Glutamine transport by rat basolateral membrane vesicles

Glutamine, a neutral amino acid, is unlike most amino acids, has two amine moieties which underlies its importance as a nitrogen transporter and a carrier of ammonia from the periphery to visceral organs. The gastrointestinal tract utilizes glutamine as a respiratory substrate. The intestinal tract receives glutamine from the luminal side and from the arterial side through the basolateral membranes of the enterocyte. This study characterizes the transport of glutamine by basolateral membrane vesicles of the rat. Basolateral membranes were prepared by a well validated technique of separation on a percoll density gradient. Membrane preparations were enriched with Na⁺/K⁺-ATPase and showed no 'overshoot' phenomena with glucose under sodium-gradient conditions. Glutamine uptake represented transport into the intravesicular space as evident by an osmolality study. Glutamine uptake was temperature sensitive and driven by an inwardly directed sodium gradient as evident by transient accumulation of glutamine above the equilibrium values. Kinetics of glutamine uptake under both sodium and potassium gradients at glutamine concentrations between 0.01 and 0.6 mM showed saturable processes with V_max of 0.39 ± 0.008 and 0.34 ± 0.05 nmol/mg protein per 15 s for both sodium-dependent and sodium-independent processes, respectively. K_m values were 0.2 ± 0.01 and 0.55 ± 0.01 mM, respectively. pH optimum for glutamine uptake was 7.5. Imposition of negative membrane potential by valinomycin and anion substitution studies enhanced the sodium-dependent uptake of glutamine suggesting an electrogenic process, whereas the sodium-independent uptake was not enhanced suggesting an electroneutral process. Other neutral amino acids inhibited the initial uptake of glutamine under both sodium-dependent and sodium-independent conditions. We conclude that glutamine uptake by basolateral membranes occurs by carrier-mediated sodium-dependent and sodium-independent processes. Both processes exhibit saturation kinetics and are inhibited by neutral amino acids. The sodium-dependent pathway is electrogenic whereas the sodium-independent pathway is electroneutral.