HETEROGENEITY OF GLUCOSE-TRANSPORT IN RAT-LIVER MICROSOMAL VESICLES

Glucose transport across the membrane of rat liver microsomal vesicles was studied by a rapid filtration method in three different experimen tal systems: (i) inward transport in the presence of extravesicular gl ucose, (ii) efflux from passively preloaded vesicles, and (iii) efflux of glucose generated intravesicularly by glucose-6-phosphatase upon a ddition of glucose 6-phosphate were investigated. The apparent intrave sicular glucose space estimated with the rapid filtration method was l ower than the total microsomal glucose accessible space both the in th e steady-state phase of uptake and at the starting point of efflux: 0. 5 versus 2.3 mu l/mg protein. The initial rate of influx/efflux was de pendent on the extravesicular/intravesicular glucose concentration and was much lower than the rate of influx estimated previously by the li ght-scattering technique. Both influx and efflux could be inhibited by N-ethylmaleimide and possibly became saturable at high (>100 mM) gluc ose concentration. Known inhibitors of GLUT transporters (genistein, c ytochalasin B, phloretin, and hexoses) did not affect glucose influx T he time course of glucose efflux from vesicles preincubated in the pre sence of glucose g-phosphate was similar to that from glucose-loaded v esicles. These data together with that obtained previously (by a light -scattering technique; Marcolongo, P., Fulceri, R, Giunti, R, Burchell , A, and Benedetti, A (1996) Biochem, Biophys. Res. Commun. 219, 916-9 22) indicate that microsomal vesicles are heterogeneous regarding thei r glucose-transporting properties and that glucose transport is bidire ctional and its feature meets the requirements of a facilitative trans port. (C) 1998 Academic Press.