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

Glucocorticoids increase osmotic water permeability (Pf) of neonatal rabbit renal brush border membrane vesicles

During postnatal maturation, there is an increase in renal brush border membrane vesicle (BBMV) osmotic water permeability and a parallel increase in aquaporin-1 (AQP1) protein abundance. The mechanisms responsible for these changes remain unknown. Because serum glucocorticoid levels rise postnatally and have previously been linked to other maturational changes in renal function, we examined the effects of glucocorticoids on osmotic (Pf) and diffusional (P(DW)) water permeability and AQP1 protein abundance of renal BBMV. Neonatal rabbits were treated with dexamethasone (10 microg/100 g) for three days and compared with control neonates and adults. Pf and P(DW) were measured at 20 degrees C with a stopped-flow apparatus using light-scattering and aminonaphthalene trisulfonic acid (ANTS) fluorescence, respectively. Pf was significantly higher in BBMV from dexamethasone-treated neonates compared with vehicle-treated neonates, but remained lower than in BBMV from adults (P<0.05). P(DW) in dexamethasone and vehicle-treated neonatal BBMV was lower than in adult BBMV. Pf/P(DW) ratio increased from neonate (5.1+/-0.3) to dexamethasone (7.0+/-0.1) and adult BBMV (6.3+/-0.1). AQP1 expression was increased by dexamethasone treatment to adult levels. Membrane fluidity, which is inversely related to generalized polarization (GP) of steady-state laurdan fluorescence, was significantly higher in neonatal BBMV than both dexamethasone and adult BBMV (GP: neonate 0.285+/-0.002, dexamethasone treatment 0.302+/-0.006, and adult 0.300+/-0.005; P<0.05). These combined results show that dexamethasone-treatment during days 4-7 of life increases BBMV water permeability despite a decrease in membrane fluidity. This occurs by increasing channel-mediated water transport, as reflected in an increase in AQP1 protein abundance and a higher Pf/P(DW) ratio. This mimics the maturational changes and suggests a physiological role for glucocorticoids in maturation of proximal tubule water transport.

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.

Ontogeny of water transport in the rabbit proximal tubule

Water transport across cell membranes is a fundamental biological problem. In the kidney, many nephron segments have mechanisms for transporting large quantities of water with minimal energy input. The proximal tubule reabsorbs two-thirds of the glomerular filtrate with a small transepithelial osmotic gradient as the driving force. In the adult proximal tubule, this is accomplished by the expression of aquaporin 1 (AQP1), the water channel located on the apical and basolateral membranes of the proximal tubule. The neonatal tubule has a much lower expression of AQP1, yet can still transport water with a small osmotic gradient. Thus, tubule properties other than AQP1 expression must allow for this to occur. There are two primary differences that account for this unexpectedly high osmotic water permeability of the neonatal proximal tubule. First, the lipid membrane of the neonatal tubule is more fluid than the adult tubule and therefore a larger fraction of the water can pass through the lipid bilayer. The second property is the fact that the neonatal tubule cells have a smaller cell volume, and thus, the intracellular compartment provides less resistance for the movement of water. This review will discuss postnatal maturation of proximal tubule water transport.

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.

Hypothyroidism increases osmotic water permeability (Pf) in the developing renal brush border membrane

