Na+-glycine cotransport in canalicular liver plasma membrane vesicles

By use of purified rat canalicular liver plasma membrane (cLPM) vesicles, the present study determined the driving forces for glycine transport across this membrane domain. Initial rates of [3H]glycine uptake (10 microM) in cLPM vesicles were stimulated by an inwardly directed Na+ gradient but not by a K+ gradient. Na+ gradient-dependent uptake of glycine demonstrated cation specificity for Na+, dependence on extravesicular Cl-, stimulation by an intravesicular-negative membrane potential, and inhibition by dissipation of the Na+ gradient with gramicidin D. Na+ gradient-dependent glycine cotransport also demonstrated greater sensitivity to inhibition by sarcosine than 2-(methylamino)-isobutyric acid. Accelerated exchange diffusion of [3H]glycine was demonstrated in the presence of Na+ when cLPM vesicles were preloaded with glycine but not with L-alanine or L-proline. Substrate velocity analysis of net Na+-dependent [3H]glycine uptake over the range of amino acid concentrations from 5 microM to 5 mM demonstrated two saturable transport systems, one of high capacity (2.2 +/- 0.2 nmol.mg protein-1.15 s-1) and low affinity (11.2 +/- 1.7 mM) and one of low capacity (51 +/- 14 pmol.mg protein.15 s-1) and comparatively high affinity (66 +/- 12 microM). These results indicate that, in addition to previously described neutral and anionic amino acid transport systems, Na+ gradient-dependent glycine transport mechanisms are present on the canalicular domain of the liver plasma membrane. These canalicular reabsorptive mechanisms may serve to reclaim some of the glycine generated within the canalicular lumen from the intrabiliary hydrolysis of glutathione.