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

We have shown that there is a maturational increase in osmotic water permeability (P_f) 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 (P_DW) 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 P_DW in red cell ghosts and liposomes, to examine the maturational changes in P_DW in BBMV. This method utilizes the sensitivity of 8-amino-naphtalene-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°-37°C. P_DW was lower in neonatal than adult BBMV at 5° (3.77 ± 0.34 vs. 5.35 ± 0.43 μm/sec, respectively, p < 0.05) and 20°C (7.03 ± 0.40 vs. 9.04 ± 0.25 μm/sec, respectively, p < 0.001), but was not different at 30° and 37°C. The activation energy (E_a) 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, P_DW 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 P_DW 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.