Short-term studies of taurocholate uptake by ileal brush border membrane vesicles: anion effects

Transport of bile acids in a human intestinal epithelial cell line, Caco-2

The transport of taurocholic acid (TA) across Caco-2 cell monolayers was dependent on time in culture and reached a plateau after 28 days, at which time the apical (AP)-to-basolateral (BL) transport was 10-times greater than BL-to-AP transport. The amounts of TA inside the cells following application of 10 nM [14C]TA to the AP or BL side of the monolayers (30 min) were approximately equal (54.4 +/- 2.7 and 64.6 +/- 2.8 fmol/mg protein, respectively). AP-to-BL transport of TA was saturable and temperature-dependent. Vmax and Km for transport were 13.7 pmol/mg protein per min and 49.7 microM, respectively. The transport of TA had an activation energy of 13.2 kcal.mol-1, required Na+ and glucose. AP-to-BL transport of [14C]TA was inhibited by the co-administration (on the AP side) of either unlabeled TA or deoxycholate, but it was not reduced by the presence of unlabeled TA on the BL side.

Characterizing mechanisms of hepatic bile acid transport utilizing isolated membrane vesicles

Utilizing the above-outlined approaches, mechanisms of hepatic bile acid transport have been characterized in membrane vesicles of rat liver, particularly for the conjugated trihydroxy bile acid, taurocholic acid. Uptake across the sinusoidal membrane is carrier mediated and coupled to the transmembrane sodium gradient. This carrier has an apparent Km between 30 to 50 microM and a Vmax between 4 to 6 nmol mg-1 protein min-1. Furthermore, Na+ gradient-dependent sinusoidal uptake of taurocholate can be stimulated by low concentrations of albumin. There is controversy as to whether the process is electrogenic. Although photoaffinity labeling studies indicate that an additional carrier for Na(+)-dependent bile acid uptake is also present at the sinusoidal membrane, this carrier has so far not been characterized in membrane vesicles. The proposition that pH gradient-driven furosemide-sensitive cholic acid uptake into sinusoidal membrane vesicles may represent carrier-mediated hydroxyl/cholate exchange must be revised on the basis of the recent findings that (1) true initial uptake rates are not saturable; (2) pH gradient-driven cholate uptake is also found in liposomes; and (3) furosemide also inhibits pH gradient-driven cholate uptake in liposomes. The mechanisms of transcellular transport of bile acids have been studied less extensively, but Na(+)-independent carrier-mediated taurocholic acid transport has been demonstrated in purified subcellular fractions such as rat liver microsomes and Golgi membranes. Finally, transport studies in canalicular rat liver plasma membrane vesicles indicate that canalicular excretion of bile acids is also a carrier-mediated process that may be driven, at least in part, by the physiologic electrical potential gradient, and that preferentially transports trihydroxy and conjugated dihydroxy bile acids.

Transport of glutamine in rat intestinal brush-border membrane vesicles

Transport of glutamine across the brush-border membrane of the rat intestine was examined using brush-border membrane vesicle (BBMV) technique. Osmolarity and temperature studies indicated that the uptake of glutamine by BBMV is mostly the result of transport of the substrate into the intravesicular space. Transport of glutamine was Na+-gradient dependent (out greater than in) with a distinct 'overshoot' phenomenon. Initial rate of transport of glutamine as a function of concentration was saturable both in the presence and absence of a Na+ gradient (out greater than in). Apparent Km of 3.50 and 3.34 mM and Vmax of 707 and 282 pmol/mg protein per 7 s, were calculated for the Na+-dependent and the Na+-independent transport processes of glutamine. The transport of [3H]glutamine by the Na+-dependent and the Na+-independent processes was severely inhibited by the addition to the incubation medium of other amino acids and unlabelled glutamine. Inducing a relatively negative intravesicular compartment with the use of valinomycin and an outwardly directed K+ gradient stimulated glutamine transport. This indicates that transport of the substrate by the Na+-dependent process is electrogenic in nature. Transport of glutamine by the Na+-independent process, however, appeared to be electroneutral in nature. These results demonstrate the existence of two carrier-mediated transport processes for glutamine in the rat intestinal BBMV, one is Na+-dependent and the other is Na+-independent. Furthermore, the results suggest that glutamine transport by the Na+-dependent process probably occurs by a glutamine/Na+ cotransport mechanism.

