Neonatal rabbit juxtamedullary proximal convoluted tubule acidification

Developmental changes in renal tubular transport-an overview

The adult kidney maintains a constant volume and composition of extracellular fluid despite changes in water and salt intake. The neonate is born with a kidney that has a small fraction of the glomerular filtration rate of the adult and immature tubules that function at a lower capacity than that of the mature animal. Nonetheless, the neonate is also able to maintain a constant extracellular fluid volume and composition. Postnatal renal tubular development was once thought to be due to an increase in the transporter abundance to meet the developmental increase in glomerular filtration rate. However, postnatal renal development of each nephron segment is quite complex. There are isoform changes of several transporters as well as developmental changes in signal transduction that affect the capacity of renal tubules to reabsorb solutes and water. This review will discuss neonatal tubular function with an emphasis on the differences that have been found between the neonate and adult. We will also discuss some of the factors that are responsible for the maturational changes in tubular transport that occur during postnatal renal development.

Therapeutic effect of prenatal alkalization and PTC124 in Na(+)/HCO3(-) cotransporter 1 p.W516* knock-in mice

We created Na(+)/HCO3(-) cotransporter 1 (NBCe1) p.W516* knock-in mice as a model of isolated proximal renal tubular acidosis showing early lethality associated with severe metabolic acidosis to investigate the therapeutic effects of prenatal alkalization or posttranscriptional control 124 (PTC124). NBCe1(W516*/W516*) mice were treated with non-alkalization (control, n=12), prenatal alkalization postcoitus (prenatal group, n=7) and postnatal alkalization from postnatal day 6 (postnatal group, n=12). Mutation-specific therapy, PTC124 (60 mg kg(-1)) or gentamicin (30 mg kg(-1)), was administered intraperitoneally from postnatal day 6. Blood and urine biochemistry, acid-base analysis, survival rate and renal histology were examined. NBCe1 protein, mRNA abundance and activity ex vivo were assessed after PTC124 and gentamicin treatment. Prenatal group mice had similar initial body weight to wild-type mice and achieved significant weight gain thereafter compared with controls. They had higher serum bicarbonate level (15.5 ± 1.4 vs 5.5 ± 0.1 mmol l(-1), P<0.05) on postnatal day 14 and better renal function, histology and survival rates (60.8 ± 23.5 vs 41.1 ± 15.8 days, P<0.05) than the postnatal group. Compared with the control and gentamicin therapies, PTC124 therapy significantly increased NBCe1 protein abundance despite unchanged mRNA transcription. Only PTC124 therapy significantly increased survival rate and partially rescued NBCe1 activity ex vivo. In NBCe1(W516*/W516*) mice, prenatal alkali therapy achieved higher survival rates and ameliorated organ dysfunction. PTC124 therapy for this nonsense mutation was partially effective in increasing NBCe1 expression and activity.

Renal NHE expression and activity in neonatal NHE3- and NHE8-null mice

Na(+)/H(+) exchanger (NHE)3 is the predominant NHE on the brush-border membrane of the proximal tubule in adult animals. NHE8 has been localized to the brush-border membrane of proximal tubules and is more highly expressed in neonates than in adult animals. However, the relative role of NHE8 in neonatal renal acidification is unclear. The present study examined if there was a compensatory increase in NHE3 in NHE8-null neonatal mice and whether there was a compensatory increase in NHE8 in NHE3-null neonatal mice. In addition, we examined whether wild-type, NHE3-null, and NHE8-null mice had an increase in NHE activity in response to metabolic acidosis. We found that at baseline, there was comparable renal NHE3 mRNA, total protein, and brush-border membrane protein abundance as in neonatal control and NHE8-null mice. There was comparable renal NHE8 mRNA, total protein, and brush-border membrane protein abundance in NHE3-null neonatal and control mice. Both NHE3- and NHE8-null mice had a comparable but lower rate of NHE activity than control mice. We next imposed metabolic acidosis in wild-type, NHE3-null, and NHE8-null mice. Acidemic NHE8-null mice had an increase in brush-border membrane vesicle NHE3 protein abundance and NHE activity compared with vehicle-treated mice. Likewise, NHE3-null mice had an increase in NHE8 brush-border membrane protein abundance and NHE activity in response to metabolic acidosis. In conclusion, both NHE3 and NHE8 likely play a role in neonatal acidification.

Glucocorticoids Reduce Renal NHE8 Expression

The proximal tubule reabsorbs most of the filtered bicarbonate which is mediated in large part by Na⁺/H⁺ exchange (NHE). We have previously demonstrated that there is an isoform switch during postnatal maturation from NHE8 to NHE3 that is concordant with the postnatal increase in serum glucocorticoid levels. To examine if glucocorticoids may be responsible for this isoform switch, we administered dexamethasone daily to mice at 7–10 days of age, a time prior to the normal isoform switch. We show that compared to vehicle-treated controls, dexamethasone caused a premature increase in renal NHE3 and decrease in NHE8 mRNA, total protein, and brush border membrane protein abundance. To examine if there was a direct epithelial action of dexamethasone on NHE8, we studied normal rat kidney (NRK) cells in vitro which express NHE8 on their apical membrane. Dexamethasone decreased NHE8 mRNA, total protein, and apical protein abundance. Dexamethasone also decreased Na⁺/H⁺ exchanger activity. These studies provide evidence that glucocorticoids may play a role in the developmental isoform switch from NHE8 to NHE3 and cause a decrease in NHE8 expression and activity.

