Stimulation of muscle glucose disposal by insulin in humans is a function of the preexisting plasma insulin level.

Abstract

To examine whether tissue sensitivity to insulin is dependent upon the prevailing plasma insulin concentration, the ability of acute hyperinsulinemia to stimulate glucose disposal was investigated in six normal subjects before and after prolonged reduction of the plasma insulin concentration. Glucose turnover ([6,6-2H2]glucose), whole body glucose oxidation and nonoxidative glucose disposal (indirect calorimetry), and glycogen synthase activity in muscle were determined in the postabsorptive and in the insulin-stimulated states (euglycemic hyperinsulinemic clamp: 3 mU.kg-1.min-1) before and after a 4-day subcutaneous infusion of the somatostatin analogue octreotide (200 micrograms/24 h). Constant octreotide infusion 1) decreased postabsorptive and meal-stimulated plasma insulin levels by approximately 30-40% but did not significantly alter overall glucose tolerance, free fatty acid, growth hormone, and glucagon levels and 2) was associated with significant increases in insulin-mediated whole body glucose disposal (pre-drug: 10.29 +/- 0.49 vs. postdrug: 11.42 +/- 0.72 mg.kg-1.min-1, P < 0.04), nonoxidative glucose disposal (6.82 +/- 0.57 vs. 7.68 +/- 0.62, P < 0.03), and fractional glycogen synthase activity (0.14 +/- 0.03 vs. 0.20 +/- 0.04 mU/mg protein, P < 0.02). In contrast, infusion of saline instead of octreotide for 4 days to control subjects did not alter any of the metabolic parameters. We conclude that lowering the plasma insulin concentration over a prolonged period of time increases insulin sensitivity. The effects of insulin to stimulate whole body glucose utilization, nonoxidative glucose disposal, and glycogen synthase activity in muscle are therefore functions of the preexisting plasma insulin concentration.