An Na(+)-dependent and an Na(+)-independent system for glutamine transport in rat liver basolateral membrane vesicles

In the present study the transport of glutamine across rat liver basolateral membrane was examined with special emphasis on the existence of an Na(+)-independent system and on the characteristics of the Na(+)-dependent system with respect to stoichiometry of glutamine to Na+. Well-validated and purified liver basolateral membrane vesicles were used in the study. Results of studies on the effect of incubation medium osmolarity and incubation temperature indicated that glutamine uptake by liver basolateral membrane vesicles is largely the result of transport of the substrate into the intravesicular compartment with little binding to basolateral membrane vesicles. Transport of glutamine with time was Na+ gradient dependent (out greater than in) with a distinct "overshoot" phenomenon. Replacing Na+ with an equivalent concentration of K+, NH4+, choline, or mannitol caused significant inhibition of the initial rate of glutamine transport; on the other hand, Li+ could partially substitute for Na+. The initial rate of transport of glutamine as a function of concentration (0.05-12 mmol/L) was saturable both in the presence and in the absence of an inwardly directed Na+ gradient. Apparent Km values of 2.95 and 3.35 mmol/L and Vmax values of 11,565 and 6663 pmol.mg protein-1.10s-1 were calculated in the presence and absence of a Na+ gradient, respectively. Both in the presence and absence of an Na+ gradient (out greater than in), transport of [3H]glutamine was significantly inhibited by the addition to the incubation medium of unlabeled glutamine as well as histidine, asparagine, and serine. Transport of glutamine by the Na(+)-dependent process was significantly inhibited or stimulated, respectively, by inducing a relatively positive or negative intravesicular space. On the other hand, glutamine transport by the Na(+)-independent process was not affected by changes in transmembrane electrical potential. Using the "activation method," the stoichiometry of glutamine Na+ transport was found to be 1:1. These results show that glutamine transport in rat liver basolateral membrane vesicles is carrier mediated both in the presence and absence of an Na+ gradient. Furthermore, the Na(+)-dependent process is electrogenic in nature (net positive) and cotransports one glutamine molecule with one Na+. Transport of glutamine by the Na(+)-independent system, on the other hand, is electroneutral in nature.