Proximal tubule Na+/H+ exchanger activity in adult NHE8-/-, NHE3-/-, and NHE3-/-/NHE8-/- mice

NHE3 is the predominant Na(+)/H(+) exchanger on the brush-border membrane (BBM) of the proximal tubule in adults. However, NHE3 null mice still have significant renal BBM Na(+)/H(+) activity. NHE8 has been localized to the BBM of proximal tubules and is more highly expressed in neonates than adult animals. The relative role of NHE8 in adult renal H(+) transport is unclear. This study examined whether there was compensation by NHE8 in NHE3(-/-) mice and by NHE3 in NHE8(-/-) mice. NHE3(-/-) mice had significant metabolic acidosis, and renal BBM NHE8 protein abundance was greater in NHE3(-/-) mice than control mice, indicating that there may be compensation by NHE8 in NHE3(-/-) mice. NHE8(-/-) mice had serum bicarbonate levels and pH that were not different from controls. NHE3 protein expression on the BBM was greater in NHE8(-/-) mice than in wild-type mice, indicating that there may be compensation by NHE3 in NHE8(-/-) mice. Both BBM NHE3 and NHE8 protein abundance increased in response to acidosis. Blood pressure and Na(+)/H(+) exchanger activity were comparable in NHE8(-/-) mice to that of controls, but both were significantly lower in NHE3(-/-) mice compared with control mice. Compared with NHE3(-/-) mice, NHE3(-/-)/NHE8(-/-) mice had lower blood pressures. While serum bicarbonate was comparable in NHE3(-/-) mice and NHE3(-/-)/NHE8(-/-) mice, proximal tubule Na(+)/H(+) exchange activity was less in NHE3(-/-)/NHE8(-/-) mice compared with NHE3(-/-) mice. In conclusion, NHE3 is the predominant Na(+)/H(+) exchanger in adult mice. NHE8 may play a compensatory role in renal acidification and blood pressure regulation in NHE3(-/-) mice.

Acid increases NHE8 surface expression and activity in NRK cells

We previously demonstrated that there is a paucity of brush-border membrane NHE3 in neonates, the predominant Na(+)/H(+) exchanger in the adult proximal tubule, while NHE8 is relatively highly expressed in neonates compared with adults. We recently showed that metabolic acidosis in neonatal rodents can increase brush-border membrane NHE8 protein expression and Na(+)/H(+) exchange activity. To further examine the regulation of NHE8 by acid, we incubated NRK cells, which express NHE8 but not NHE3, with either acid or control media (6.6 vs. 7.4). There was an increase in Na(+)/H(+) exchanger activity within 6 h of incubation with acid media assessed as the rate of sodium-dependent recovery of pH from an acid load (dpH(i)/dt). The acid stimulation persisted for at least 24 h. The increase in Na(+)/H(+) exchange activity was paralleled by an increase in surface expression of NHE8, assessed by surface biotinylation and streptavidin precipitation. The increase in both apical membrane NHE8 protein expression and Na(+)/H(+) exchange activity with pH 6.6 media compared with 7.4 media was not affected by actinomycin D or cycloheximide consistent with an increase in surface expression independent of mRNA or protein synthesis. Furthermore, there was no increase in total cellular NHE8 protein abundance or mRNA abundance with acid media. Finally, we demonstrate that the increase in surface expression of NHE8 with acid media was blocked by colchicine and cytochalasin D and mediated by acid increasing the rate of exocytosis. In conclusion, NHE8 surface expression and activity are regulated by acid media by increasing the rate of trafficking to the apical membrane.

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.

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.

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.

Prenatal programming of rat proximal tubule Na+/H+ exchanger by dexamethasone

Prenatal administration of dexamethasone causes hypertension in rats when they are studied as adults. Although an increase in tubular sodium reabsorption has been postulated to be a factor programming hypertension, this has never been directly demonstrated. The purpose of this study was to examine whether prenatal programming by dexamethasone affected postnatal proximal tubular transport. Pregnant Sprague-Dawley rats were injected with intraperitoneal dexamethasone (0.2 mg/kg) daily for 4 days between the 15th and 18th days of gestation. Prenatal dexamethasone resulted in an elevation in systolic blood pressure when the rats were studied at 7-8 wk of age compared with vehicle-treated controls: 131 +/- 3 vs. 115 +/- 3 mmHg (P < 0.001). The rate of proximal convoluted tubule volume absorption, measured using in vitro microperfusion, was 0.61 + 0.07 nl.mm(-1).min(-1) in control rats and 0.93+ 0.07 nl.mm(-1).min(-1) in rats that received prenatal dexamethasone (P < 0.05). Na(+)/H(+) exchanger activity measured in perfused tubules in vitro using the pH-sensitive dye BCECF showed a similar 50% increase in activity in proximal convoluted tubules from rats treated with prenatal dexamethasone. Although there was no change in abundance of NHE3 mRNA, the predominant luminal proximal tubule Na(+)/H(+) exchanger, there was an increase in NHE3 protein abundance on brush-border membrane vesicles in 7- to 8-wk-old rats receiving prenatal dexamethasone. In conclusion, prenatal administration of dexamethasone in rats increases proximal tubule transport when rats are studied at 7-8 wk old, in part by stimulating Na(+)/H(+) exchanger activity. The increase in proximal tubule transport may be a factor mediating the hypertension by prenatal programming with dexamethasone.

