Growth Hormone Release During and Following Acute Resistance Exercise Includes Disulfide-linked Aggregates

Abstract

Growth hormone (GH) exists as a family of nearly 100 different molecular variants, which can include oligomers linked via non-covalent or covalent (disulfide) bonds. Previous data (Hymer, et al., 2001) have demonstrated that chemically reducing post exercise serum [via glutathione (GSH)] results in elevated GH concentrations, indicating the presence of disulfide-linked aggregates. PURPOSE: To provide a further characterization of GH aggregates in the exercise-induced hormonal milieu before and after 8 wks of chronic exercise training. METHODS: Thirteen males (27.5 ± 1.4 yr) underwent two different 8-wk resistance training programs designed to improve military performance. Prior to and following the training programs, subjects performed an acute exercise test (AET; 6 sets of 10 repetition maximum squat with 2-min inter-set rest periods), and had venous blood drawn before (Pre), during (Mid), and post AET (0, 15, and 30 min post exercise). Plasma GH concentrations were determined in the non-reduced (−GSH) and reduced (+GSH; [1 0mM GSH] for 18 h) states using a commercially available IRMA assay. Data were analyzed using repeated measures ANOVA with a LSD post-hoc test. RESUITS: No differences were observed in the GH responses of the 2 training programs; therefore, training group data were collapsed for analysis. Significant main effects were observed for GSH reduction (p<0.01), suggesting that +GSH increased the GH signal (+GSH: 1.7 ±0.3 vs. -GSH: 1.4 ±0.3 ?g/L), and time (p<0.01), indicating that GH concentrations increased during the AET (Pre: 0.1 ± 0.0 <Mid: 1.3 ± 0.4 < Post: 2.9 ± 0.6 ?g/L) and declined in recovery (15-minpost: 2.2 ± 0.4 > 30-min post: 1.4 ± 0.4 ?g/L). A significant reduction × time interaction effect indicated that serum contained disulfide-linked GH aggregates at Mid, 0, 15 and 30 min post (p<0.01). For integrated GH AUC concentrations, a reduction main effect suggested that total GH released contained disulfide-linked molecular variants (+GSH: 7.7 ± 1.4 vs. -GSH: 6.4 ± 1.3 min*μg/L; p<0.01). No effects of chronic resistance exercise on the GH response were observed. CONCLUSIONS: Disulfide-linked GH aggregates, which may sustain the biological half-life of GH, are released during and following acute exercise; however, this response does not appear to be altered with chronic training.