Age-related changes in albumin binding by renal brush-border membrane vesicles.

A selective proteinuria occurs with normal aging. We investigated the contribution of a defect in the receptor-mediated endocytosis to the age-related albuminuria by measuring albumin binding by renal brush-border membrane vesicles from young and old female Wistar rats using a filtration method. Old (24 months) rats had a significantly higher proteinuria (13.29 +/- 5.25 mg prot/24 h/100 g bw) than did young (3 months) rats (1.23 +/- 0.55 mg prot/24 h/100 g bw). Scatchard analysis of the kinetic parameters of 125I-albumin binding revealed a decrease in the binding capacity of brush-border membrane vesicles from old rats. The number of binding sites, N (pmol/mg protein/min) was 236.84 +/- 97.50 in old rat preparations and 380.27 +/- 178.36 in young rat vesicles (P < 0.05). By contrast, Km did not change significantly with age (478.86 +/- 259.29 nM in old rat vesicles and 498.00 +/- 220.36 nM in young rat preparations). Consequently the index of adsorptive endocytosis efficiency (the N/Km ratio) decreased drastically with age from 0.782 +/- 0.238 at 3 months to 0.547 +/- 0.199 at 24 months (P < 0.05). These data indicate that defective receptor-mediated endocytosis could, at least partly, explain the age-dependent rise in urinary albumin excretion.