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.

Ontogeny of renal sodium transport

One of the main functions of the adult kidney is to maintain a constant extracellular fluid balance. The adult kidney does this, by and large, by filtering a massive quantity of fluid and reabsorbing the solutes needed to maintain volume and electrolyte homeostasis, while leaving the waste products to be excreted in the urine. One of the most precisely regulated functions of the adult kidney is to maintain sodium balance. The challenge of the neonatal kidney is even greater. It must maintain a positive salt balance for growth while the neonate is fed a diet that is very low in sodium. This review focuses on how the neonatal kidney reabsorbs NaCl with a special emphasis on the differences between the neonatal and adult kidney.

Neonatal acid base balance and disturbances

Maintaining acid base balance presents a considerable challenge to the growing neonate. The infant must ingest protein for growth and development. The metabolism of sulfur containing amino acids leads to the production of protons that must be secreted by the kidney. In addition, the formation of hydroxyapatite for the mineralization of growing bone also leads to acid production. Thus, the growing infant must excrete approximately 2 to 3 mEq of acid per kilogram of body weight per day to avoid becoming acidotic. The mechanisms for excreting acid undergo complex maturational changes that predispose the neonate, and the premature neonate in particular, to a great risk for the development of acidosis. In addition, infants are susceptible to gastrointestinal disturbances that can lead to acidosis due to acute loss of bicarbonate in the stool. The kidney is then responsible for the production of new bicarbonate to restore the body's acid base balance. There are also a number of inherited disorders in the kidney that affect acid secretion and lead to acid base disturbances in neonates. This review discusses the mechanisms by which the kidney is capable of excreting acid as well as the developmental regulation of these processes and the basis of inherited disorders of acidification.

Identification of an apical Cl-/HCO-3 exchanger in rat kidney proximal tubule

SLC26A6 (or putative anion transporter 1, PAT1) is located on the apical membrane of mouse kidney proximal tubule and mediates Cl-/HCO3- exchange in in vitro expression systems. We hypothesized that PAT1 along with a Cl-/HCO3- exchange is present in apical membranes of rat kidney proximal tubules. Northern hybridizations indicated the exclusive expression of SLC26A6 (PAT1 or CFEX) in rat kidney cortex, and immunocytochemical staining localized SLC26A6 on the apical membrane of proximal tubules, with complete prevention of the labeling with the preadsorbed serum. To examine the functional presence of apical Cl-/HCO3- exchanger, proximal tubules were isolated, microperfused, loaded with the pH-sensitive dye BCPCF-AM, and examined by digital ratiometric imaging. The pH of the perfusate and bath was kept at 7.4. Buffering capacity was measured, and transport rates were calculated as equivalent base flux. The results showed that in the presence of basolateral DIDS (to inhibit Na+-HCO3- cotransporter 1) and apical EIPA (to inhibit Na+/H+ exchanger 3), the magnitude of cell acidification in response to addition of luminal Cl- was approximately 5.0-fold higher in the presence than in the absence of CO2/HCO3-. The Cl--dependent base transport was inhibited by approximately 61% in the presence of 0.5 mM luminal DIDS. The presence of physiological concentrations of oxalate in the lumen (200 microM) did not affect the Cl-/HCO3- exchange activity. These results are consistent with the presence of SLC26A6 (PAT1) and Cl-/HCO3- exchanger activity in the apical membrane of rat kidney proximal tubule. We propose that SLC26A6 is likely responsible for the apical Cl-/HCO3- (and Cl-/OH-) exchanger activities in kidney proximal tubule.

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.

Water transport in neonatal and adult rabbit proximal tubules

We have recently demonstrated that although the osmotic water permeability (P(f)) of neonatal proximal tubules is higher than that of adult tubules, the P(f) of brush-border and basolateral membrane vesicles from neonatal rabbits is lower than that of adults. The present study examined developmental changes in the water transport characteristics of proximal convoluted tubules (PCTs) in neonatal (9-16 days old) and adult rabbits to determine whether the intracellular compartment or paracellular pathway is responsible for the maturational difference in transepithelial water transport. The permeability of n-butanol was higher in the neonatal PCT than the adult PCT at all temperatures examined, whereas the diffusional water permeability was identical. Increasing the osmotic gradient increased volume absorption in both the neonatal and the adult PCT to the same degree. The P(f) was not different between the neonatal and the adult PCT at any osmotic gradient studied. To assess solvent drag as a measure of the paracellular transport of water, the effect of the osmotic gradient on mannitol and chloride transport were measured. There was no change in chloride or mannitol transport with the increased osmotic gradient in either group, indicating that there was no detectable paracellular water movement. In addition, the mannitol permeability of the neonatal PCT was found to be lower than that of the adult PCT with the isotonic bath (8.97 +/- 4.01 vs. 40.49 +/- 13.89 microm/s, P < 0.05). Thus the intracellular compartment of the neonatal PCT has a lower resistance for water transport than the adult PCT and is responsible for the higher than expected P(f) in the neonatal PCT.

