Title

Regulation of high-affinity growth hormone binding protein in resistance-trained and untrained men

Date of Completion

January 2003

Keywords

Biology, Animal Physiology|Health Sciences, Recreation

Degree

Ph.D.

Abstract

The purpose of this investigation was to examine the effects of acute and chronic heavy resistance exercise on circulating concentrations of growth hormone binding protein (GHBP), immunoreactive growth hormone (iGH), and insulin-like growth factor-1 (IGF-1), with the hypothesis that resistance training enhances GH sensitivity at the liver via an increase in GH receptor, as indicated by elevated GHBP, and thus induces elevated IGF-1 concentrations in resistance-trained men. Using a cross-sectional experimental design, a group of resistance-trained men (N = 9) with at least five years of resistance training experience (mean ± SE years: 7.94 ± 1.31; actual range: 5–18 years) and a group of untrained men (N = 10) performed an acute heavy resistance exercise protocol (6 sets of 10 repetition maximum (10 RM) parallel squats). Resting blood samples were obtained 72 hours prior to exercise and immediately prior to exercise, as well as 0, 15, 30, 45, and 60 minutes post-exercise. Significant (P < 0.05) resistance-exercise induced increases in GHBP, iGH, and IGF-1 were observed in both subject groups. No significant differences in GHBP were observed between groups either at rest or post-exercise. Both groups experienced a significant increase in iGH in response to exercise, with the resistance-trained group exhibiting a significantly greater magnitude of response as well as higher area under the curve (AUC). Both groups experienced a significant increase in IGF-1 in response to exercise. The resistance-trained men exhibited significantly higher IGF-1 than untrained subjects at rest and post-exercise, as well as a significantly greater AUC than the untrained group. Significant positive correlations were observed between GHBP and BMI, GHBP and body fat, GHBP and leptin, as well as IGF-1 and fat-free mass. A significant positive correlation was also observed between resting Leptin and GHBP in the untrained subjects, whereas this relationship was not observed in the trained subjects. These data indicate that resistance training does not increase blood GHBP, thus indicating no change in GH receptor expression after training despite elevated IGF-1 in resistance-trained subjects, which may indicate greater hepatic GH sensitivity. However, other factors that strongly influence blood GHBP, including leptin and adipose cells, may mask training-induced increases in circulating GHBP. ^