Effect of exogenous insulin on plasma free carnitine levels during exercise in normal man

without a constant 0.13 mU kg-' min-I i.v. insulin infusion. Blood samples were taken at rest, during exercise, and after exercise for measurement of plasma glucose, insulin, C-peptide, free fatty acids, and carnitine. Plasm glucose dropped significantly (p < 8.81) from basal during both infusions, but values at 30,45, and $0 rnin of exercise were lower (p < 0.05) during insulin infusion compared with the saline infusion. Exercise produced a significant (p < 00.81) fall in plasma insulin in both infusions. However, from 30 to 90 min of exercise, the plateau insulin level was higher during the insulin infusion compared with the saline infusion (9 1.4 t 3.0 vs. 32.9 + 3.0 pmol/L; p < 0.001). Plasma C-peptide decreased significantly (p < 0.01) during exercise and recovery in both infusions, but values between infusions were not significantly different. Plasma free fatty acids increased significantly (p < 0.8%) at 90 min of exercise during the saline infusion, while during the insulin infusion this was noted during recovery only. Free fatty acid values at 75 (p < 0.05) and 90 min (p < 0.01) of exercise, and during recovery (p < 0.0%), were lower during the insulin infusion than during the saline infusion. Plasma free carnitine declined (p < 6.01) during exercise and recovery. However, at 60 min of exercise and at 30 min sf recovery, free cmitine levels were significantly (p < 0.05) lower during the saline infusion compared with the insulin infusion. The decrement in surface area below baseline in free carnitine was greater during the saline infusion than in the insulin infusion (-298 f 52 vs. - 133 k 29 j~.moI. L-' . min-I; p < 0.01). Esterified cmitine increased (p < 0.01) similarly during both infusions. These data indicate that the fall in plasma free cmitine during exercise can be reduced by an exogenous insulin infusion in nomal man. Indirectly, this suggests that the absence sf a decline in plasma free cmitine during exercise in diabetics can be explained by their high insulin levels.