Role of glucocorticoids in the maturation of the rat renal Na+/H+ antiporter (NHE3)

BACKGROUND

Neonates have a lower Na+/H+ antiporter activity on the apical membrane of proximal tubule than that of adults. The maturational increase in Na+/H+ antiporter activity occurs at the time when there is a rise in serum glucocorticoid levels in rats. The purpose of the present study was to examine whether glucocorticoids are responsible for the postnatal increase in Na+/H+ antiporter activity.

METHODS

Nine-day-old Sprague-Dawley rats were compared with rats studied at 30 days of age who had either a sham operation or adrenalectomy (ADX) at nine days of age and with rats that had an adrenalectomy and physiologic corticosterone replacement (ADX-Cort) to determine whether glucocorticoid deficiency prevented the maturational increase in Na+/H+ antiporter activity. Na+/H+ antiporter activity was measured in proximal convoluted tubules perfused in vitro by the change in cell pH (pHi) following luminal sodium removal. NHE3 mRNA abundance was measured using Northern blot analysis, and NHE3 protein abundance was measured by immunoblot.

RESULTS

Na+/H+ antiporter activity was 93.8 +/- 17.7, 157.0 +/- 18.0, 356.7 +/- 29.9, and 402.5 +/- 14.5 pmol/mm. min in nine-day-old, ADX, ADX-Cort, and sham control groups, respectively. The ADX-Cort and sham control were higher than the 9-day-old and the 30-day-old ADX group (P < 0.05). Brush-border membrane NHE3 protein abundance in the nine-day-old and ADX groups were sixfold less than ADX-Cort and sham control groups (P < 0.001). Nine-day-old neonates had fivefold less renal cortical NHE3 mRNA than the ADX, ADX-Cort, and sham-operated control groups (P < 0.01).

CONCLUSIONS

These data demonstrate that glucocorticoids play a role in the postnatal maturation of the proximal tubule Na+/H+ antiporter activity and brush-border membrane NHE3 protein abundance. Glucocorticoid deficiency does not completely prevent the maturational increase in Na+/H+ antiporter activity and does not affect NHE3 mRNA abundance.

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.

The moth and the aspen tree: sodium in early postnatal development

Over the past 25 years, our perception of the neonatal kidney has changed markedly from its being a "limited" organ compared with that of the adult to being extraordinarily well adapted in its role in maintaining homeostasis and making possible the rapid somatic growth necessary during this critical period of life. The present review focuses on the physiologic adaptations by the neonatal kidney in the maintenance of a positive sodium balance, which is necessary for normal growth not only in mammals but also in moths. There is a fine interplay between the developing brain, heart, thyroid, adrenals, and sympathetic nervous system, all converging on the kidney to conserve sodium, which is limited in the diet. The renin-angiotensin system plays a central role in this response and is balanced by developmental changes in the renal response to atrial natriuretic peptide, all of which contribute to sodium conservation. Over the next 25 years, advances in molecular genetics will doubtless elucidate many more facets of the mechanisms underlying neonatal sodium homeostasis. This will be particularly important as the survival of ever smaller preterm infants improves steadily.

Diuretic therapy of heart failure in infants and children

Diuretics are the mainstay of traditional therapy for congestive heart failure. The syndrome of heart failure is now understood to involve complex interactions of neurohumoral substances released in response to poor cardiac function. Developmental changes during infancy and childhood will affect both the activation of systemic neurohumoral responses and the pharmacokinetic and pharmacodynamic actions of diuretics. Few human studies directly evaluate the efficacy of diuretic therapy in heart failure in adults. The pharmacodevelopmental aspects of diuretic therapy in infants and children are also incompletely studied. This review will describe the kidney's role in the pathogenesis of sodium and water retention in heart failure and the developmental changes in the kidney related to fluid retention. Known principles of diuretic therapy in congestive heart failure will be described. All these factors can then be used by the reader to evaluate the role of diuretic therapy in the complex syndrome of heart failure in infants and children.

Ontogeny of Na+/H+ antiporter activity in rat proximal convoluted tubules

Neonates have a lower serum bicarbonate level than adults, which is caused by a lower renal threshold for bicarbonate. Eighty percent of bicarbonate reabsorption occurs in the proximal tubule, in which proton secretion is predominantly mediated by a luminal Na+/H+ antiporter. Previous studies have demonstrated that there is a maturational increase in apical membrane rabbit proximal convoluted tubule Na+/H+ antiporter activity. However, in rat brush border membrane vesicles, Na+/H+ activity was higher in neonates than that in adult rats. To examine the maturation of Na+/H+ antiporter activity in rat proximal convoluted tubules, we perfused rat proximal convoluted tubules in vitro. Na+/H+ antiporter activity was assayed as the proton secretory rate on luminal sodium removal. Na+/H+ antiporter activity was 121.2 +/- 18.4 pmol/mm x min in neonatal and 451.8 +/- 40.6 pmol/mm x min in adult proximal convoluted tubules (p < 0.001). We next examined whether the increase in Na+/H+ antiporter activity was associated with changes in renal cortical NHE3 mRNA and brush border membrane NHE3 protein abundance. Adult renal cortical NHE3 mRNA abundance was 10-fold greater than that in 1-d-old neonates (p < 0.001). There was a comparable developmental increase in renal brush border membrane vesicle NHE3 protein abundance (p < 0.001). In summary, this study demonstrates that there is a maturational increase in rat apical membrane Na+/H+ antiporter activity, renal cortical NHE3 mRNA, and brush border membrane vesicle NHE3 protein abundance.

