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To get some insight into the mechanisms of insulin resistance in obesity, insulin binding and biological effects were investigated in soleus muscles isolated from normal and obese mice. Basal and insulin-stimulated 2-deoxyglucose uptake were measured at the steady state of insulin binding. The results were consistent with the concept of spare receptors, i.e., maximal insulin effect was achieved when only about 20% of total receptors was occupied. When similar studies were applied to muscles of gold thioglucose obese or genetically obese (ob/ob) mice, and compared to lean controls: a) insulin binding was decreased; b) the insulin dose-response curve of 2-deoxyglucose uptake was shifted to the right; c) maximally insulin-stimulated 2-deoxyglucose uptake, glycolysis, and glycogen synthesis were markedly decreased. Insulin binding and effects returned toward normal after a 40-h fast in obese mice. These results point to two loci for the insulin resistance of skeletal muscle in obesity: 1) a decrease in the number of insulin receptors, which results in a diminished insulin sensitivity; and 2) one or more alterations beyond receptor that are responsible for the decreased responsiveness of the tissue to insulin and appear to play a major role in the insulin resistance of muscle in obesity.

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