DEVELOPMENTAL-CHANGES OF RENAL BRUSH-BORDER MEMBRANE IONIC PERMEABILITY

Renal brush-border membrane vesicles (BBMV) show an age-dependent incr ease in height of the Na+-gradient driven overshoot for glucose and pr oline uptake. Conversely, early uptake of Na-22(+) is more rapid in BB MV from kidney of 7-day-old vs. adult rats. To understand the mechanis ms responsible for these observations, ionic permeability characterist ics of BBMV from different aged animals were determined using an elect rical potential sensitive fluorescent dye, diS-C-3(5). Absolute and re lative ionic permeabilities were determined after a 3-h incubation in 100 mM KCl. Intravesicular K+ ([K+](in)), a measure of absolute K+ per meability, was calculated from the extravesicular K+ at which valinomy cin produced no potential difference (PD). [K+](in) was significantly lower in vesicles from 7-day, compared to adult (P < 0.01). While Cl- permeability, relative to that of K+ (P-Cl-/P-K+) was similar, P-Na+/P -K+ decreased significantly with age (P < 0.05: 7 day vs. adult). In t he presence of an inwardly directed NaCl gradient, the lower P-Na+ rel ative to P-Cl- of the adult vesicles would result in a less positive i ntravesicular charge, which would therefore augment Na+-solute co-tran sport. Fluorescence polarization studies also show that lipids from BB M vesicles of 7-day-old rats are more fluid than those from adult. The se differences are likely due to developmental lipid compositional cha nges, which influence membrane transport and permeability characterist ics. These findings would explain, in part, the age-dependent alterati ons of renal BBMV solute transport.