The osmotic water permeability (Pf) of the rabbit proximal tubule brush border membrane vesicles (BBMV) increases during maturation and is mediated by an increase in aquaporin-1 (AQP1) protein expression. Serum thyroid hormone levels increase after birth and have been shown to play a role in the maturation of other renal transport functions. We examined the hypothesis that thyroid hormone plays a role in the maturational increase in osmotic water permeability. Hypothyroidism was induced by addition of 0.1% propylthiouracil (PTU) to the drinking water of pregnant rabbits (starting 9 d before delivery) and was continued until the rabbits were studied as adults (9-11 wk). Some animals received thyroid hormone replacement by daily injection with triiodothyronine (T3; 10 microg/100 g body weight) for three days before study. Pf was found to be higher in BBMV from hypothyroid (82.7 +/- 5.5 microm/s) than from euthyroid (60.6 +/- 4.0 microm/s) and T3-replacement rabbits (69.0 +/- 5.0 microm/s) (p < 0.05). The activation energy (Ea; in kcal/deg.mol) of Pf was not different among the three experimental groups (euthyroid 5.6 +/- 0.9, hypothyroid 4.9 +/- 0.8, T3-replacement 5.0 +/- 1.0; p = NS), nor was the percentage mercury inhibition of Pf (euthyroid 66.5 +/- 5.3, hypothyroid 74.2 +/- 3.2 and T3-replacement 73.1 +/- 4.3; p = NS). AQP1 expression, measured by immunoblotting, was highest in BBMV from hypothyroid rabbits (p < 0.05). Membrane fluidity, measured as steady-state generalized polarization (GP) of Laurdan, which is inversely related to membrane fluidity, was significantly different between the three groups (GP: euthyroid 0.307 +/- 0.004, hypothyroid 0.271 +/- 0.004 and T3-replacement 0.287 +/- 0.003; for all p < 0.05). These data demonstrate that the maturational increase in thyroid hormone levels is not responsible for the maturational increase in water transport. Surprisingly, congenital hypothyroidism in rabbits is associated with an increased Pf when rabbits are studied as adults. The higher Pf in hypothyroid adult rabbits is due to a higher expression of AQP1 protein as well as a greater membrane fluidity than in euthyroid rabbits.

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.

Diffusional water permeability (PDW) of adult and neonatal rabbit renal brush border membrane vesicles

We have shown that there is a maturational increase in osmotic water permeability (Pf) of rabbit renal brush border membrane vesicles (BBMV). The purpose of the present study was to further investigate the changes in proximal tubule water transport that occur during postnatal development. Diffusional water permeability (PDW) has not been measured directly in adult or neonatal BBMV. We validated the method described by Ye and Verkman (Simultaneous optical measurement of osmotic and diffusional water permeability in cells and liposomes. Biochemistry 28:824-829, 1989) to measure PDW in red cell ghosts and liposomes, to examine the maturational changes in PDW in BBMV. This method utilizes the sensitivity of 8-aminonaphtalene-1,3,6-trisulfonic acid (ANTS) fluorescence to the D2O-H2O content of the solvent. ANTS-loaded neonatal (11 days old) and adult BBMV were rapidly mixed with two volumes of isoosmotic D2O solution using a stopped-flow apparatus at 5 degrees -37 degrees C. PDW was lower in neonatal than adult BBMV at 5 degrees (3.77 +/- 0.34 vs. 5.35 +/- 0.43 mm/sec, respectively, p<0.05) and 20 degrees C (7.03 +/- 0.40 vs. 9.04 +/- 0.25 mm/sec, respectively, p<0.001), but was not different at 30 degrees and 37 degrees C. The activation energy (Ea) was higher in neonatal than in adult BBMV (9.29 +/- 0.56 kcal/mol vs. 6.46 +/- 0.56 kcal/mol, p<0.001). In adult BBMV, PDW was inhibited by 0.5 mM HgCl2 by 46.6 +/- 3.6%, while it was not affected in neonatal BBMV (p<0.001). The results indicate that PDW can be measured in rabbit renal BBMV. There are significant changes in water transport across the apical membrane during postnatal development, consistent with a maturational increase in channel-mediated water transport.