Biotin transport in rat intestinal brush-border membrane vesicles

Transport of biotin across rat intestinal brush-border membrane was examined using the brush-border membrane vesicle (BBMV) technique. Uptake of biotin by BBMV is the result of transport of the substrate into the intravesicular space with negligible binding to membrane surfaces. In the presence of a Na+ gradient (out greater than in), transport of biotin was higher with a transient 'overshoot' phenomenon. In comparison, transport of biotin in the presence of a choline gradient (out greater than in) was lower with no 'overshoot' phenomenon. In both jejunal and ileal BBMV, the transport of biotin as a function of concentration was saturable in the presence of a Na+ gradient (out greater than in) but was linear in the presence of a choline gradient (out greater than in). Vmax of the Na+-dependent transport system was 0.88 and 0.37 pmol/mg protein per s and apparent Kt was 7.57 and 7.85 microM in jejunal and ileal BBMV, respectively. Structural analogues inhibited the transport process of biotin. Unlike the electrogenic transport of D-glucose, the transport of the anionic biotin was not affected by imposing a relatively positive intravesicular potential with the use of valinomycin and an inwardly-directed K+ gradient, suggesting that biotin transport is most probably an electroneutral process. This suggestion was further supported by studies on biotin transport in the presence of anions of different lipid permeability. The results of this study demonstrate that biotin transport across rat intestinal brush-border membrane is by a carrier-mediated, Na+-dependent and electroneutral process. Furthermore, transport of biotin is higher in the jejunum than the ileum.

Taurocholate transport by human ileal brush border membrane vesicles

Human ileal brush border membrane vesicles were prepared from intestines obtained from cadaveric renal allograft donors. The energetics and kinetics of taurocholate transport were studied. Fifty-five percent of equilibrium uptake (picomoles per mg protein) resulted from binding to the vesicle surface or incorporation into an osmotically insensitive compartment. The initial rate of transport was stimulated fourfold by an inwardly directed Na+ gradient when compared with a K+ gradient, and cation gradient-dependent differences persisted throughout the initial 5 min of incubation (p less than 0.05). Taurocholate uptake was half-maximally stimulated by a Na+ concentration of 23 +/- 4 mM. A Hill transformation of this plot gave a slope (n) of 0.97, indicating a 1:1 (mol/mol) Na+-taurocholate coupling ratio. Generation of a negative inside diffusion potential by anion substitution or valinomycin-induced K+ diffusion potential failed to alter bile salt uptake, suggesting an electroneutral transport mechanism. When Na+-dependent uptake velocity (10 s) was examined over a range of taurocholate concentration (0.036-0.9 mM), the plot described a rectangular hyperbola. The mean apparent Michaelis constant was 0.037 +/- 0.007 mM and maximum velocity was 1093 +/- 329 pmol taurocholate per milligram protein per 10 s. These data confirm and extend animal studies of ileal bile salt transport. Taurocholate uptake by the human ileal brush border occurs by a Na+-dependent, carrier-mediated electroneutral mechanism. According to this model, a single Na ion is coupled with a single taurocholate anion and transported across the brush border by a carrier mechanism that is driven by a transmembrane Na+ gradient.

Ontogenesis of taurocholate transport by rat ileal brush border membrane vesicles

Developmental aspects of taurocholate transport into ileal brush border membrane vesicles were studied in 2-wk-old (suckling), 3-wk-old (weanling), and 6-wk-old (adolescent) rats. Taurocholate uptake (picomoles per milligram protein) into brush border membrane vesicles prepared from 2-wk-old rats was similar under Na+ and K+ gradient conditions (outside greater than inside). By contrast, uptake in 3- and 6-wk-old rats was significantly enhanced at 20 s, and at 1, 2, and 5 min of incubation in the presence of a Na+ gradient when compared with a K+ gradient incubation (P less than 0.05). Under isotope exchange conditions, a plot of active uptake velocity versus taurocholate concentration (0.10-1.0 mM) in 2-wk-old rat membrane vesicles was linear and approached the horizontal axis, suggesting the absence of active transport. However, similar plots in 3- and 6-wk-old rats described a rectangular hyperbola, indicating a Na+-dependent, saturable cotransport system. Woolf-Augustinsson-Hofstee plots of the uptake velocity versus concentration data from 3- and 6-wk-old rat brush border membrane vesicles yielded Vmax values that were not significantly different, 844 and 884 pmol uptake/mg protein per 120 s, respectively. The respective Km values were 0.59 and 0.66 mM taurocholate. The induction of an electrochemical diffusion potential by incubating K+-loaded vesicles with valinomycin did not significantly enhance taurocholate uptake in 2-, 3-, or 6-wk-old rat vesicle preparations. These data indicate that taurocholate transport into rat ileal brush border membrane vesicles is mediated by an electroneutral, sodium-coupled, cotransport system that is incompletely developed in the 2-wk-old suckling rat but fully developed by the time of weaning at 3 wk of age.