbonate in the process. Filtered organic solutes such as amino acids, oligopeptides, and proteins are also retrieved by the proximal tubule. Luminal membrane Na(+)/H(+) exchangers either directly mediate or indirectly contribute to each of these processes. Na(+)/H(+) exchangers are a family of secondary active transporters with diverse tissue and subcellular distributions. Two isoforms, NHE3 and NHE8, are expressed at the luminal membrane of the proximal tubule. NHE3 is the prevalent isoform in adults, is the most extensively studied, and is tightly regulated by a large number of agonists and physiological conditions acting via partially defined molecular mechanisms. Comparatively little is known about NHE8, which is highly expressed at the lumen of the neonatal proximal tubule and is mostly intracellular in adults. This article discusses the physiology of proximal Na(+)/H(+) exchange, the multiple mechanisms of NHE3 regulation, and the reciprocal relationship between NHE3 and NHE8 at the lumen of the proximal tubule.

Effect of thyroid hormone on the postnatal renal expression of NHE8

We previously demonstrated that there are developmental changes in proximal tubule Na(+)/H(+) exchanger (NHE) activity. There is a maturational increase in postnatal brush-border membrane (BBM) vesicle NHE3 protein abundance and decrease in NHE8 protein abundance. The purpose of this study was to determine whether thyroid hormone plays a role in the rat renal maturational isoform switch from NHE8 to NHE3 and whether thyroid hormone regulates NHE8. Administration of thyroid hormone to neonatal rats, before the normal postnatal increase in serum thyroid hormone levels at 3 wk of age, resulted in a premature increase in NHE3/beta-actin BBM protein abundance and mRNA abundance. Thyroid hormone also caused a premature decrease in BBM NHE8/beta-actin protein abundance, whereas there was no change in mRNA expression (standardized to 28s). Rats made hypothyroid from birth were studied at 28 days, after the normal maturational increase in thyroid hormone. In these hypothyroid adult rats, the maturational increase in BBM NHE3 protein abundance and NHE3 mRNA expression was prevented. In contrast, the developmental decrease in BBM NHE8 protein abundance was prevented in hypothyroid adults, but mRNA expression was unchanged in hypothyroid rats. To determine whether the effect of thyroid hormone was due to a direct epithelial effect, we studied normal rat kidney cells in culture. We recently showed that this cell line expresses NHE8, but does not express NHE3. Thyroid hormone caused a decrease in surface expression of NHE8, determined by biotinylation, but total cellular abundance remained unchanged. NHE8 activity, measured as the sodium-dependent rate of intracellular pH recovery from an acid load, was less with thyroid treatment than control. In conclusion, thyroid hormone plays a potential role in the developmental isoform change from NHE8 to NHE3 and decreases NHE8 activity.

Origin and fate of pendrin-positive intercalated cells in developing mouse kidney

Pendrin is an apical anion exchanger found in type B and nonA-nonB intercalated cells that is involved in bicarbonate secretion. The purpose of this study was to establish the origin and fate of pendrin-positive intercalated cells in the mouse kidney. Using immunohistochemistry, we found that pendrin-positive cells first appeared in the connecting tubule at embryonic day 14 (E14) and subsequently in the medullary collecting duct at E18. Most of the pendrin-positive cells in the connecting tubule were nonA-nonB intercalated cells, wheras those in the medullary collecting duct were type B intercalated cells. In the cortical collecting duct, pendrin-positive cells appeared in the inner part at day 4 after birth and in the outer part at day 7. Pendrin-positive cells gradually disappeared by apoptosis from the inner part of the medullary collecting duct two weeks after birth. Using 5-bromo-2'deoxy-uridine (BrdU) to follow cell proliferation, we determined that selective proliferation of pendrin-positive intercalated cells does not occur; instead, these cells may arise from undifferentiated precursor cells from separate foci, one in the connecting tubule and one in the collecting duct.

Ontogeny of NHE8 in the rat proximal tubule

Proximal tubule bicarbonate reabsorption is primarily mediated via the Na(+)/H(+) exchanger, identified as NHE3 in adults. Previous studies have demonstrated a maturational increase in rat proximal tubule NHE3 expression, with a paucity of NHE3 expression in neonates, despite significant Na(+)-dependent proton secretion. Recently, a novel Na(+)/H(+) antiporter (NHE8) was identified and found to be expressed on the apical membrane of the proximal tubule. To determine whether NHE8 may be the antiporter responsible for proton secretion in neonates, the present study characterized the developmental expression of NHE8 in rat proximal tubules. RNA blots and real-time RT-PCR demonstrated no developmental difference in the mRNA of renal NHE8. Immunoblots, however, demonstrated peak protein abundance of NHE8 in brush border membrane vesicles of 7- and 14-day-old compared with adult rats. In contrast, the level of NHE8 expression in total cortical membrane protein was higher in adults than in neonates. Immunohistochemistry confirmed the presence of NHE8 on the apical membrane of the proximal tubules of neonatal and adult rats. These data demonstrate that NHE8 does undergo maturational changes on the apical membrane of the rat proximal tubule and may account for the Na(+)-dependent proton flux in neonatal proximal tubules.

