Taurocholate Transport by Brush‐Border Membrane Vesicles from the Developing Rabbit Ileum

Abstract

SummaryTo examine the ontogenesis of bile acid transport in the rabbit ileum, brush‐border membrane vesicles (12− to 20‐fold purified) were prepared from 14− to 49‐day‐old animals. Taurocholate uptake was characterized by the emergence of secondary active, Na+ ‐dependent transport at the start of weaning (21 days). Transient intravesicular accumulation (overshoot) of taurocholate occurred at 5–10 s of incubation, and the overshoot maximum increased significantly from 21 days (349.2 ± 22.4 nmol/mg protein) to 35 days (569.0 ± 84.3 nmol/mg protein; p < 0.001), without further increase at maturity (49 days, ± 607.6 ± 136.7 nmol/mg protein). No significant taurocholate active uptake component was noted at 14 days; however, ileal vesicles from sucklings showed carrier‐mediated, Na+ D‐glucose cotransport. In ± 35‐day‐old rabbits, osmolarity studies at 20 s of incubation showed that only 12% of [14C]taurocholate uptake was secondary to bile acid‐to‐membrane binding. Conversely, at 20 min, >95% of radiolabel incorporation represented solute bound to the external and/or internal membrane surface. Arrhenius plots establish brush‐border membrane taurocholate uptake as an intrinsic, lipid‐dependent process, with a slope discontinuity between 24 and 28°C, similar to the membrane lipid thermotropic transition region. Steady‐state fluorescence polarization studies (1,6‐diphenyl‐1,3,5‐hexatriene) demonstrate a temporal association between the maturation of taurocholate uptake and age‐related decreases in ileal brush‐border membrane fluidity. These data indicate that maturation of bile acid secondary active transport in the rabbit ileum may be regulated, at least in part, by changes in brush‐border membrane lipid dynamics.