Insulin binding and glucose transport activity in cardiomyocytes of a diabetic rat

We studied insulin binding and glucose transport in isolated adult cardiomyocytes from rats with 2-wk streptozotocin-induced diabetes. At 37 degrees C, cells from diabetic rats bound less 125I-insulin and exhibited lower rates of 3-O-methylglucose transport than cells from control rats. In contrast, the amount of 125I-insulin bound to myocytes at 4 degrees C was the same in both groups. Preincubation of cells from both groups with 10-10,000 ng/ml insulin significantly increased their basal rates of glucose transport by approximately 40%. However, the augmented rates in diabetics were still approximately 36% lower than the corresponding insulin-stimulated rates in the controls. When the glucose transport data were expressed as percent maximal insulin effect and plotted as a function of the amount of insulin bound, the curves obtained from both diabetic and nondiabetic controls were superimposable. These data demonstrate that 1) heart cells from diabetic rats bind less insulin than from control rats under conditions in which they exhibit impaired glucose transport rates, 2) there is no apparent difference in total receptor number between the two groups, but internalization of intact insulin appears to be diminished in diabetes, 3) coupling exists between insulin binding and glucose transport in both groups, and 4) these impaired processes are completely reversed by insulin treatment in vivo but not in vitro.