Comparison of growth in primary Fanconi syndrome and proximal renal tubular acidosis

To compare the difference between primary proximal renal tubular acidosis (PRTA) and Fanconi syndrome (FS), and to find out possible risk factors for growth retardation, we studied the long-term growth, clinical, laboratory, and radiological findings associated with the treatment of six children with primary FS and 15 children with PRTA. The ages of the children with FS were much older than those with PRTA at initial diagnosis (7.03+/-3.82 vs. 1.63+/-1.56 years). The height standard deviation score (SDS) at the start of treatment was significantly lower in FS than in PRTA. Catch-up growth was noted in PRTA at the end of follow-up (initial height SDS -2.13+/-1.10 vs. last height SDS -1.33+/-1.43, P=0.023 by paired t-test), whereas apparent linear growth impairment was found in FS in terms of overall growth velocity index (82.70+/-8.37%) and height SDS (initial -3.25+/-0.95 vs. last -3.15+/-0.31, P=0.791). There was also a higher rate of rickets occurrence in FS (3/6 vs. 0/15 in PRTA). Hypophosphatemia during the follow-up period was more frequent for FS than PRTA (69.2+/-26.1% vs. 7.0+/-25.8%, P<0.001), whereas metabolic acidosis (blood HCO(3)<20 mmol/l) was less efficiently corrected in PRTA (49.1+/-20.5% vs. 25.2+/-21.6% in FS, P=0.028). Moreover, the height Delta SDS correlated well with the mean serum P level during the treatment period in these patients (R=0.528, P=0.014 for all children; R=0.917, P=0.01 for FS patients). Our data suggest that metabolic acidosis may not be the sole factor causing growth impairment in FS. Correction of metabolic acidosis may indeed improve growth in PRTA but not in FS. This study indicates that factors other than metabolic acidosis, such as phosphate depletion and delayed diagnosis/treatment, should be considered to be important causes of growth retardation in FS.

Maturation of the Na+/H+ antiporter (NHE3) in the proximal tubule of the hypothyroid adrenalectomized rat

In previous studies examining the role of glucocorticoids and thyroid hormone on the maturation of the Na(+)/H(+) antiporter (NHE3), we found attenuation in the maturational increase in proximal tubule apical Na(+)/H(+) antiporter activity but no change in NHE3 mRNA abundance in either glucocorticoid-deficient or hypothyroid rats. In addition, prevention of the maturational increase in either hormone failed to totally prevent the maturational increase in Na(+)/H(+) antiporter activity. We hypothesized that one hormone played a compensatory role when the other was deficient. The present study examined whether combined deficiency of thyroid and glucocorticoid hormones would completely prevent the maturation of the Na(+)/H(+) antiporter. Adrenalectomy was performed in 9-day-old hypothyroid Sprague-Dawley rats, a time before the normal postnatal maturational increase in these hormones occurs. Nine- and 30-day-old adrenalectomized (ADX), hypothyroid rats had comparable NHE3 mRNA abundance, which was 5- to 10-fold less than 30-day-old ADX, hypothyroid rats that received corticosterone-thyroxine replacement and 30-day-old sham control rats (P < 0.05). Brush-border membrane NHE3 protein abundance was comparable in 9- and 30-day-old ADX, hypothyroid groups and approximately 20-fold lower than both the 30-day replacement and 30-day sham groups (P < 0.05). Similarly, the replacement and sham groups had higher sodium-dependent proton secretion than 9- and 30-day-old ADX, hypothyroid groups (P < 0.05). We conclude that combined deficiency of both hormones totally prevents the maturational increase in NHE3 mRNA and protein abundance and Na(+)/H(+) antiporter activity.

Postnatal expression of transport proteins involved in acid-base transport in mouse kidney

The kidney plays a major role in maintaining and controlling systemic acid-base homeostasis by reabsorbing bicarbonate and secreting protons and acid-equivalents, respectively. During postnatal kidney development and adaptation to changing diets, plasma bicarbonate levels are increasing, the capacity for urinary acidification maturates, and the final morphology and distribution of intercalated cells is achieved. In adult kidney, at least two types of intercalated cells (IC) are found along the collecting duct characterised either by the expression of AE1 (type A IC) or pendrin (non-type A IC) where non-type A IC are found only in the convoluted distal tubule, connecting tubule and cortical collecting duct. Here we investigated in mouse kidney the relative mRNA abundance, protein expression levels and distribution of several proteins involved in renal acid-base transport, namely, the Na(+)/HCO(3)(-) cotransporter NBC1 (SLC4A4), the Na(+)/H(+)-exchanger NHE3 (SLC9A3), two subunits of the vacuolar H(+)-ATPase [ATP6V0A4 (a4), ATP6V1B1 (B1)], the Cl(-)/HCO(3)(-) exchangers AE1 (SLC4A1) and pendrin (SLC26A4). Relative mRNA abundance of all transport proteins was lowest at day 3 after birth and increased thereafter in parallel with protein levels. The numbers of type A and non-type A IC in the cortical collecting duct (CCD) increased from day 3 to days 18 and 24, whereas the number of IC in the CCD with apical staining for the vacuolar H(+)-ATPase subunits a4 and B1 decreased from day 3 to days 18 and 24, respectively. In addition, cells with characteristics of non-type A IC (pendrin expression, basolateral expression of vacuolar H(+)-ATPase subunits) were found in the inner and outer medulla 3 days after birth but were absent from the medulla of 24-day-old mice. Taken together, these results demonstrate massive changes in mRNA and protein expression levels of several acid-base transporters during postnatal kidney maturation and also show changes in intercalated cell phenotype in the medulla during these processes.

