Increased affinity predominates in insulin stimulation of glucose transport in the adipocyte

The kinetics for transport of glucose and 3-O-methyl glucose (MeGlc) were determined in preparations of rat adipocytes characterized by low basal rates which could be increased consistently more than 30-fold by addition of insulin (10 nM). In basal cells, the K(m) of [¹⁴C]glucose uptake was about 75 mM. The K(i) for glucose inhibition of [¹⁴C]MeGlc uptake was similarly high (105 mM). These results were further confirmed by studies of basal glucose consumption which remained linear up to the highest glucose concentration used (30 mM) and by measurements of MeGlc transport kinetics (net and equilibrium exchange K(m) were both about 35 mM). Basal glucose uptake was determined to be stereospecific and cytochalasin sensitive (>90%) and thus could not be dismissed as nonmediated diffusion. Insulin treatment decreased the transport K(m) for glucose and MeGlc to one-tenth the values measured in basal cells. The V(max) for glucose was doubled and that for MeGlc quadrupled. We conclude that insulin brings about a great reduction in the K(m) of hexose transport in adipocytes. This can only be perceived if activation of basal cells by experimental manipulations is avoided. The decrease in K(m) is the major kinetic factor involved in the stimulation of glucose transport by insulin.