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

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Abstract
Elimination of 8 U 125I-insulin and 99mTc-pertechnetate from a s.c. depot on the thigh or the abdomen was studied at rest and during intense bicyce execise in healthy postabsorptive volunteers. Disappearance rates of the tracers and 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 2-fold increase in the rate of 125I-insulin disappearance from leg depot (1st-order rate constants rose from 0.68 .+-. 0.15 to 1.12 .+-. 0.12% min-1, P < 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 s.c. 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-1) was significantly greater (P < 0.05) than that from the abdominal group (298 .+-. 251% min-1). When the data from the 2 study groups were pooled, a direct relationship existed between the change in 125I-insulin disappearance rate and the change in plasma insulin concentration (r = 0.61, P < 0.02). Plasma glucose levels fell throughout the observation period during both the exercise and the control study, following leg and abdominal injection. The glucose decremental area was greater during exercise than at rest both following leg (P < 0.05) and abdominal injection (P < 0.01). The exercise-induced mean reduction in plasma glucose was 60% lower following abdominal injection, but this difference was not significant. In healthy subjects in the postabsorptive state intense physical exercise of short duration apparently can accelerate the absorption of s.c. insulin; the effect is more pronounced at injection sites near the exercising parts; and an increase in s.c. blood flow is not the main reason for this effect.