Maturational changes in renal tubular transport

PURPOSE OF REVIEW

This review examines the maturational changes that occur in renal tubules during postnatal development.

RECENT FINDINGS

The ability to study transport in neonatal tubules and the use of molecular techniques have allowed studies that not only examine the mechanism of solute and water transport in neonates but also what causes the maturational changes in transport at a molecular and cellular level.

SUMMARY

This review demonstrates that there are significant quantitative and qualitative differences in transport during postnatal maturation in every nephron segment. In some segments the maturational changes involve simply a change in abundance of transporters, while in others the difference in transport is due to changes in transporter isoforms, changes in paracellular permeability or changes in intracellular signaling that regulate the transporter. This review focuses on these changes and what is known about what causes the maturational changes in transport.

Urinary acidification in extremely low birth weight infants

Premature infants often present metabolic acidosis without protein load in the early neonatal period, around days 4-6. In order to elucidate the cause of acidosis, we investigated urinary acidification of infants in the early neonatal period. Urine pH, fractional excretion of HCO(3)(-) (FEHCO(3)), excretion of HCO(3)(-) and NH(4)(+) of the appropriate-for-date infants were measured on days 0-2 and on days 4-6 of life. Extremely low birth weight (ELBW) infants showed higher urine pH than more than 1500 g birth weight infants. FEHCO(3) and HCO(3)(-) excretion were of high values in ELBW infants on days 0-2, but decreased on days 4-6. Urine NH(4)(+) excretion rate was lower in ELBW infants than in birth weight more than 1000 g on days 0-2 of life and still remained at a low rate on days 4-6. These data indicated that insufficiency of NH(4)(+) excretion is the main cause for metabolic acidosis of ELBW infants in the early neonatal period.

Renal tubular acidosis: a new look at an old problem

Although the definition of renal tubular acidosis (RTA) is simple, understanding the physiologic basis underlying the various types of this clinical entity is much more difficult. The pathophysiology of this disorder is reviewed using the normal acid-base functions of the involved segments of the nephron as a guide to understanding. Clinical and laboratory features of the subtypes of RTA are addressed, and diagnosis and treatment discussed. New developments in the knowledge and understanding of the associated growth disturbances, mineral metabolism, and molecular biology of RTA are also reviewed to provide the most current view of this relatively common pediatric entity.

Maturation of carbonic anhydrase IV expression in rabbit kidney

Carbonic anhydrase (CA) IV facilitates renal acidification by catalyzing the dehydration of luminal H(2)CO(3). CA IV is expressed in proximal tubules, medullary collecting ducts, and A-intercalated cells of the mature rabbit kidney (Schwartz GJ, Kittelberger AM, Barnhart DA, and Vijayakumar S. Am J Physiol 278: F894-F904, 2000). In view of the maturation of HCO transport in the proximal tubule and collecting duct, the ontogeny of CA IV expression was examined. During the first 2 wk, CA IV mRNA was expressed in maturing cortex and medulla at ~20% of adult levels. The maturational increase was gradual in cortex over 3-5 wk of age but surged in the medulla, so that mRNA levels appeared higher than those in the adult medulla. In situ hybridization showed very little CA IV mRNA at 5 days, with increases in deep cortex and medullary collecting ducts by 21 days. Expression of CA IV protein in the cortex and medulla was minimal at 3 days of age but then apparent in the juxtamedullary region, A-intercalated cells and medullary collecting ducts by 18 days; there was little labeling of the proximal straight tubules of the medullary rays. Thus CA IV expression may be regulated to accommodate the maturational increase in HCO absorption in the proximal tubule. In the medullary collecting duct, there is a more robust maturation of CA IV mRNA and protein, commensurate with the high rate of HCO absorption in the neonatal segment.

Hypomagnesaemia-hypercalciuria-nephrocalcinosis: a report of nine cases and a review

BACKGROUND

The cardinal characteristics of primary hypomagnesaemia-hypercalciuria-nephrocalcinosis include renal magnesium wasting, marked hypercalciuria, renal stones, nephrocalcinosis, a tendency towards chronic renal insufficiency and sometimes even ocular abnormalities or hearing impairment.

METHODS

As very few patients with this syndrome have been described, we provide information on nine patients on follow-up at our institutions and review the 42 cases reported in the literature (33 females and 18 males).

