Characterization of growth hormone disulfide-linked molecular isoforms during post-exercise release vs nocturnal pulsatile release reveals similar milieu composition
Introduction
The molecular heterogeneity of human growth hormone (GH) has been well recognized for some time [3,4,19,23,28], but only recently considered in regard to exercise responses. The fact that over 100 molecular isoforms exist in the circulation offers a partial explanation for the pleiotropic physiological effects that GH exerts [3,12,28]. The numerous functional effects of GH on local tissues include lipid, carbohydrate and protein metabolism, tissue anabolism, cartilaginous growth stimulation, increased mobilization and utilization of fat for energy, and insulin-like growth factor release from the liver [12,28]. The primary form of GH, and the most oft-studied, is 22 kDa GH, expressed by the hGH-N gene [3,22,28]. This molecular isoform accounts for approximately 21% of plasma circulating GH, while the second most prevalent is the 20 kDa monomer (representing 6% of circulating plasma GH) [3,10,22,28]. Human GH can also form variants and aggregates (i.e. dimers, trimers, and oligomers) and fragments which exist in the circulation [2,4]. Lewis et al. were the first to isolate a disulfide-linked GH dimer and to demonstrate its chemical, biological, and immunological properties. Interestingly, the growth-promoting activity of disulfide-linked GH dimer is greatly diminished when compared to the 22 kD monomer. Thus, with the two main forms comprising <30% of circulating plasma GH, combined with the fact that different isoforms have a multitude of downstream bioactivities, there is a need for evaluating molecular heterogeneity of GH [1,3,5,13,25]. This need may be particularly relevant following stimuli known to induce a robust release of GH, such as exercise and sleep.
GH is known to mediate many metabolic and somatogenic actions, thus being highly pertinent to physically active populations. The stress of physical exercise is a potent stimulus for eliciting acute GH release from the anterior pituitary [18]. Previous studies have shown that both aerobic and resistance exercise can result in acute, transient increases in GH concentrations [9,14,15,20,21,23,26,36] and that exercise intensity and duration can alter the magnitude of this elevation. A vast majority of exercise studies have been limited to short, post-exercise sampling schemes (<2 h) that may not fully characterize the extent of the exercise-induced changes in GH secretion due to the pulsatility of its release. For instance, the pattern of GH release differs between daytime and nighttime [12,16]. It would therefore seem important to also sample GH during the sleeping hours (i.e., a time of dynamic GH release) to fully examine the impact of exercise on GH release. The few studies that have examined overnight GH concentrations after exercise are contradictory, either reporting no change [20,26,39], an increase, or a decrease [15]. Additionally, exercise's potential effect on GH heterogeneity has been previously investigated. Wallace et al. [34] demonstrated changes in the proportions of GH isoforms following aerobic exercise. Variations in immunodetectability and molecular heterogeneity have also been demonstrated following both acute and chronic resistance training [19,23]. Although these studies demonstrated important differences in GH isoform detections, they were limited by a single or short post-exercise sampling period and by only investigating one mode of exercise.
As aerobic versus resistance exercise is known to elicit diverse training effects, it is plausible to hypothesize that different modes of exercise may also exert dissimilar influences on GH molecular isoform concentrations. While 22 kDa GH accounts for ~75% of total GH during acute exercise, the proportion of non-22 kDa-GH to the 22 kDa-GH isoform changes post-exercise [34]. Authors have suggested that, in response to exercise, differential pituitary isoform secretion may be responsible for the increased proportion of non-22-kDa isoforms in the circulation [34]. Although these studies demonstrated important differences in GH isoform detections, they were limited by a single or short post-exercise sampling period and by only investigating one mode of exercise. There may also exist different isoform secretory patterns during sleep when exercise has occurred earlier. This may, in turn, result in an increased nocturnal pulse response following exercise. The first aim of this study was to confirm the results of previous studies in a cohort with a prolonged overnight sampling period: that is, to characterize the influence of mode of exercise (aerobic vs. resistance) and volume of exercise (moderate vs. high) on circulating GH immediately post-exercise as well as following the onset of sleep (i.e. the nocturnal). However, the primary purpose of this study was to evaluate any differences in the presence of GH dimeric isoforms both at post-exercise and nocturnal pulse using gluthathione reduction. The current study reports similar milieu composition between post-exercise vs nocturnal pulsatile release disulfide linked molecular isoforms. The increased presence of disulfide-linked GH isoforms (i.e. dimers) after both exercise and nocturnal pulse release suggest both of these stimuli produce GH isoforms with extended half-lives and sustained physiological actions.
