Age-related differences in growth hormone (GH) regulation during strenuous exercise.

This study was designed to investigate the central neuroendocrine mechanisms by which exercise (EX) stimulates growth hormone (GH) release as a function of age. Twelve male subjects, six in their early-to-mid twenties and six in their late sixties or seventies, received a strong GH stimulus either as incremental EX until volitional exhaustion or by administration of GHRH alone or Hex alone two hours after a presumed maximal GH response to combined administration of GHRH plus hexarelin (Hex). Total GH availability was calculated as area under the curve (AUC) over time periods 0 - 120 and 120 - 240 min. The mean AUC in micro g/l x 120 min to GHRH+Hex in the younger group was approximately twice that in the older group (11,260, range 3,947 - 19,007 vs. 5,366, range 2,262 - 8,654). In younger males, the mean AUC to EX (509, range 0 - 1,151) was larger than to GHRH (119, range 0 - 543), but less than that to Hex (919, range 0 - 1,892). In the older group, GH responses to EX and GHRH were abolished (mean AUC: 112, range 0 - 285, and 156, range 30 - 493), respectively) in contrast to the response to Hex (1,077, range 189 - 1,780). These data indicate that maximal GH stimulation by GHRH+Hex results in greater desensitization of GHRH compared to Hex, irrespective of age. We postulate that the abolished responsiveness of GH to EX in older group is due to insufficient disinhibition of hypothalamic somatostatin activity and desensitization of GHRH, while the preserved activity of a central Hex-related pathway is not involved. The GH response to EX in younger males is due to complete inhibition of somatostatin activity and stimulation of a central Hex-related pathway in spite of GHRH desensitization. We conclude that a central Hex-related pathway is the primary factor for EX-induced GH release only in younger males.

Involvement of endogenous growth hormone-releasing hormone (GHRH) in the exercise-related response of growth hormone.

The aim of this study was to investigate the involvement of endogenous growth hormone-releasing hormone (GHRH) in the growth hormone (GH) release during strenuous exercise (EX). Eight healthy male subjects (age: 22.1 +/- 0.8 yr, body mass index: 22.2 +/- 0.9 kg/m 2, .VO 2 max: 52.2 +/- 0.5 ml/min/kg [mean +/- SEM]) were exposed to incremental EX until volitional exhaustion (cycle ergometry), and in random order to a maximally stimulating bolus injection of 100 microg GHRH, or to combined administration of 100 microg GHRH and EX (GHRH+EX). Serial blood samples in the fasted state were taken immediately before the start of each trial, and at appropriate intervals over 2 h. Total GH availability was calculated as area under the response curve (AUC), corrected for differences in baseline values. The results showed that peak serum GH levels to GHRH alone and EX alone were not significantly different: 41.5 +/- 9.0 microg/l and 64.1 +/- 8.1(mean +/- SEM). Peak GH level to GHRH+EX was 156.1 +/- 19.9 microg/l, which was significantly greater than to either stimulus alone (p < 0.02) or additively (105.6 +/- 17.1 microg/l, p < 0.02). AUC's to GHRH alone and EX alone were not significantly different (3242 +/- 839 vs. 2472 +/- 408 microg/l x 120 min). AUC to GHRH+EX (7807 +/- 1221 microg/l x 120 min) was greater than to either stimulus alone (p < 0.02) or additively (5714 +/- 1247 microg/l x 120 min, p < 0.02). This indicates a potentiating (synergistic) effect between GHRH and EX. We postulate that GH responses to strenuous EX are only partially due to maximal GHRH activation. Next to complete inhibition of hypothalamic somatostatin activity, which is achieved by strenuous exercise, activation of endogenous GH-releasing peptides, such as Ghrelin, must be operative.

Growth hormone responses during strenuous exercise: the role of GH-releasing hormone and GH-releasing peptide-2.

PURPOSE AND METHODS: This study was designed to investigate the role of two effective releasers of growth hormone (GH): GHRH and GHRP-2 during exercise (EX). Eight healthy male subjects (ages: 22 +/- 1.2 (mean +/- SD) yr, BMI: 22.5 +/- 2.2 kg x m(-2)) were exposed to maximally stimulating dose of 100 microg GHRH iv, and 200 microg GHRP-2 iv, during incremental EX on a cycle ergometer to exhaustion. GH responses after EX alone were compared with the responses after the combined administration of the same EX plus GHRH, EX plus GHRP-2, and EX plus GHRH plus GHRP-2. Blood samples were obtained in the fasted state at intervals for 2 h postexercise and the area under the GH response curve (AUC) was calculated by trapezoidal integration. RESULTS: Significant differences (P < 0.003) were observed between the AUCs after administration of EX alone (mean +/- SEM): 2,324 +/- 312 microg x L(-1) 120 min, after EX plus GHRH: 6,952 +/- 1,083, after EX plus GHRP-2: 14,674 +/- 2,210, and after the combination EX plus GHRH plus GHRP-2: 17,673 +/- 1,670. However, AUCs after each combination did not differ significantly from those after arithmetical addition of each separate stimulus, indicating that the mechanisms of the respective stimuli do not interact. Linear regression analysis on mean GH responses between 20 and 30 min after the start of EX showed that EX alone and GHRH alone explain about 59% (adj. R2) of the GH response to the combination EX plus GHRH. The ratio of the respective regression coefficients (GHRH vs EX) was about 2:1 (instead of 1:1), indicating that EX seems to potentiate the activity of GHRH. GHRH alone and EX alone also explained about 74% of the response to the combination EX plus GHRP-2. In the latter response, a synergistic action of GHRP-2 on GHRH could be observed. CONCLUSIONS: The data indicate that under strenuous EX conditions, endogenous GHRH activity causes a further increase of GH release. A GHRP-2 mediated mechanism in the central neuroendocrine regulation acts as a "booster," possibly by stimulating the effects of GHRH and/or an unknown hypothalamic factor, as well as by stimulating the pituitary GH release directly.