RESULTS

Urinary tract infections, polyuria-polydipsia, renal stones and tetanic convulsions were the main clinical findings at diagnosis. The clinical course was highly variable; renal failure was often reported. The concomitant occurrence of ocular involvement or hearing impairment was reported in a large subset of patients. Parental consanguinity was noted in some families.

CONCLUSIONS

The results indicate an autosomal recessive inheritance. The diagnosis of primary hypomagnesaemia-hypercalciuria-nephrocalcinosis deserves consideration in any patient with nephrocalcinosis and hypercalciuria.

Renal citrate metabolism and urinary citrate excretion in the infant rat

BACKGROUND

Although hypercalciuria has the same prevalence in children as adults, children rarely develop renal stones. This may be explained by a greater urinary citrate excretion in infants compared with adults. The present study examines the renal excretion of citrate and renal cortical citrate metabolism in infant and adult rats.

METHODS

Adult male and newly weaned infant rats were acclimated to metabolic cages and fed synthetic diets. Urine was collected after two days, and renal cortical citrate metabolism was assayed.

RESULTS

Infant rats had a lower plasma [HCO3-] and higher plasma [K+] and had a fourfold higher urinary citrate:creatinine ratio and a twofold higher concentration of citrate in their urine compared with adult rats. This higher urinary citrate excretion was not due to a difference in renal proximal tubular Na/citrate cotransporter activity, nor renal cortical citrate synthase or ATP citrate lyase activities in infants as compared with adults. However, infant rat kidneys had significantly lower mitochondrial aconitase (m-aconitase) activity. Renal cortical citrate concentrations were comparable in infant and adult rats. Manipulation of plasma [K+] to adult levels did not affect the higher urinary citrate excretion in infant rats.

CONCLUSIONS

Urinary citrate excretion in infant rats is greater than in adults but does not parallel tissue [citrate]. Thus, this higher urinary citrate is likely due to maturational differences in the proximal tubule, other than Na/citrate cotransport, that directly affect citrate transport.

Postnatal development of carbonic anhydrase IV expression in rabbit kidney

Carbonic anhydrase (CA) IV activity facilitates renal acidification by catalyzing the dehydration of luminal carbonic acid. CA IV has been localized to the proximal tubules and medullary collecting ducts. Maturation of CA IV expression has been considered to be important in the development of renal acid excretion. The purpose of the present study was to determine the maturational expression of CA IV in rabbit kidney. A guinea pig polyclonal antibody to purified rabbit lung microsomal membrane CA IV was generated. Immunoblotting of membrane proteins after peptide-N-glycosidase F treatment revealed two N-glycosylation sites and reduction in size from approximately 52 to 35 kDa; there appeared to be heavier glycosylation in the medulla. In membrane and total proteins from the kidney cortex, CA IV was 15-30% of the adult level during the first 2 wk of life but increased to mature levels by 5 wk of age. The maturational pattern in the cortex was confirmed by measuring SDS-resistant CA hydratase activity. In the medulla, both membrane and total proteins were generally less than one-fourth of the adult level of CA IV during the first 2 wk of life before reaching mature levels by 5 wk of age. Immunohistochemistry showed staining in proximal tubules (apical > basolateral), with maximal label in the S2 segment. CA IV also appeared on the apical membranes of a minority cell type of the cortical collecting duct, presumably the alpha-intercalated cell. Several labeled cells also appeared to be the process of being extruded from medullary collecting ducts of 1- to 2-wk rabbits. The antibody did not reliably detect medullary CA IV expression in sections from mature rabbits. These studies indicate that there is a substantial postnatal increase in expression of CA IV in the maturing kidney in both the cortex and medulla. The disappearance of intercalated cells in the maturing rabbit medullary collecting duct may be part of a normal renal developmental program as previously reported [J. Kim, J.-H. Cha, C. C. Tisher, and K. M. Madsen. Am. J. Physiol. 270 (Renal Fluid Electrolyte Physiol. 39): F575-F592, 1996]. It is likely that the maturation of CA IV expression contributes to the increase in renal acidification observed early in postnatal life.

Effects of thyroid hormone on the neonatal renal cortical Na+/H+ antiporter

The neonatal proximal tubule has a lower rate of bicarbonate absorption than that of adults. This is due, in part, to a lower rate of apical membrane Na+/H+ antiporter activity. The purpose of these studies was to examine if thyroid hormone could be a factor in the maturational increase in Na+/H+ antiporter activity. Hypothyroid (0.01% propylthiouracil in drinking water starting at day 14 gestation and throughout the postnatal period), euthyroid, and hyperthyroid (intraperitoneal triiodothyronine, 10 micrograms/100 g body wt, once daily on days 17 to 20 of postnatal life) rats were all studied at 21 days of life. Renal cortical brush border Na+/H+ antiporter activity was 453 +/- 24, 527 +/- 30 and 608 +/- 25 pmol/mg protein in the hypothyroid, euthyroid and hyperthyroid groups, respectively (P < 0.001). Hyperthyroid neonates had approximately twofold greater renal cortical NHE-3 mRNA abundance than euthyroid and hypothyroid neonates (P < 0.05). Brush border membrane NHE-3 protein abundance in hypothyroid and hyperthyroid neonates was one-third and twofold that of euthyroid 21-day-old rats, respectively (P < 0.001). These data are consistent with a potential role of thyroid hormone in the postnatal increase in Na+/H+ antiporter activity.

