Characterization of glutamine transport by liver plasma membrane vesicles

Abstract

Plasma membrane vesicles were prepared from livers of fed normal and diabetic rats and used to characterize the membrane transport process responsible for glutamine uptake by the liver cell. In vesicles from normal rats the initial velocity of glutamine uptake was fourfold more rapid (0.20 +/- 0.02 vs. 0.05 +/- 0.02 nmol X mg protein-1 X 10 s-1) when Na+ replaced K+ in the extravesicular buffer. In the presence of a Na+-gradient glutamine uptake by vesicles was saturable, with a Km of 1.3 +/- 0.5 mM and a Vmax of 10 +/- 2.3 nmol X mg-1 X min-1. Lithium could fully substitute for Na+ in stimulating glutamine entry. In the presence of an imposed K+-gradient glutamine uptake was a linear function of its extravesicular concentration. In accord with the sodium-stimulated uptake of glutamine occurring via a sodium symport process, we observed that glutamine stimulated the initial rate of 22Na+ entry into vesicles by four- to fivefold. We further observed that glutamine entry was more rapid when lipophilic anions accompanied sodium in the incubation buffer, suggesting that Na+-glutamine flux is electrogenic. Preloading of vesicles with glutamine did not effect subsequent entry of labeled glutamine (no transstimulation), whereas intravesicular alanine did enhance alanine but not glutamine entry. Alloxan diabetes, which is known to stimulate the Na+-alanine cotransporter in these vesicles did not increase glutamine entry at any concentration tested.(ABSTRACT TRUNCATED AT 250 WORDS)