PEAK OXYGEN-UPTAKE, MUSCLE VOLUME, AND THE GROWTH HORMONE-INSULIN-LIKE GROWTH-FACTOR-I AXIS IN ADOLESCENT MALES

Purpose: The growth effects of exercise appear to be mediated in part by central neuroendocrine control reflected in circulating levels of g rowth hormone (GH), insulin-like growth factor-I (IGF-I), and their bi nding proteins (BP). In previous studies positive correlations between peak (V) over dot O-2 and circulating IGF-I have been demonstrated. T he relationship between peak oxygen uptake and these potential regulat ing factors has not been examined in adolescent males where patterns o f GH pulsatility and levels of IGF-I are rapidly changing. Methods: Fo rty-three healthy adolescent males (age 16 +/- 0.7 yr, 70% at Tanner V ) performed cycle ergometry to determine p oxygen uptake (peak (V) ove r dot O-2), and magnetic resonance images to determine the thigh muscl e volume. Baseline blood samples were collected for GHBP, the extracel lular portion of the GH tissue receptor (by ligand mediated immunofunc tional assay), ICF-I (by RIA), and IGFBPs 1-5 (by RIA). Mean GH was de termined from samples obtained every 20 min overnight. Results: Peak ( V) over dot O-2/kg was positively correlated with mean overnight GH le vels (r = 0.41, P < 0.005). Both peak (V) over dot O-2/kg and thigh mu scle volume/kg were negatively correlated with GHBP(r = -0.33, P < 0.0 2) and IGFBP-4 (r = -0.52, P < 0.005). There were no correlations betw een peak (V) over dot O-2/kg and IGF-I or IGFBPs 1-3, and 5. Conclusio ns: GH pulsatility is increased adolescent males who have higher peak (V) over dot O-2, bur this did not translate into increases in IGF-I. We speculate that in the fitter males, lower GHBP levels may reduce he patic sensitivity to GH. Thus, circulating IGF-I was unchanged despite higher mean GH in subjects with higher peak (V) over dot O-2. IGFBP-4 which is known to inhibit IGF-I was negatively correlated with peak ( V) over dot O-2 leading, possibly, to increased IGF-I bioactivity. Fit ness las assessed by muscle mass and peak (V) over dot O-2) does modul ate the GH-IGF-I axis, but not solely through circulating IGF-I; both GHBP and IGFBPs play important roles.