A Comparison of Basal and Insulin stimulated Glucose Transport in Rat Adipocyte Plasma Membranes

Abstract

Specific D-glucose transport in plasma membranes prepared from control and insulin-treated rat adipocytes was measured using a recently developed dual isotope ([³H]-D-glucose and [¹⁴C]-L-glucose), rapid filtration assay which allowed measurements of initial rates at 1 s and 37δ.¹ Plasma membranes from insulintreated adipocytes showed an increase in glucose transport compared with control cells. Saturation kinetic data revealed that the plasma membranes from insulin-treated and control cells had the same K_m (26 mM) for glucose transport, whereas insulin treatment increased the V_max from 4433 pmol/mg protein/s to 9465 pmol/mg protein/s. Arrhenius plots showed no difference in the energy of activation between control and insulin-stimulated glucose transport states. The optimum pH of both control and insulin-stimulated glucose transport was 7.4. Lower or higher pHs progressively decreased both control and insulin-stimulated glucose transport proportionately. Calcium in the transport assay media did not affect basal or insulinstimulated glucose transport. However, omission of calcium from the adipocyte incubation media significantly lowered the insulin stimulation by 24% while basal levels were not significantly affected. Insulin specifically bound to the plasma membrane was carried through the fractionation procedure, but removal of this insulin did not alter the stimulated glucose transport. Glucose transport by plasma membranes from control or insulin-treated adipocytes was equally (percentage) inhibited by N-ethylmaleimide, dithiothreitol, reduced glutathione, or cytochalasin B. No inhibition of control or insulin-stimulated transport was seen with cytochalasin D or oxidized glutathione. The data presented are consistent with insulin causing the formation of new transport sites similar to the existing basal Sites.