Direct and Indirect Effects of Insulin to Inhibit Hepatic Glucose Output in Obese Subjects

The effects of small increases in plasma insulin on hepatic glucose production are incompletely understood. To partially elucidate this issue we have studied seven obese subjects with the euglycemic clamp technique with a low-dose insulin infusion rate of 15 mU · m⁻² · min⁻¹ over 3 h. Basal insulin levels were 24 ± 7 μU/ml and increased to steady-state levels of 35 ± 3 μU/ml during insulin infusion. Endogenous insulin secretion, quantitated by C-peptide measurements, decreased by 58% of the basal value after peripheral insulin infusion. Based on C-peptide measurements and the contribution of the peripheral insulin infusion to the circulating insulin concentrations, calculated portal insulin levels either decreased or remained unchanged during the clamp studies. Basal glucagon levels were 165 ± 18 and did not change during the insulin infusion. The basal glucose disposal rate was 86 ± 2 mg · m⁻² · min⁻¹ and did not increase significantly during the clamp studies. In contrast, hepatic glucose output (HGO) was suppressed by 82 ± 5% of the basal value. In summary, in a group of insulin-resistant obese subjects, glucose-clamp studies were performed at peripheral insulin levels of 35 ± 3 μU/ml; glucose disposal did not increase, whereas HGO was suppressed by 82%. At the same time, glucagon levels remained constant and estimated portal insulin levels either decreased or remained unchanged. These findings suggest that insulin can suppress HGO through indirect extrahepatic actions.