H-13R Free Communication/Poster Miscellaneous
Activation of the growth hormone/insulin-like growth factor (GH/IGF) axis during exercise has been considered potentially favorable for anabolic metabolism. The purpose of this study was to establish the response of the IGF axis, particularly IGF-binding protein (IGFBP-3) proteolysis during submaximal exercise and recovery. Eight GH-deficient adults (GHDA) and 8 age-matched controls performed 45 min of exercise on a cycle ergometer at the lactate threshold. The GHDA were studied on 2 occassions with GH adminstered during exercise (0.4 IU GH – (+GH)) or no GH was given (-GH). Controls were only studied on one occasion. Blood samples were drawn at baseline, 15, 30, 45 min of exercise, 15 and 75 min post exercise. IGFBP-3 proteolysis was measured by an in vitro proteolytic activity assay and other hormones by RIA. GH levels increased (P < 0.01) with exercise in the +GH and controls with similar peak values (+GH: 9.8±2.4, controls: 11.4±3.6 ng/ml). Total IGF-I concentrations were lower in the control group but were only significantly lower than the +GH day. Neither total IGF-I nor IGF-II changed over time, IGFBP-1 demonstrated a time effect (P < 0.01) in all groups, and a time x group interaction with a rise at 75 min post exercise, which was greater in the GHDA subjects than control subjects (absolute increase from rest: +GH 14.9±4.8, -GH 17.8±4.8, controls 9.8±4.8 μg/L). Similarly a time x group interaction was found for IGFBP-2 with a significant increase (P < 0.01) over time in the GHDA subjects only. In the GHDA subjects, IGFBP-3 levels showed a significant change over time (P < 0.01), which was not observed in the controls. No change in IGFBP-3 proteolysis (%) was found during exercise or recovery. Submaximal exercise induces some changes in the GH/IGF axis and to IGFBP but IGFBP-3 proteolysis was unchanged with exercise, suggesting that during exercise it is not regulating IGF-I bioactivity.