Introduction. Endogenous Growth Hormone (GH) and Insulin-Like Growth Factor I (IGF-I) super families are known to mediate many of the growth and repair processes of skeletal muscle in response to long term resistance training. No data exist to describe the dynamics of the IGF-I family within the molecular weight ranges in which GH variants exist. In addition, gender differences in the responses of IGF-I and immunoreactive GH to an acute bout of resistance exercise within these molecular weight ranges have not been examined. Purpose. The primary purpose of this investigation was to assess IGF-I and GH in three different molecular weight ranges (i.e., 0-30 kD (C fraction), 30-60 kD (B fraction) and > 60 kD (A fraction) plasma fractions which contain different GH variants to an acute resistance exercise bout in well-trained men and women. Methods. Eight men (BM: 87.0±18.5 kg; Ht: 175.3±6.7 cm; Age: 21±1 yrs) and seven women (BM: 76.4±8.8 kg; Ht: 164.6±6.7 cm; Age: 24±5 yrs) volunteered to participate in this study. Each subject performed an acute resistance exercise bout and blood was collected pre-, immediately post- (IP), and 70 minutes (+70) post-exercise. Results. Women had significantly higher resting values of ICE-I, GH, and IGFBP-3. Exercise-induced increases in circulating IGF-I and GH were observed in both genders, with women demonstrating a significantly (P ≤ 0.05) higher concentration of IGF-I at the IP and +70 time points. Significant exercise-induced increases were observed for IGF-I binding proteins 1 and 2 (IGFBP-1 and 2) for the women at IP and +70 and +70 for the men (IGFBP-1), and at IP for both genders (IGFBP-2). Increases IP exercise were observed in medium size plasma fractions (Fraction B, 30-60 kD) for IGF-I and GH in both genders, but the only gender was observed in IGF-I Fraction B. Conclusion. The results from this research demonstrate the dynamic nature of the pituitary-hepatic axis of the GH responses in the different molecular weight fractions. Furthermore, IGF-I in the circulation may be complexed in a higher molecular weight fraction, suggesting biological activity apart from the free hormone hypothesis. Finally, women demonstrated higher resting concentrations of IGF-I.
