Impaired Insulin-Stimulated Nonoxidative Glucose Metabolism in Pancreas-Kidney Transplant Recipients: Dose-Response Effects of Insulin on Glucose Turnover

Insulin resistance is a characteristic feature in recipients of a pancreas transplant, but the relative contribution of the liver and peripheral tissues to this abnormality within a spanning range of insulin concentrations is unknown. To assess the impact of insulin action on glucose metabolism after pancreas transplantation, a euglycemic-hyperinsulinemic clamp with sequential insulin infusions (5, 40, and 200 mU m⁻²·min⁻¹ for 120 min each), combined with isotopic determinations of the rates of hepatic glucose production and extrahepatic glucose uptake, indirect calorimetry, and measurements of glycogen synthase and hexokinase activities in vastus lateralis muscle, were performed in six pancreas-kidney transplant recipients (Px group) and compared with those performed in six nondiabetic kidney transplant recipients with similar immunosuppression (Kx group) and six nondiabetic control subjects. The overall effects of insulin on whole-body glucose metabolism, determined as the glucose infusion rates versus the corresponding steady-state serum insulin concentrations, demonstrated a rightward shift in the dose-response curves of the transplanted groups compared with those of normal subjects. The dose-response curve for glucose disposal rates (R_d) was shifted to the right in the Px and Kx groups, and the maximal glucose disposal rate was reduced by 40% in the Px group (11.7 ±1.1 mg·kg⁻¹ fat-free mass·min⁻¹) and 30% in the Kx group (13.9 ± 1.2 mg·kg⁻¹ fat-free mass·min⁻¹) compared with that in control subjects (19.1 ±2.2 mg·kg⁻¹ fat-free mass·min⁻¹) (P < 0.05). The dose-response curve for suppression of hepatic glucose output rates was similar at increasing hepatic sinusoidal insulin concentrations.Glucose oxidation rates were similar in all groups, whereas nonoxidative glucose rates were reduced by 50% in the Px group and by 30% in the Kx group compared with those in the control group (P < 0.05). In the Px group, an impaired activation of the fractional velocity and absent decrease in the half-maximal stimulation of muscle glycogen synthase occurred during the insulin infusion. However, this finding could only explain in part the degree of impairment in nonoxidative glucose metabolism. No differences were found in total hexokinase activity in muscle between normal subjects and the transplant groups at basal insulinemia or after insulin stimulation. During hyperinsulinemia, glucagon and nonesterified fatty acids were not suppressed as much in the transplanted groups as they were in normal control subjects (P < 0.05). In conclusion, pancreas transplantation causes impaired peripheral action of insulin as compared with that in normal subjects and kidney transplant recipients. The main course of insulin resistance in the two transplant groups is explained by the immunosuppressive treatment, but the augmented insulin resistance in pancreas transplant recipients could partly be explained by the chronic peripheral hyperinsulinemia. The principal site of insulin resistance was a reduced insulin-stimulated nonoxidative glucose metabolism of peripheral tissues, which resulted in decreased capacity to store glucose as glycogen. The impaired peripheral insulin action could only partly be explained by a reduced activation of the glycogen synthase enzyme in skeletal muscle.