A new classification for renal defects in net acid excretion

The traditional classification of the group of disorders called renal tubular acidosis (RTA) into proximal and distal subclasses is based on which nephron segment is thought to have an abnormal function. Nevertheless, such a distinction may not be correct and also does not characterize the pathophysiology of the renal acidosis in each patient. In this article, we propose an alternative classification, one that is based on the component of net acid excretion that is abnormal. We also suggest expanding the definition of net acid excretion to include a term that describes the renal handling of metabolizable organic anions because their loss in the urine represents the loss of "potential bicarbonate." Because a low rate of excretion of ammonium (NH4+) is present in patients with both distal and isolated proximal RTA, our initial clinical step in patients with hyperchloremic metabolic acidosis (HCMA) is to evaluate the rate of excretion of NH4+. The basis for a low rate of excretion of NH4+ is shown by examining the urine pH. If the urine pH is low, further studies are performed to determine why the availability of NH3 is low; if the urine pH is high, further investigations are initiated to examine if the defect in H+ secretion involves the proximal or the distal nephron. Conversely, if the rate of excretion of NH4+ is high in a patient with HCMA, a component of the degree of acidosis could be attributable to a high rate of excretion of metabolizable organic anions. Case examples are provided to illustrate the approach and its implications for future molecular studies.

Postnatal differentiation of rabbit collecting duct intercalated cells

The newborn has a limited ability to regulate H+/HCO3- homeostasis, due in part to immaturity of the intercalated cells in the distal nephron. We traced the postnatal differentiation of the intercalated cells of the rabbit cortical collecting duct (CCD) and outer medullary collecting duct (OMCD) using MAb to the 31-kD subunit of the vacuolar H(+)-ATPase, membrane portion of erythrocyte band 3, and apical surface of B-intercalated cells (peanut agglutinin [PNA], MAb B63). In the most superficial CCD of the newborn there was no binding to these probes, although deeper in the cortex there was faint apical staining with PNA and MAb B63 and a few patterns of H(+)-ATPase and band 3 labeling of neonatal intercalated cells. The OMCD showed mostly apical H(+)-ATPase and both cytoplasmic and basolateral band 3 labeling but B-intercalated cell markers were not seen. By 3 wk of age the staining of the CCD and OMCD was more polarized, resembling those in the adult. Band 3 positive cells (as a percentage of total cells) in the CCD increased from 13 to 17% during maturation, and in the OMCD they increased from 22 to 37%. Some basolateral band 3 and apical H(+)-ATPase staining was also seen in the inner medullary collecting duct of 3-wk-old rabbits to a greater extent than in newborn or adult rabbits. Labeling of intercalated cells in the CCD and OMCD was weakest and least numerous in the newborn, greater in the 3 wk old, and greatest in the adult. Most maturing cortical intercalated cells bound both PNA and H(+)-ATPase MAb, comparable to what has been observed in the adult CCD. PNA-negative cells showing apical H(+)-ATPase labeling, consistent with the classic A-intercalated cell phenotype, comprised only 5% of identified intercalated cells in the newborn CCD compared with 12% in older animals. In or near the developing renal vesicles and ampullary structures were occasional cytoplasmically staining PNA- and B63-positive cells. Whether these cells are precursors of specific renal tubular cells cannot yet be established. Staining for principal cells (ST.9) was less intense in the neonatal cortex than in more mature cortex, but the deep cortex and outer medulla were heavily labeled at all ages. These data indicate that immature intercalated cells, in the CCD and OMCD, may undergo significant postnatal proliferation and differentiation, acquiring mature phenotypes during the first month of life. The A-intercalated cell appears more differentiated than the B cell during the 1st wk of life, suggesting that A-intercalated cells contribute more than B cells to the maintenance of acid-base homeostasis in the newborn.

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.