Novel amiloride-sensitive sodium-dependent proton secretion in the mouse proximal convoluted tubule

The proximal convoluted tubule (PCT) reabsorbs most of the filtered bicarbonate. Proton secretion is believed to be mediated predominantly by an apical membrane Na(+)/H(+) exchanger (NHE). Several NHE isoforms have been cloned, but only NHE3 and NHE2 are known to be present on the apical membrane of the PCT. Here we examined apical membrane PCT sodium-dependent proton secretion of wild-type (NHE3(+/+)/NHE2(+/+)), NHE3(-/-), NHE2(-/-), and double-knockout NHE3(-/-)/NHE2(-/-) mice to determine their relative contribution to luminal proton secretion. NHE2(-/-) and wild-type mice had comparable rates of sodium-dependent proton secretion. Sodium-dependent proton secretion in NHE3(-/-) mice was approximately 50% that of wild-type mice. The residual sodium-dependent proton secretion was inhibited by 100 microM 5-(N-ethyl-N-isopropyl) amiloride (EIPA). Luminal sodium-dependent proton secretion was the same in NHE3(-/-)/NHE2(-/-) as in NHE3(-/-) mice. These data point to a previously unrecognized Na(+)-dependent EIPA-sensitive proton secretory mechanism in the proximal tubule that may play an important role in acid-base homeostasis.

Maturation of proximal straight tubule NaCl transport: role of thyroid hormone

We have recently demonstrated that the rates of both active and passive proximal straight tubule (PST) NaCl transport in neonatal rabbits were less than in adults. In this segment NaCl entry across the apical membrane is via parallel Na(+)/H(+) and Cl(-)/OH(-) exchangers, which increases in activity with maturation. The present in vitro microperfusion study examined whether thyroid hormone plays a role in the maturational increase in PST NaCl transport. Neonatal and adult PST were perfused with a high-chloride-low bicarbonate solution without organic solutes, simulating late proximal tubule fluid. Thyroid hormone-treated neonates had a higher rate of PST total and passive NaCl transport. In 8-wk-old animals that were hypothyroid since birth, the maturational increase in total and passive NaCl transport was prevented. Thyroid treatment for 4 days in hypothyroid 8-wk-old rabbits increased the rate of both total and passive NaCl transport. The maturational increases in both Na(+)/H(+) and Cl(-)/OH(-) exchange activities were blunted in 8-wk-old hypothyroid animals and increased to control levels with thyroid treatment. This study demonstrates that thyroid hormone is a factor responsible for the maturational increase in both active and passive PST NaCl transport.

Maturational changes in rabbit renal basolateral membrane vesicle osmotic water permeability

We have recently demonstrated that while the osmotic water permeability (Pf) of neonatal proximal tubules is higher than that of adult tubules, the Pf of brushborder membrane vesicles from neonatal rabbits is lower than that of adults. The present study examined developmental changes in the water transport characteristics of proximal tubule basolateral membranes by determining aquaporin 1 (AQP1) protein abundance and the Pf in neonatal (10-14 days old) and adult rabbit renal basolateral membrane vesicles (BLMV). At 25 degrees C the Pf of neonatal BLMV was significantly lower than the adult BLMV at osmotic gradients ranging from 40 to 160 mOsm/kg water. The activation energies for osmotic water movement were identical in the neonatal and adult BLMV (8.65 +/- 0.47 vs. 8.86 +/- 1.35 kcal x deg(-1) x mol(-1). Reflection coefficients for sodium chloride and sodium bicarbonate were identical in both the neonatal and adult BLMV and were not different from one. Mercury chloride (0.5 mM) reduced osmotic water movement by 31.3 +/- 5.5% in the adult BLMV, but by only 4.0 +/- 4.0% in neonatal vesicles (P < 0.01). Adult BLMV AQP1 abundance was higher than that in the neonate. These data demonstrate that neonatal BLMV have a lower Pf and AQP1 protein abundance than adults and that a significantly greater fraction of water traverses the basolateral membrane lipid bilayer and not water channels in neonates compared to adults. The lower Pf of the neonatal BLMV indicates that the basolateral membrane is not responsible for the higher transepithelial Pf in the neonatal proximal tubule.