Ontogeny of rabbit proximal tubule urea permeability

Urea transport in the proximal tubule is passive and is dependent on the epithelial permeability. The present study examined the maturation of urea permeability (P(urea)) in in vitro perfused proximal convoluted tubules (PCT) and basolateral membrane vesicles (BLMV) from rabbit renal cortex. Urea transport was lower in neonatal than adult PCT at both 37 and 25 degrees C. The PCT P(urea) was also lower in the neonates than the adults (37 degrees C: 45.4 +/- 10.8 vs. 88.5 +/- 15.2 x 10(-6) cm/s, P < 0.05; 25 degrees C: 28.5 +/- 6.9 vs. 55.3 +/- 10.4 x 10(-6) cm/s; P < 0.05). The activation energy for PCT P(urea) was not different between the neonatal and adult groups. BLMV P(urea) was determined by measuring vesicle shrinkage, due to efflux of urea, using a stop-flow instrument. Neonatal BLMV P(urea) was not different from adult BLMV P(urea) at 37 degrees C [1.14 +/- 0.05 x 10(-6) vs. 1.25 +/- 0.05 x 10(-6) cm/s; P = not significant (NS)] or 25 degrees C (0.94 +/- 0.06 vs. 1.05 +/- 0.10 x 10(-6) cm/s; P = NS). There was no effect of 250 microM phloretin, an inhibitor of the urea transporter, on P(urea) in either adult or neonatal BLMV. The activation energy for urea diffusion was also identical in the neonatal and adult BLMV. These findings in the BLMV are in contrast to the brush-border membrane vesicles (BBMV) where we have previously demonstrated that urea transport is lower in the neonate than the adult. Urea transport is lower in the neonatal proximal tubule than the adult. This is due to a lower rate of apical membrane urea transport, whereas basolateral urea transport is the same in neonates and adults. The lower P(urea) in neonatal proximal tubules may play a role in overall urea excretion and in developing and maintaining a high medullary urea concentration and thus in the ability to concentrate the urine during renal maturation.

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 and adult rabbit renal brush border membrane vesicle solute reflection coefficients

The interaction between solute and water in epithelial transport is represented by the solute reflection coefficient. Because the osmotic water transport process changes in the rabbit proximal tubule during maturation, there is a potential for the solute reflection coefficients to also undergo maturational changes. In the present study, we directly examined solute reflection coefficients in neonatal and adult brush border membrane vesicles (BBMV) using the stop-flow light-scattering technique. Reflection coefficients for NaCl, KCl, NaHCO3 and urea were found to be identical in the neonatal and adult BBMV and were not different from 1. Thus, although the water transport pathway undergoes changes in the proximal tubule during maturation, there is no evidence for changes in solute and water interaction. Because the reflection coefficients are not different from 1, there is no evidence for solvent drag in the proximal tubule apical membrane in either the neonatal or adult tubule.

Maturational changes in rabbit renal brush border membrane vesicle urea permeability

Urea transport in the proximal tubule is thought to occur by passive diffusion through the lipid bilayers of the cell membranes. The lipid composition of cell membranes changes during maturation and may directly affect urea permeability of proximal tubule membranes. The present study examined the maturation of urea transport in rabbit renal brush border membrane vesicles (BBMV). BBMV from adult and neonatal (9- to 11-d-old) New Zealand white rabbits were loaded with 500 mM urea and mixed with an iso-osmotic mannitol solution using a stop-flow instrument. Vesicle shrinkage, due to efflux of urea, was followed with light scattering and urea permeability was calculated from an exponential fit of the data. Urea permeability was significantly lower in the neonatal BBMV than the adult at 25 degrees C (0.34+/-0.04 x 10(-6) versus 0.56+/-0.03 x 10(-6) cm/sec;p < 0.001, n=7) and 37 degrees C (0.45+/-0.04 x 10(-6) versus 0.66+/-0.03 x 10(-6) cm/sec; p=0.001, n=7). There was no effect of 250 microM phloretin on urea permeability in either adult or neonatal BBMV at either temperature. The activation energy for urea diffusion was higher in the neonatal than the adult BBMV. Because the maturational increase in urea permeability could potentially be due to a sodium-dependent urea transporter in the adult BBMV, the sodium dependence of urea uptake in adult BBMV was examined. There was no difference in urea permeability in the presence or absence of 20 mM NaCl. Permeability of the lipid-soluble molecule, glycerol, was also found to be the same in the neonatal and adult BBMV. Urea transport in the apical membrane of neonatal and adult proximal tubules is not phloretin sensitive, a finding consistent with diffusion of urea via the lipid bilayer. The rate of urea diffusion is lower in neonatal membranes and may be an important factor in overall urea excretion. This may also play a role in developing and maintaining a high medullary urea concentration and thus the ability to concentrate the urine during renal maturation.