CHRONIC HYPERINSULINEMIA DECREASES INSULIN ACTION BUT NOT INSULIN SENSITIVITY

Hyperinsulinemia and insulin resistance are commonly seen in obese and non-insulin-dependent diabetes mellitus (NIDDM) patients, suggesting a causal link exists between hyperinsulinemia and insulin resistance. In a previous study, we demonstrated that chronic (28 days) intraportal hyperinsulinemia (50% increase in basal insulin levels) resulted in a decrease in insulin action as assessed by a one-step euglycemic hyperinsulinemic clamp. Since only one dose of insulin was used during the clamp, it was not possible to determine if the decrease in insulin action was due to a change in insulin sensitivity and/or maximal insulin responsiveness. In the present study, insulin resistance was induced as before, but insulin action was assessed in overnight fasted conscious dogs using a four-step euglycemic hyperinsulinemic clamp (1, 2, 10, and 15 mU/kg/min). Insulin responsiveness was assessed before the induction of chronic hyperinsulinemia (day 0), and after 28 days of hyperinsulinemia (day 28). Transhepatic glucose balance and whole-body glucose utilization were measured to allow assessment of both the hepatic and peripheral effects of insulin. Chronic hyperinsulinemia increased basal insulin levels from 13 ± 2 to 21 ± 4 μU/mL. After 4 weeks of chronic hyperinsulinemia, maximal insulin-stimulated glucose utilization was decreased 23% ± 4% (P < .05) and insulin sensitivity (ED50) was not significantly altered. During the four-step clamp, the liver was a major site of glucose utilization. The liver was responsible for 13% of the total glucose disposal rate on day 0 (2.9 mg/kg/min) at the highest insulin infusion rate (15 mU/kg/min; 2,000 μU/mL). On day 28, insulin suppression of endogenous glucose production and stimulation of hepatic glucose uptake was similar (3.3 mg/kg/min) to that seen on day 0, suggesting that the decrease in maximal insulin responsiveness did not involve the liver. In conclusion, insulin resistance produced by a mild hyperinsulinemia is due to a suppression of maximal extrahepatic insulin-stimulated glucose uptake, suggesting that a postreceptor defect is present in extrahepatic tissues. © 1990.