Effect of bicycle exercise on insulin absorption and subcutaneous blood flow in the normal subject

Elimination of 8 units 125I-insulin and 99mTc-pertechnetate from a subcutaneous depot on the thigh or the abdomen was studied at rest and during intense bicycle exercise in healthy postabsorptive volunteers. Disappearance rates of the tracers as well as plasma insulin and glucose concentrations were determined before, during and after the 20 min exercise period, and compared to corresponding values obtained during a non-exercise, control study on another day. Leg exercise caused a two-fold increase in the rate of 125I-insulin disappearance from a leg depot (first-order rate constants rose from 0.68 +/- 0.15 to 1.12 +/- 0.12% . min-1, P less than 0.05), but had no significant effect on the rate of disappearance from an abdominal depot (rate constants were 0.75 +/- 0.17 and 0.87 +/- 0.18% . min-1 at rest and during exercise, respectively). 99mTC-pertechnetate clearance from leg or abdomen showed no significant change during exercise, indicating that subcutaneous blood flow was unaltered. Leg, but not abdominal, injection of insulin was associated with a greater rise in plasma insulin during exercise than at rest. The average difference between exercise and control insulin area-under-curve in the leg group (1426 +/- 594% . min) was significantly greater (P less than 0.05) than that from the abdominal group (298 +/- 251% . min). When the data from the two study groups were pooled, a direct relationship was found to exist between the change in 125I-insulin disappearance rate and the change in plasma insulin concentration (r = 0.61, P less than 0.02). Plasma glucose levels fell throughout the observation period both during the exercise and the control study, following leg as well as abdominal injection. The glucose decremental area was greater during exercise than at rest both following leg (P less than 0.05) and abdominal injection (P less than 0.01). The exercise-induced mean reduction in plasma glucose was 60% lower following abdominal injection, but this difference was not significant. In conclusion, the present results demonstrate that, in healthy subjects in the postabsorptive state (a) intense physical exercise of short duration can accelerate the absorption of subcutaneously injected insulin; (b) the effect is more pronounced at injection sites near the exercising parts; (c) an increase in subcutaneous blood flow is not the main reason for this effect.