Insulin resistance in Cushing's syndrome

It is well established that cortisol excess causes insulin resistance in man, but the mechanisms responsible for this insulin resistance are poorly understood. We studied 5 women with Cushing's syndrome with impaired oral glucose tolerance tests and 7 normal subjects, plotting the shape of the insulin-induced disposal dose-response curve obtained by means of the euglycemic clamp procedure during four different plasma insulin plateaus at four infusion rates of 21, 73, 760, and 1200 mU/M².min. Glucose disposal (M = mg/M².min) was calculated as glucose amount infused to maintain euglycemia. In Cushing's syndrome the dose-response curve was shifted to the right in comparison with normal subjects, with a significantly lower M (337 ± 35 vs. 657 ± 76 P <0.01) during the highest insulin infusion rate [maximal glucose disposal (MGD)] without any significant difference in the levels of insulin half-maximally effective in the stimulation of glucose utilization. Neither erythrocyte nor monocyte maximum insulin receptor binding were different between the two populations. Four Cushing's syndrome patients were studied again after surgical treatment. A marked improvement of MGD was observed without any significant change in insulin-binding capacity. These results, particularly the marked decrease in MGD, a typical feature of postreceptor defects, indicate that cortisol-induced insulin resistance in man is due to an impairment of peripheral insulin action located beyond the hormone-receptor binding step.