Maturation of proximal tubular acidification

Neonatal juxtamedullary proximal convoluted tubules (PCTs) transport bicarbonate at one-third the rate of adult rabbit PCTs. The lower rate of bicarbonate transport could be due to a greater permeability of the neonatal PCT to bicarbonate or to a lower rate of active bicarbonate transport. This review discusses potential factors which could result in a lower rate of bicarbonate transport by the neonatal PCT. In isolated perfused PCT, bicarbonate permeability is lower in neonatal than adult PCT, and thus it does not account for the lower rate of bicarbonate transport in neonatal PCT. In the adult PCT, apical proton secretion occurs via the Na+/H+ antiporter and H(+)-ATPase; basolateral bicarbonate exit occurs via the Na(HCO3)3 symporter. The activity of transporters can be ascertained by measuring intracellular pH with the fluorescent dye BCECF. Apical Na+/H+ antiporter, apical H(+)-ATPase and basolateral Na(HCO3)3 symporter activity are all significantly lower in neonatal PCT. The factors which stimulate PCT maturation are unknown, however glucocorticoids have been postulated to play an important role in this process. Administration of dexamethasone to pregnant does results in higher rates of PCT volume absorption, bicarbonate transport, Na+/H+ antiporter and Na(HCO3)3 symporter activities than in PCT from vehicle-treated controls. Thus, the lower rate of neonatal PCT bicarbonate transport is due to lower activities of the apical Na+/H+ antiporter, apical H(+)-ATPase and basolateral Na(HCO3)3 symporter. Glucocorticoids may be an important factor in the maturation of PCT acidification.

Short-term primary cultures in studies of post-natal maturation of the rat proximal tubule-proton and bicarbonate transport

This is a review of recent work based on an in vitro model which has allowed us to investigate the postnatal maturation of renal epithelial cells. Renal proximal tubule cells from 8- to 40-day-old Sprague-Dawley rats were studied after 48 h of primary culture. The regulation of intracellular pH (pHi) was measured by quantitative fluorescence microscopy using 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Recordings were made under basal conditions and after imposing a cytoplasmic alkalosis or acidosis using 15 mM ammonium salt. The ability of the cells to recover from both acidosis and alkalosis improved during post-natal maturation. The improvement in recovery from intracellular acidosis could be entirely accounted for by an increase in the rate of Na+/H+ exchange. The capacity for Na+/H+ exchange was independent of the cellular growth rate, but depended on cellular differentiation. A developmental increase in the activity of Cl-/HCO3- exchange between 12 and 14 days of age was also demonstrated. No developmental change was seen in either steady-state pHi (7.27-7.35) or in cytoplasmic buffer capacity (37.6-44.4 mM/pHi). The characteristics of transporter maturation revealed by these experiments are very similar to those observed in isolated perfused proximal tubules of developing rabbits.

Developmental expression of acid-base-related proteins in the rabbit kidney

The newborn is limited in its ability to respond to acid-base perturbations. To investigate the development of renal H+/HCO3- transport mechanisms, we probed acid-base-related epitopes in the mesonephric and developing metanephric kidneys of rabbits. Using immunofluorescence with monoclonal antibodies to the vacuolar H+ATPase, band 3-like Cl-/HCO3- exchanger, and apical surface of fully differentiated beta-intercalated cells, and peanut lectin cytochemistry (another marker of beta-intercalated cells), we found that these epitopes were poorly expressed in the nephrogenic zone of the newborn kidney cortex. Deeper in the cortex, collecting ducts showed weak apical staining with beta-intercalated cell antibodies and two patterns of staining with the H+ATPase and band 3 antibodies: polar and circumferential or diffuse. Some cells showed apical staining with H+ATPase while others showed diffuse staining, similar to that observed in the mature cortical collecting duct. Band 3 labeling was basolateral, as observed in the adult, and diffuse, which was rarely seen in mature kidney sections. Newborn outer medullary collecting ducts showed apical labeling with H+ATPase and basolateral staining with band 3 antibodies, similar to the mature outer medulla. Surprisingly, the mesonephric collecting tubule showed cells with apical H+ATPase staining or basolateral band 3 labeling and, less frequently, cells with positive staining for beta-intercalated cells. The relative maturity of the mesonephric collecting tubule and similarity to what is observed in mature metanephric collecting ducts indicates that intercalated cells may be present and functioning in both organs. Thus, the lineage of intercalated cells may be more intricate than previously believed.

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.

Use of calcium excretion values to distinguish two forms of primary renal tubular hypokalemic alkalosis: Bartter and Gitelman syndromes

Clinical or biochemical findings were reevaluated in 34 pediatric patients with primary renal tubular hypokalemic metabolic alkalosis. The patients were subdivided into two groups. Bartter syndrome (primary renal tubular hypokalemic metabolic alkalosis with normocalciuria or hypercalciuria) was diagnosed in 18 patients with molar urinary calcium/creatinine ratios greater than 0.20, and Gitelman syndrome (primary renal tubular hypokalemic metabolic alkalosis with magnesium deficiency and hypocalciuria) was diagnosed in 16 patients with molar urinary calcium/creatinine ratios less than or equal to 0.20 and plasma magnesium levels less than 0.75 mmol/L. Some clinically important differences between the groups were observed. Patients with Bartter syndrome were often born after pregnancies complicated by polyhydramnios (8/18) or premature delivery (7/18) and had short stature (11/18) or polyuria, polydipsia, and a tendency to dehydration (16/18) during infancy (12/18) or before school age (18/18). Patients with Gitelman syndrome had tetanic episodes (12/16) or short stature (3/16) at school age (14/16). We conclude that the Bartter and Gitelman syndromes represent two distinct variants of primary renal tubular hypokalemic metabolic alkalosis and are easily distinguished on the basis of urinary calcium levels.