Neonatal rabbit proximal tubule basolateral membrane Na+/H+ antiporter and Cl-/base exchange

The present in vitro microperfusion study examined the maturation of Na+/H+ antiporter and Cl-/base exchanger on the basolateral membrane of rabbit superficial proximal straight tubules (PST). Intracellular pH (pHi) was measured with the pH-sensitive fluorescent dye 2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein in neonatal and adult superficial PST. Na+/H+ antiporter activity was examined after basolateral Na+ addition in tubules initially perfused and bathed without Na+. Neonatal Na+/H+ antiporter activity was approximately 40% that of adult segment (9.7 +/- 1.5 vs. 23.7 +/- 3.2 pmol. mm-1. min-1; P < 0.001). The effect of bath Cl- removal on pHi was used to assess the rates of basolateral Cl-/base exchange. In both neonatal and adult PST, the Cl-/base exchange activity was significantly higher in the presence of 25 mM HCO-3 than in the absence of HCO-3 and was inhibited by cyanide and acetazolamide, consistent with Cl-/HCO-3 exchange. The proton flux rates in the presence of bicarbonate in neonatal and adult tubules were 14.1 +/- 3.6 and 19.5 +/- 3.5 pmol. mm-1min-1, respectively (P = NS), consistent with a mature rate of Cl-/HCO-3 exchanger activity in neonatal tubules. Basolateral Cl-/base exchange activity in the absence of CO2 and HCO-3, with luminal and bath cyanide and acetazolamide, was greater in adult than in neonatal PST and inhibited by bath DIDS consistent with a maturational increase in Cl-/OH- exchange. We have previously shown that the rates of the apical membrane Na+/H+ antiporter and Cl-/base exchanger were approximately fivefold lower in neonatal compared with adult rabbit superficial PST. These data demonstrate that neonatal PST basolateral membrane Na+/H+ antiporter and Cl-/base exchanger activities are relatively more mature than the Na+/H+ antiporter and Cl-/base exchangers on the apical membrane.

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.

Epidermal growth factor inhibits Na-Pi cotransport in weaned and suckling rats

In the present study, we determined the effect of epidermal growth factor (EGF; 10 microgram/100 g body wt) on sodium gradient-dependent phosphate transport (Na-Pi cotransport) regulation in suckling (12-day-old) and weaned (24-day-old) rats. Weaned rats had higher proximal tubular brush border membrane vesicle (BBMV) Na-Pi cotransport activity (232 +/- 16 in weaned vs. 130 +/- 9 pmol. 10 s-1. mg protein-1 in suckling rats, P < 0.05). Chronic treatment with EGF induced inhibition of BBMV Na-Pi cotransport in both suckling (130 +/- 9 vs. 104 +/- 7 pmol. 10 s-1. mg protein-1, P < 0. 05) and weaned rats (232 +/- 16 vs. 145 +/- 9 pmol. 10 s-1. mg protein-1, P < 0.005). The inhibitory effect was selective for Na-Pi cotransport as there was no inhibition of Na-glucose cotransport. Weaned rats had a higher abundance of BBMV NaPi-2 protein than suckling rats (increase of 54%, P < 0.001) and a twofold increase in NaPi-2 mRNA. The EGF-induced inhibition of Na-Pi transport was paralleled by decreases in NaPi-2 protein abundance in both weaned (decrease of 26%, P < 0.01) and suckling (decrease of 27%, P < 0.01) animals. In contrast, there were no changes in NaPi-2 mRNA abundance. We conclude that proximal tubule BBMV Na-Pi cotransport activity, NaPi-2 protein abundance, and NaPi-2 mRNA abundance are higher in weaned than in suckling rats. EGF inhibits Na-Pi cotransport activity in BBMV isolated from suckling and weaned rats, and this inhibition is mediated via a decrease in NaPi-2 protein abundance, in the absence of a change in NaPi-2 mRNA.

Maturational changes in rabbit renal brush border membrane vesicle osmotic water permeability

We have recently shown that the osmotic water permeability (Pf) of proximal tubules from neonatal rabbits is higher than that of adults (AJP 271:F871-F876, 1996). The developmental change in Pf could be due to differences in one or more of the components in the path for transepithelial water transport. The present study examined developmental changes in water transport characteristics of the proximal tubule apical membrane by determining Pf and aquaporin 1 (AQP1) expression in neonatal (10-14 days old) and adult rabbit renal brush border membrane vesicles (BBMV). AQP1 abundance in the adult BBMV was higher than the neonatal BBMV. At 25 degrees C the Pf of neonatal BBMV was found to be significantly lower than the adult BBMV at osmotic gradients from 50 to 250 mOsm/kg water. The activation energy for osmotic water movement was higher in the neonatal BBMV than the adult BBMV (9.19 +/- 0.37 vs. 5.09 +/- 0.57 kcal . deg-1 . mol-1, P < 0.005). Osmotic water movement in neonatal BBMV was inhibited 17.9 +/- 1.3% by 1 mm HgCl2 compared to 34.3 +/- 3.8% in the adult BBMV (P < 0.005). These data are consistent with a significantly greater fraction of water traversing the apical membrane lipid bilayer in proximal tubules of neonates than adults. The lower Pf of the neonatal BBMV indicates that the apical membrane is not responsible for the higher transepithelial Pf in the neonatal proximal tubule.

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.

