GLUCOSE-METABOLISM DURING EXERCISE IN MAN - THE ROLE OF INSULIN IN THE REGULATION OF GLUCOSE-UTILIZATION

The present study was designed to characterize further the role of ins ulin in the regulation of glucose utilization during a 2-h exercise at 40% Vo(2)max in 14 h fasted, healthy subjects. Endogenous insulin and glucagon were suppressed by somatostatin infusion and replaced singly or in combination to match the hormonal concentrations observed durin g similar exercise in saline-treated control subjects. Glucose kinetic s were determined by a tracer method using D-[2,3,4,6,6-H-2]glucose. I n the exercising controls, during the last how of the exercise, plasma glucose remained stable (4.26 +/- 0.06 mmol/L) and glucose utilizatio n (R-d) increased significantly (p < 0.05) from 12.2 +/- 0.2 to 28.6 /- 1.3 mu mol . kg(-1). min(-1). During insulin deficiency without glu cagon replacement, plasma glucose was maintained at 3.74 +/- 0.10 mmol /L by dextrose infusion, but with glucagon replacement plasma glucose increased to 6.69 +/- 0.24 mmol/L (p < 0.05). These hormonal changes w ere associated with an increase in R-d to 18.6 +/- 1.1 mu mol . kg(-1) . min(-1) (p = ns versus resting controls) and to 37.9 +/- 1.9 mu mol . kg(-1). min(-1) (p < 0.05 versus resting controls), respectively. Wh en insulin was replaced without glucagon replacement, plasma glucose w as maintained at 3.85 +/- 0.06 mmol/L by dextrose infusion and R-d inc reased significantly (p < 0.05) from the resting value to 25.9 +/- 0.7 mu mol . kg(-1). min(-1). When insulin was replaced together with glu cagon, the plasma glucose (4.29 +/- 0.15 mmol/L) and the R-d (32.1 +/- 0.9 mu mol . kg(-1). min(-1), p < 0.05 versus the resting value) obta ined were similar to the values from the saline exercising control. Gl ucose metabolic clearance rate (MCR) significantly increased (p < 0.05 ) during exercise in all protocols. When insulin was made deficient, M CR increased 2-fold (p < 0.05) during exercise (2.7 to 4.8 and 5.4 mL . g(-1). min(-1), respectively, with and without glucagon deficiency). However, when insulin was present, with and without glucagon deficien cy, it increased further to 6.7 and 7.5 ml . kg(-1). min(-1), respecti vely, and values were different (p < 0.05) from glucose MCRs during in sulin deficiencies. It is concluded that in 14 h fasted, healthy subje cts, exercise per se can stimulate whole body glucose uptake even when insulin is made deficient. Insulin is necessary, however, for optimal glucose utilization during prolonged mild intensity exercise.