GLUCOSE-TRANSPORT AND GLUCOSE-6-PHOSPHATE HYDROLYSIS IN INTACT RAT-LIVER MICROSOMES

Glucose transport was investigated in rat liver microsomes in relation to glucose 6-phosphatase (Glu-6-Pase) activity using a fast sampling, rapid filtration apparatus. 1) The rapid phase in tracer uptake and t he burst phase in glucose 6-phosphate (Glu-6-P) hydrolysis appear sync hronous, while the slow phase of glucose accumulation occurs during th e steady-state phase of glucose production. 2) [C-14]Glucose efflux fr om preloaded microsomes can be observed upon addition of either cold G lu-6-P or Glu-6-Pase inhibitors, but not cold glucose. 3) Similar stea dy-state levels of intramicrosomal glucose are observed under symmetri cal conditions of Glu-6-P or vanadate concentrations during influx and efflux experiments, and those levels are directly proportional to Glu -6-Pase activity. 4) The rates of both glucose influx and efflux are c haracterized by t(1/2) values that are independent of Glu-6-P concentr ations. 5) Glucose efflux in the presence of saturating concentrations of vanadate was not blocked by 1 mM phloretin, and the initial rates of efflux appear directly proportional to intravesicular glucose conce ntrations. 6) It is concluded that glucose influx into microsomes is t ightly linked to Glu-6-Pase activity, while glucose efflux may occur i ndependent of hydrolysis, so that microsomal glucose transport appears unidirectional even though it can be accounted for by diffusion only over the accessible range of sugar concentrations.