Maturation of rabbit proximal straight tubule chloride/base exchange

The present in vitro microperfusion study compared the mechanism and rates of NaCl transport in neonatal and adult rabbit proximal straight tubules. In proximal straight tubules perfused with a late proximal tubular fluid and bathed in a serumlike albumin solution, the rate of volume absorption (JV) was 0.54 +/- 0.10 and 0.12 +/- 0.05 nl.mm-1.min-1 in adults and neonates, respectively (P < 0.05). With the addition of 10(-5) M bath ouabain, JV decreased to 0.27 +/- 0.07 and -0.03 +/- 0.04 nl.mm-1.min-1 in adult and neonatal tubules, respectively (P < 0.05), consistent with lower rates of active and passive NaCl transport in the neonatal proximal straight tubule. The effect of luminal sodium and chloride removal on intracellular pH was used to assess the relative rates of Na+/H+ and Cl-/base exchange. The rates of Na+/H+ and Cl-/base exchange were approximately fivefold less in neonatal proximal straight tubules than adult tubules. In both neonatal and adult proximal straight tubules, the rate of Cl-/base exchange was not affected by formate, bicarbonate, or cyanide and acetazolamide, consistent with Cl-/OH- exchange. These data demonstrate an increase in proximal straight tubule NaCl transport during postnatal renal development.

Axial heterogeneity of sodium-bicarbonate cotransporter expression in the rabbit proximal tubule

It is generally accepted that Na(HCO3)n cotransport is the most important mechanism mediating basolateral bicarbonate efflux in the early proximal tubule. The presence of basolateral Na(HCO3)n cotransport in the late proximal tubule (S3 segment) and in the juxtamedullary S1 and S2 segments has been controversial. The renal sodium-bicarbonate cotransporter (NBC) has been recently cloned from rat (M. F. Romero, M. A. Hediger, E. L. Boulpaep, and W. F. Boron. J. Am. Soc. Nephrol. 7: 1259, 1996), salamander (M. F. Romero, M. A. Hediger, E. L. Boulpaep, and W. F. Boron. Nature 387: 409-413, 1997), and human (C. E. Burnham, H. Amlal, Z. Wang, G. E. Shull, and M. Soleimani. J. Biol. Chem. 272: 19111-19114, 1997). The localization of NBC in the kidney is unknown. The present study was designed to localize NBC mRNA expression in the rabbit proximal tubule. In situ hybridization studies were combined with functional studies of basolateral Na(HCO3)n cotransport in superficial and juxtamedullary S1, S2, and S3 segments of the rabbit proximal tubule. The results demonstrate that NBC mRNA is localized predominantly to the cortex, with less expression in the outer medulla. NBC expression was not detected in the inner medulla. The highest level of NBC mRNA is in the S1 proximal tubule. NBC is expressed at a low levels in the S3 segment, with intermediate expression in the S2 segment. In bicarbonate-buffered solutions, the rate of base efflux mediated by Na(HCO3)n cotransport followed a similar pattern in superficial and juxtamedullary proximal tubule segments, i.e., S1 > S2 > S3. The juxtamedullary S1 segment had the greatest rate of basolateral Na(HCO3)n cotransport and the highest level of NBC expression in the proximal tubule.

Age dependence of tolerance to anoxia and changes in cytosolic calcium in rabbit renal proximal tubules

Calcium(Ca2+)-dependent processes mediate, in part, anoxic cell injury. These may account for the difference in sensitivity to anoxia between certain immature and mature renal cells. To address this question, we studied the effects of anoxia on cytosolic free Ca2+ concentration ([Ca2+]i), cell integrity, and transport functions in microdissected proximal convoluted tubules (PCT) of < 3-week-old (newborn) and > 12-week-old (adult) rabbits. Tubules were loaded with 10 microM fura-2 AM by incubation for 60 min at 37 degrees C, and then superfused with isosmotic saline solution gassed with either 95%O2-5%CO2 (control group) or 95%N2-5%CO2 (anoxia group) for 30 min. [Ca2+]i was measured ratiometrically; cell damage was assessed by nuclear binding of propidium iodide (PI). Anoxia resulted in a fourfold increase in [Ca2+]i in adult tubules (from resting values of 245 +/- 10 to 975 +/- 100 nM, P < 0.001), whereas in newborn tubules the rise was significantly less (from resting values of 137 +/- 5 to 165 +/- 5 nM, P < 0.001 between anoxic groups). Transient exposure to 100 mM potassium chloride, which depolarizes the PCT cells, induced increases in [Ca2+]i from baseline, to 920 +/- 90 nM in tubules from adult and to 396 +/- 16 nM in those from newborn rabbits (P < 0.001 between age groups). After exposure to ligands such as parathyroid hormone (PTH) and ATP, [Ca2+]i increased in both newborn and adult tubules, but to lower levels in newborn tubules. The response to PTH and ATP was transient in both age groups, [Ca2+]i returning to baseline levels after 2 min. Following anoxia, tubules from adult animals exhibited staining of all cell nuclei by 1 min exposure to PI, indicative of gross permeabilization of the cells. Nuclei of anoxic immatures tubules did not stain with PI. The sodium-dependent uptakes of a glucose analogue (14C-alpha-methyl-glucopyranoside) and phosphate (32Pi) were preserved in agarose-filled tubules of newborns after anoxia, whereas in those of adults recovery from anoxia was associated with drastic reduction in the uptake of these solutes. Overall, our results suggest that: (1) during anoxia, cell Ca2+ rises to critical levels in PCTs of adults compared with those of < 3-week-old animals, (2) Ca2+ influx occurs via a pathway activated by exposure to high [K+]o, presumably voltage-sensitive Ca2+ channels or reversal of Na(+)-Ca2+ exchange, (3) these pathways are either less active or less abundant in proximal tubules of newborn compared with adult rabbits, and (4) secondary active transport activity and cellular integrity are well preserved after anoxia in PCT cells of newborn but not of adult rabbits.

