Bile acid transport in isolated brush border vesicles

Abstract

BILE ACID TRANSPORT IN ISOLATED BRUSH BORDER VESICLES. F.A. Wilson and L.L. Treanor. Department of Medicine, Vanderbilt University, Nashville, Tennessee Bile acids(BA) are transported across the intact jejunum and ileum via passive and sodium-dependent active processes, respectively. Previous studies in our laboratory indicated no significant difference in total BA binding betweenjejunal and ileal brush borders (J.Memb. Bio!. 33:213,1977). One possible explanation is that such a demonstration requires the presence of an energy source. Active glucose transport occurs in brush border vesicles (BBV) when free energy is supplied by the asymmetric distribution of Nat and an electrical potential difference across the vesicle wall (Hopfer et al. J. Biol. Chem. 248:25, 1973). To determine whether BA transport differences exist, BBV were prepared from rat iejunum and ileum. Purity of preaarations was assessed byelectronmicroscopy and sucrase activity. Vesicle uptake was determined by membrane filtration. Validity studies revealed that d-glucose displayed the overshoot phenomenon of stimulated uptake in the presence, but not absence, of a Nat gradient. Maximal d-glucose uotake was higher in jejunal than ileal BBV. The time of maximal d-glucose uptake correlated with the absolute ratio of maximal/equilibrium uptake. Lglucose failed to show the overshoot. Transport of BA in ileal BBV was linearwith resoect to membrane protein and demonstrated the overshoot phenomenon with resoect to time in the presence of a Na+ gradient, but not after Nat equilibrium. The maximal uptake of glycodeoxycholate (GDC) occurred at 10 set and the ratio of maximal/equilibrium uptake was > 1.9. The overshoot phenomenon was not seen for GDC in the presence or absence of a Nat gradient across the wall of jejunal BBV. GDC uptake at 15 min (Nat equilibrium) was subtracted from maximal uptake at 10 set (Nat gradient) to yield a saturable transport component. Kinetic analysis revealed a Km of O.lmM, a value similar to that described for isolated whole epithelial cells (Biochim.Biophys. Acta. 406: 280, 1975). These studies support the existence of a transport system in the brush border membrane that: 1) reflects the kinetics and characteristics of BA transport in isolated epithelial cells and intact intestinal tissue and 2) catalyzes the co-transport of Nat and BA across this membrane in a manner analogous to d-glucose transport.