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.