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.

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.

Na/H antiporter mRNA expression in single nephron segments of rat kidney cortex

Renal cortical tubules consist of polarized epithelial cells where Na/H antiport activity has been demonstrated on the apical and/or basolateral membrane. Apical Na/H antiport activity plays an important role in transcellular bicarbonate (HCO3-) reabsorption, whereas basolateral Na/H antiport activity could be involved in transcellular HCO3- secretion as well as cell volume and pH control. To determine whether this heterogeneity in both localization and function is due to the existence of more than one Na/H antiporter, we studied the tissue distribution of Na/H antiporter mRNA by use of reverse transcription (RT) and polymerase chain reaction (PCR) in isolated nephron segments from rat renal cortex. The primers used were directed against the rat renal cortical Na/H antiporter cDNA which is homologous to the human growth factor-activatable Na/H antiporter. RT/PCR of beta-actin mRNA were performed as positive controls. Na/H antiporter mRNA expression in the proximal tubule was not detectable in S1 and S2 segments from superficial and most midcortical nephrons, which exhibit exclusively luminal Na/H antiport activity. It was expressed in S1 and S2 segments from juxtamedullary nephrons which have also basolateral Na/H antiport activity. Beta-actin mRNA was expressed uniformly in all segments of the proximal tubule. Na/H antiporter mRNA was also expressed in cortical thick ascending limb and cortical collecting duct, segments with basolateral Na/H antiport activity as well as in the glomeruli. In conclusion, at least two different Na/H antiporters exist in the renal cortex, i.e., the proximal tubule. The close correlation between functional localization of basolateral Na/H antiport activity and mRNA expression suggests that the rat kidney Na/H antiporter DNA homologous to the human growth factor activatable Na/H antiporter encodes a basolateral exchanger. The observed expression in a minority of midcortical proximal tubules could reflect a certain heterogeneity in these nephron segments.

Renal growth in infancy and childhood--experimental studies of regulatory mechanisms

During the peri- and early postnatal period, nephrogenesis is completed and kidney growth is accomplished both by cellular proliferation and enlargement. The number of nephrons in a given species is predetermined, whereas cellular growth can be influenced by environmental factors in an age-dependent manner. Unilateral nephrectomy or a high-protein diet stimulates renal growth more in the young than in the adult. Conversely, pyelonephritis inhibits renal growth in infancy but not in adulthood. The relative importance of hyperplasia and hypertrophy for renal growth also changes with renal maturation. The mechanisms behind these developmental changes in regulation of renal growth are largely unknown, but age-dependent changes in the expression of several proto-oncogene products have been demonstrated. These include growth factor receptors as well as components of the intracellular system that transfers the signal from an activated growth factor receptor to the cell nucleus. Studies on rat proximal tubule cells in primary culture might be of great value in expanding our knowledge of growth regulation in the developing kidney. Such studies have already shown that under identical environmental conditions the basal proliferative rate is age dependent, that the proliferative response to growth stimulation changes postnatally, and that this is associated with changes of both the response of the Na+/H(+)-exchanger and the expression of the c-fos proto-oncogene.

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.

Salt and the newborn kidney

Renal function differs in term infants from that in adults, with lower glomerular filtration rate (GFR) and reduced proximal tubular reabsorption of sodium (Na) and water: nevertheless, it is adequate for their needs. This is not true of very preterm infants in whom hyponatraemia is common. Animal studies have shown that Na+, K(+)-ATPase and the Na+/K+ exchanger are poorly expressed at birth with rapid postnatal rises. Cell receptors for hormones that influence tubular Na transport are less numerous in the premature infant than later in life: intracellular second messenger systems may also be immature. The low GFR is due to vasoconstriction and may be necessary to prevent water and electrolyte wasting due to tubular overload. The hyponatraemia of prematurity could, in principle, be due either to Na loss or water excess and can be prevented either by giving additional Na or by restricting water intake. Na supplementation causes relative volume expansion (VE), water restriction volume contraction (VC); this is demonstrated by the effect of the two approaches on weight gain and on the levels of vasoactive hormones in the blood. We argue that moderate VE is more physiological than VC, both in attempting to simulate intrauterine conditions and in consideration of the infant's nutritional needs. The much less common complication of hypernatraemia is usually due to abnormal water loss and should be prevented by increasing water intake appropriately. The above applies to well, preterm babies: sick preterm infants are much more variable in their Na and water requirements than well infants of comparable gestation and weight and each needs an individually tailored regimen based on frequent clinical assessment and laboratory measurement.