Variations in insulin responsiveness in rat fat cells are due to metabolic differences rather than insulin binding

Insulin resistance was studied by comparing insulin response and insulin binding in four groups of rats. Glucose metabolism in isolated fat cells from male Wistar rats weighing 340 g was less responsive to a supramaximal dose of insulin than glucose metabolism in fat cells from rats weighing 200 g. Induction of streptozotocin-diabetes in rats weighing 200 g resulted in a marked decrease in the insulin responsiveness of fat cells. Ventromedial hypothalamic lesions of 340 g rats had the opposite effect and restored the insulin responsiveness of fat cells. The responsiveness in the four groups was correlated to the rate of glucose conversion to fatty acids in fat cells. The binding of 125I-insulin was the same in both 340 and 200 g rats. The ventromedial hypothalamic lesioned rats and the diabetic rats showed, in spite of their great difference in insulin responsiveness, the highest binding of 125I-insulin to fat cells. Insulin binding was not correlated to the plasma insulin level which however was reflected in the lipoprotein lipase activity in the adipose tissue. In conclusion, these results indicate that variations in insulin responsiveness in fat cells are due to alterations in cellular metabolism rather than in insulin binding.