Taurocholate Transport by Brush-Border Membrane Vesicles from the Developing Rabbit Ileum: Structure/Function Relationships

To 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 [C]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.