CHARACTERIZATION OF GLUTAMINE AND GLUTAMATE TRANSPORT IN RAT LUNG PLASMA-MEMBRANE VESICLES

Insufficient glutamine for the lungs during sepsis may contribute to a n impairment in lung function. Lung glutamine metabolism is supported by both blood glutamine uptake and de novo biosynthesis using circulat ing glutamate as a precursor. Information regarding the specific plasm a membrane carriers involved in this uptake is lacking. Furthermore, t he effect of sepsis on amino acid transport in whole lung has not been studied. We isolated lung plasma membrane vesicles (LPMVs) from contr ol and LPS-treated rats and assayed glutamine and glutamate transport activity in LPMVs. Vesicle purity and functionality were confirmed by time-dependent concentrative amino acid uptake in the presence of Na+, impoverishment of microsomal enzymes, and a 25-fold enrichment in the plasma membrane marker 5'-nucleotidase. Eighty percent of glutamine u ptake in lung vesicles was mediated via the high affinity Na+-dependen t carrier System ASC (V-max = 80 +/- 10 pmole/mg protein/15 sec; K-m = 224 +/- 30 mu M) while 19% occurred via the Na+-independent System AS C (V-max = 11 +/- 2 pmole/mg/15 sec; K-m = 141 +/- 23 mu M). Ninety pe rcent of glutamate transport was mediated by the Na+-independent Syste m X-AG(-). Treatment of rats with LPS resulted in a decrease in both g lutamine and glutamate transport in LPMVs. LPMVs offer a novel method for characterizing lung amino acid transport and studying the effects of catabolic states on this activity. The effects of endotoxin on Syst em ASC and X-AG(-) activity may contribute to reduced lung glutamine a vailability during septic states which may impair cellular metabolism and function. (C) 1997 Academic Press.