Hepatic taurine transport: a Na+-dependent carrier on the basolateral plasma membrane

Abstract

Highly purified rat basolateral liver plasma membrane vesicles were used to examine the mechanism and the driving forces for hepatic uptake of the beta-amino acid, taurine. An inwardly directed 100 mM NaCl gradient stimulated the initial rate of taurine uptake and energized a transient twofold accumulation of taurine above equilibrium ("overshoot"). In contrast, uptake was slower and no overshoot was detected in the presence of a KCl gradient. A negative intravesicular electrical potential generated by the presence of permeant anions or an outwardly directed K+ gradient with valinomycin increased Na+-stimulated taurine uptake. External Cl- stimulated Na+-dependent taurine uptake independent of effects on the transmembrane electrical potential difference. Na+-dependent taurine uptake showed a sigmoidal dependence on extravesicular Na+ concentration, suggesting multiple Na+ ions are involved in the translocation of each taurine molecule. Na+-dependent taurine uptake demonstrated Michaelis-Menten kinetics with a maximum velocity of 0.537 nmol.mg protein-1.min-1 and an apparent Km of 174 microM. [3H]taurine uptake was inhibited by the presence of excess unlabeled taurine, beta-alanine, or hypotaurine but not by L-glutamine or L-alanine. In summary, using basolateral liver plasma membrane vesicles, we have shown that hepatic uptake of taurine occurs by a carrier-mediated, secondary active transport process specific for beta-amino acids. Uptake is electrogenic, stimulated by external Cl-, and requires multiple Na+ ions for the translocation of each taurine molecule.

Enhanced Uptake of Taurine by Basolateral Plasma Membrane Vesicles Isolated from Developing Rat Liver

“…2). Similar to results previously observed using membrane vesicles from adult rats (8), taurine uptake by membrane vesicles from developing rat liver decreased with increasing medium osmolarity and extrapolation of the derived linear regression curve to infinite osmolarity (theoretical intravesicular volume = 0) showed minimal binding of taurine.…”

“…adult rat liver (8). To determine whether nonspecific taurine binding to the membrane contributes to the observed age-related differences in taurine uptake, the effect of medium osmolarity on taurine uptake was assessed using membrane vesicles from developing rat liver (Fig.…”

“…Uptake of t3H]-taurine was measured as previously described (8). Briefly 20 pl of LPMV suspension (10 mM HEPES-KOH, pH 7.5, 300 mM sucrose) containing 45-85 pg of protein were preincubated at 37" C in a water bath.…”

“…The liver plays a central role in taurine homeostasis first as a respository for this substrate during the verinatal veriod and later as a maior site of biosynthesis (6,"7). ~.~a + -d e~d n d e n t cotransport mechgnism for taurine has recently been demonstrated on the basolateral plasma membrane of rat liver and on the brush border membrane of rat jejunum (8,9). Significantly higher rates of taurine transport were observed in brush border membrane vesicles isolated from suckling compared with adult rat jejunum (9).…”

Impact of SLC6A Transporters in Physiological Taurine Transport at the Blood–Retinal Barrier and in the Liver

“…Studies using rat hepatic plasma membrane vesicles showed Na + -dependent taurine uptake, of which the K m value (174-380 µM) was similar to the K m value of the mouse orthologue of GAT2 (540 µM, mouse GAT3), 5,59,60) and it is a conceivable that GATs on the sinusoidal membrane of periportal hepatocytes contribute to taurine uptake, followed by conjugation. In addition, the uptake study in isolated rat hepatocytes suggested Na + -and Cl − -dependent taurine uptake by hepatocytes, and the estimated a K m value (594 µM) was similar to that of mouse GAT3-mediated taurine transport.…”