Evaluation of carbonic anhydrase isozymes in disorders involving osteopetrosis and/or renal tubular acidosis

Carbonic anhydrase II (CA II) deficiency in man is an autosomal recessive disorder manifest by osteopetrosis, renal tubular acidosis, and cerebral calcification. Other features include growth failure and mental retardation. Complications of the osteopetrosis include frequent bone fractures, cranial nerve compression symptoms, and dental malocclusion. The anemia and leukopenia seen in the recessive, lethal infantile form of osteopetrosis are not seen in CA II deficient patients. The renal tubular acidosis usually includes both proximal and distal components. Symptoms of metabolic acidosis respond to therapy, but no specific treatment is available for the osteopetrosis or cerebral calcification. We review here the role of carbonic anhydrases in bone resorption and renal acidification, and discuss clinical features and laboratory findings which distinguish CA II deficiency from other disorders producing osteopetrosis, renal tubular acidosis, or brain calcification. Methods to evaluate patients with pure proximal renal tubular acidosis for deficiency of CA IV are also discussed.

Ontogeny of Na/H antiporter activity in rabbit renal brush border membrane vesicles

The development of the Na/H antiporter was studied in renal brush border membrane vesicles (BBMV) from fetal and adult rabbits using isotopic and fluorescent techniques. The kinetics of the antiporter studied by 22Na+ uptake revealed that the Vmax was only 25% of that in the adult; however, the Km's for Na+ were not significantly different. These data were confirmed by a fluorescent assay using the pH-sensitive probe, acridine orange: the Vmax was significantly lower in the fetal BBMV. Conductive Na+ movement was estimated from amiloride-insensitive 22Na+ uptake and the rate of alkalinization induced by K+, an ion whose relative conductance was found to be similar to that of Na+. Although relative Na+ conductance was significantly greater in fetal BBMV, the lower Vmax in fetal vesicles could not be ascribed to this factor. Maternal administration of betamethasone (50 micrograms/kg intramuscularly) for 2 d before delivery significantly increased the Vmax of the antiporter to levels observed in the adult; Km was unaffected. Na/K ATPase activity increased fourfold after betamethasone, but the specific activities of four brush border marker enzymes and the kinetics of Na(+)-glucose cotransport were unchanged. These data indicate that there is a developmental increase in brush border Na/H exchange which is the result of an increase in the number and/or the turnover number of the carriers. Further, these data suggest that the postnatal increase in antiporter activity may be related to the surge in glucocorticoid concentration that occurs perinatally.

Assessment of chemotherapy-associated nephrotoxicity in children with cancer

Assessment of the toxicity caused by chemotherapy in children with cancer has become more important as the number of long-term survivors has continued to increase. It is vital to monitor both acute life-threatening adverse effects and long-term toxicity that may impair the child's development and cause permanent morbidity. Renal damage may follow treatment with cytotoxic drugs, especially cisplatin or ifosfamide, and lead to glomerular, proximal tubular or distal tubular impairment or to any combination of these. Greater understanding of nephrotoxicity and of its prevention may enable the use of more intensive schedules or of higher doses of potentially nephrotoxic chemotherapy. However, the evaluation of cytotoxic drug-induced nephrotoxicity has frequently depended mainly on measurement of the plasma creatinine concentration, which may remain normal despite substantial glomerular impairment or severe tubular dysfunction. Detailed assessment of nephrotoxicity depends on an understanding of normal renal physiology and requires evaluation of all aspects of function. A comprehensive but simple investigatory protocol that enables assessment of the nature and severity of nephrotoxicity in children is described, which can be performed without admission to hospital. Glomerular function is assessed by measurement of the glomerular filtration rate from the plasma clearance of [51Cr]-ethylenediaminetetraacetic acid ([51Cr]-EDTA). Proximal nephron function is evaluated in three ways: by measurement of the concentration of calcium, magnesium, phosphate, glucose and urate in blood and urine along with calculations of their fractional excretion and of the renal threshold for phosphate; by determination of the excretion in urine of low-molecular-weight proteins (e.g. retinol-binding protein); and by investigation of urinary bicarbonate excretion in patients who are acidotic. Distal nephron function is initially investigated by examination of the concentration (osmolality) and acidification (pH) of an early morning sample of urine. Finally, a group of general investigations is performed, including quantitation of urinary excretion of renal tubular enzymes (e.g. N-acetylglucosaminidase) and measurement of blood pressure.

Electrolyte balance

The kidney is structurally and biochemically immature at birth. As a consequence, renal function is low (3;10;ll;18). Glomerular filtration rate (related to body surface area or to body weight) is approximately 25% of that in adults. The capacity of several different tubular transport systems is lower in the infant than in the adult (2;5;13;21;28). A low transporting capacity of the neonatal kidney will sometimes result in undesired losses of electrolytes, amino acids, and peptides. The capacity to concentrate urine is low (7;29), and disturbances of serum tonicity, therefore, are common. The low concentrating capacity can be attributed to renal immaturity. The capacity of the newborn fullterm as well as preterm infant to release antidiuretic hormone is normal 7;31).