Section snippets
Subjects and methods
Eight healthy men participated in the study, which was approved by the United States Army Research Institute of Environmental Medicine and Medical Research and Materiel Command human use review boards. Subjects were briefed on all study procedures and associated risks and could participate only after giving their free and informed voluntary consent. Each subject was medically screened by a physician prior to inclusion in the study. All subjects were determined to be non-smokers and free of any
Results
Main effects are reported as (mean ± SE ng/mL). Significant differences were revealed by reduction (p = .006), as main effects for reduced samples (2.46 ± 0.29) were larger than non-reduced samples (2.19 ± 0.26). No significant differences were observed when pooling GH responses at post-exercise (2.02 ± 0.21) and nocturnal pulse (2.63 ± 0.51; p = .32). No interaction terms within this model were statistically significant.
Table 1 displays GH concentrations for each condition at PE or NP when GSH
Discussion
The goals of this study were to characterize the effect of mode of exercise (resistance vs. aerobic) with duration of exercise (moderate vs. high) on GH release post-exercise and during sleep. Additionally, the primary aim was to evaluate the effect of GSH reduction on post-release proportions in an attempt to characterize the response of GH dimeric isoforms to exercise stimuli. The results of this study confirm those of previous studies [15,29], demonstrating that aerobic training stimulates
Conclusion
This study observed similar milieu composition between all forms of GH and dimeric isoforms in response to exercise stimuli and at the first nocturnal pulse. The results of previous studies demonstrating different GH isoform secretion following resistance and aerobic exercise led to the authors' hypothesis that the nocturnal pulse in sleep following exercise could also produce heterogenous GH concentrations. However, the lack of differences between post-exercise and nocturnal pulse
Author contributions
B.C. Nindl, R.W. Matheny, K.R. Rarick, J.R Pierce were involved in study design, data collection, data analysis and manuscript preparation/review. S.R. Eagle and B.J. Martin were involved in manuscript preparation and data analysis. M.A. Sharp, M.D. Kellogg, and J.F. Patton were involved in study design, data analysis, and manuscript review.
Acknowledgements
The opinions or assertions contained herein are the private views of the author(s) and are not to be construed as official or reflecting the views of the Army or the Department of Defense. The investigators have adhered to the policies for protection of human subjects as prescribed in Army Regulation 70-25, and the research was conducted in adherence with the provisions of 32 CFR Part 219. Any citations of commercial organizations and trade names in this report do not constitute an official
Declaration of interest
The authors have no conflicts of interest to report.
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Cited by (2)
Exercise and the hypothalamic–pituitary–adrenal axis: a special focus on acute cortisol and growth hormone responses
2019, Current Opinion in Endocrine and Metabolic ResearchCitation Excerpt :The decreases in cortisol concentration with low-intensity exercise are reported to be due to an increased metabolic clearance (i.e. cellular uptake) of cortisol, rather than a decrease in synthesis of the steroid [25]. In terms of intensity effects on GH, both aerobic and resistance exercise that acutely challenge metabolic homeostasis can induce increased GH output, but Nindl et al. [26] recently showed that both moderate and intense aerobic exercise are likely a greater stimulus for GH synthesis and secretion than moderate- or high-intensity resistance exercise. In terms of cortisol alterations after the cessation of exercise, Hooper et al. [27] followed up 22 males competing in the Ironman World Championship and analyzed multiple blood samples before the race and for 40 h after the race.
Exercise and the growth hormone–insulin-like growth factor axis
2019, Current Opinion in Endocrine and Metabolic ResearchCitation Excerpt :High-intensity functional training incorporating rowing and resistance training for 15 min has been shown to increase GH by more than 600% [6], whereas low-intensity aerobic exercise (0.5–1 h at 50% VO2max on a cycle ergometer) does not significantly elevate GH and IGF-1 [7]. On the other hand, there is indication that high-intensity aerobic exercise (six 15-min bouts at 70% VO2max on a cycle ergometer) might be superior to high-intensity resistance exercise (50 sets of exercises activating large muscle groups with 10–5 repetition maximum (RM) loads) in increasing circulating GH, with elevated nocturnal GH peak after the exercise only seen under the high-intensity aerobic exercise conditions [8]. In young sedentary women, high-intensity interval training (four 30-s all-out sprints) increased exercise-stimulated total pulsatile GH secretion without an effect on nocturnal GH [9].