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.

Suboptimal mineral composition of cow's milk formulas: a risk factor for the development of late metabolic acidosis

Late metabolic acidosis was observed in a term baby boy with renal tubular acidosis type 4 who received two cow's milk formulas in succession. Suboptimal mineral composition of the formulas turned out to be an important risk factor for the development of late metabolic acidosis.

Carbonic anhydrase isozymes IV and II in urinary membranes from carbonic anhydrase II-deficient patients

Carbonic anhydrase II (CA II) deficiency has been shown to be the primary defect in the recessively inherited syndrome of osteopetrosis with renal tubular acidosis. Until now, the absence of CA II in kidney of CA II-deficient patients has not been shown directly, and the status of the membrane-associated CA in kidney of CA II-deficient patients has been unclear. To address these questions, we analyzed urinary membranes and soluble fractions from normal and CA II-deficient subjects. The CA activity in membrane fractions of normal urine was found to comprise two components--(i) a vesicle-enclosed, sodium dodecyl sulfate (SDS)-sensitive fraction, which was shown immunochemically to be the 29-kDa CA II, and (ii) an SDS-resistant fraction, which was due to native and cleaved forms of the 35-kDa, membrane-anchored isozyme CA IV. Urinary membranes from CA II-deficient patients showed little or no SDS-sensitive activity and no immunoreactivity for CA II, providing direct evidence that their mutation, which produces CA II deficiency in erythrocytes, also affects CA II in kidney. CA IV activity and immunoreactivity were present in normal amounts in urinary membranes from CA II-deficient patients. We conclude from the enzymatic and immunological evidence presented that both CA II and CA IV are present in urinary membranes from normal subjects, that renal CA IV is present but renal CA II is absent in urinary membranes from patients with the CA II-deficiency syndrome, and that the methods presented should be useful in studying renal CA II and renal CA IV in other disorders of impaired bicarbonate reabsorption.

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.

Neonatal rabbit juxtamedullary proximal convoluted tubule acidification

The present in vitro microperfusion study examined apical membrane Na+/H+ antiporter and basolateral membrane Na(HCO3)3 symporter activity in newborn and adult juxtamedullary proximal convoluted tubules. Proton fluxes were determined from the initial rate of change of intracellular pH after a change in the luminal or bathing solution, buffer capacity, and tubular volume of newborn and adult tubules. Intracellular pH (pHi) was measured fluorometrically using the pH-sensitive dye (2',7')-bis (carboxyethyl)-(5,6)-carboxyfluorescein (BCECF). Apical Na+/H+ antiporter proton flux, assayed by the effect of sodium removal (147----0 meq/liter) on pHi, was one-third the adult level for the first 2 wk and doubled in the 3rd wk of life. Adult levels were achieved by 6 wk of age. Na+/H+ antiporter activity was not detected on the basolateral membrane of 1-wk-old newborns, indicating that polarity of this transporter was already present. Basolateral membrane Na(HCO3)3 proton flux, assayed by the effect of a bath bicarbonate change (25----5 meq/liter) and by a bath sodium change (147----0 meq/liter) on pHi, was 50-60% of adult values in 1-wk-old newborns. Basolateral membrane Na(HCO3)3 proton flux assayed by a bath bicarbonate change (25----5 meq/liter) remained at 50-60% of adult values for the 1st mo of life and increased to adult levels by 6 wk of age. This transporter not only plays a role in net acidification, but is an important determinant of cell pH in newborn juxtamedullary proximal convoluted tubules.

Variable clinical presentation of carbonic anhydrase deficiency: evidence for heterogeneity?

The clinical and biochemical findings in an Italian sibship affected by carbonic anhydrase II deficiency are described. Evidence of clinical heterogeneity and an increased frequency of the disease in the Mediterranean area and the Middle East are discussed.

The diagnosis of hypercalciuria in children

Calcium loading tests were performed in 21 children with hypercalciuria, haematuria and/or nephrolithiasis and 10 control subjects. Comparisons of 24-h calcium excretion before and after loading were evaluated rather than fasting urinary calcium to urinary creatinine ratio. The differences in calcium excretion before and after loading clearly distinguished absorptive from renal hypercalciuria. A difference higher than 0.035 mmol/kg indicated absorptive hypercalciuria in 6 of 21 patients, whereas in the remaining 15 much lower differences indicated renal hypercalciuria. Resorptive hypercalciuria caused by low serum values of 25-hydroxyvitamin D was considered in 6 of the 15 patients with renal hypercalciuria. These patients had low values of phosphate reabsorption (TmP/GFR) and could be clearly separated by high values of calcium reabsorption (TmCa/GFR), in contrast to patients with renal hypercalciuria who had normal values of TmP/GFR and low values of TmCa/GFR. The correct treatment and prevention of nephrolithiasis caused by hypercalciuria in children should be based on accurate diagnosis; this can be achieved by using the calcium loading test described in this report.