Cholinergic but not serotonergic mediation of exercise-induced growth hormone secretion

Silvestre DH, Cavalcanti-de-Albuquerque JPA, Louzada RA, Carvalho DP, Werneck-de-Castro JP. Thyroid hormone and estrogen regulate exercise-induced growth hormone release

Growth hormone (GH) regulates whole body metabolism, and physical exercise is the most potent stimulus to induce its secretion in humans. The mechanisms underlying GH secretion after exercise remain to be defined. The aim of this study was to elucidate the role of estrogen and pituitary type 1 deiodinase (D1) activation on exercise-induced GH secretion. Ten days after bilateral ovariectomy, animals were submitted to 20 min of treadmill exercise at 75% of maximum aerobic capacity and tissues were harvested immediately or 30 min after exercise. Non-exercised animals were used as controls. A significant increase in D1 activity occurred immediately after exercise (~60%) in sham-operated animals and GH was higher (~6-fold) 30 min after exercise. Estrogen deficient rats exhibited basal levels of GH and D1 activity comparable to those found in control rats. However, after exercise both D1 activity and serum GH levels were blunted compared to sedentary rats. To understand the potential cause-effect of D1 activation in exercise-induced GH release, we pharmacologically blocked D1 activity by propylthiouracil (PTU) injection into intact rats and submitted them to the acute exercise session. D1 inhibition blocked exercise-induced GH secretion, although basal levels were unaltered. In conclusion, estrogen deficiency impairs the induction of thyroid hormone activating enzyme D1 in the pituitary, and GH release by acute exercise. Also, acute D1 activation is essential for exercise-induced GH response.

Insulin signaling pathways and cardiac growth

The development, growth, function and metabolism of the heart are regulated by extracellular growth factors, cytokines and ligands. In this review, the role of insulin and insulin-like growth factors in the regulation of cardiac growth will be discussed. In addition, the role of insulin- and insulin-like growth factor-stimulated intracellular signaling proteins in cardiac growth will be reviewed.

Abuse of growth hormone among young athletes

The underground abuse of growth hormone (GH) among young athletes presents a challenge to medical professionals. Health care professionals providing knowledgeable guidance regarding healthy ways to improve performance and appearance, as well as accurate information regarding substances' perceived benefits, risks, and unknown qualities, is invaluable to the young athlete. Further research focused on the profile and motivation of young people who use GH is essential to understanding and intervening better with those who use these substances.

Exercising in the cold inhibits growth hormone secretion by reducing the rise in core body temperature

OBJECTIVE

Ambient temperature alters exercise induced GH secretion. It is unknown whether temperature affects GH secretion at exercise intensities above the anaerobic threshold when other factors may override the relationship seen at lower intensities.

DESIGN

Cross-over study of ambient temperature on exercise induced GH in swimmers and rowers.

SETTING

St Thomas Hospital, London.

SUBJECTS

Ten healthy men (age 21.7+/-0.8 yrs). Five swimmers and five rowers.

INTERVENTION

Forty-minute exercise test at 105% of anaerobic threshold at room temperature (RT) and at 4 degrees C.

MEASUREMENTS

Cutaneous and core body temperature. Serum GH concentration.

RESULTS

Cutaneous body temperature increased during exercise at RT but decreased in the cold. Although core temperature rose in both settings, the rise was greater at RT (p=0.021). GH increased at both temperatures but the onset was delayed by the cold. Peak GH tended to be higher at RT (17.4+/-3.6 microg/L vs. 9.5+/-1.5 microg/L, p=0.07). Total GH secretion was greater at RT (353.3+/-99.1 microg min/L) than 4 degrees C (128.3+/-21.0 microg min/L), p=0.038. Change in core temperature correlated with log peak GH (r=0.66, p=0.039) and log incremental GH (r=0.67, p=0.032) when exercising at 4 degrees C. There was no difference between swimmers and rowers.

CONCLUSIONS

Exercise at 4 degrees C reduces GH secretion during exercise at intensities above the anaerobic threshold. A change in core body temperature may be one mechanism by which exercise induces GH secretion. The difference in GH between swimmers and rowers during their respective events relates to the conditions under which they compete.

Growth Hormone Releaser Attenuates β-Amyloid (1 – 42)-Induced Memory Impairment in Mice

Accumulating evidence indicates that growth hormone (GH) might be effective at preventing the development of Alzheimer's disease. However, exogenous GH treatment has exhibited side effects for clinical application; thus supplementation with amino acids to promote the release of GH could be a possible alternative treatment. In this study, mice that were fed with a diet of GH-releasing supplements had significantly attenuated memory impairments and hippocampal changes in the acetylcholinesterase activity and acetylcholine level induced by amyloid beta protein (Abeta) (1 - 42). Our results suggest that the use of GH-releasing supplement exerts beneficial effects on the memory impairment induced by Abeta (1 - 42).

Growth hormone and exercise: physiology, use and abuse

This review summarizes the interactions between growth hormone (GH) and exercise. Exercise has profound effects upon the GH-insulin-like growth factor I axis per se. In addition, there is increasing evidence that such physiological perturbations might be influential in the performance responses to repeated training. However, the ergogenic effects of systemic administration of recombinant human GH by athletes and bodybuilders remain unproven. What is certain is that the prevalence of GH abuse by sportspeople will increase, not least because it is currently undetectable. The frequent and potentially severe side-effects associated with such 'doping' will be of increasing relevance to endocrinologists.

Association of depressiveness with blunted growth hormone response to maximal physical exercise in young healthy men

Blunted response of growth hormone secretion to several pharmacological challenges is present in depression, but much less is known about the relationship of depression and secretion of growth hormone elicited by physiological stimuli. Furthermore, it is not known whether blunted growth hormone response occurs in depressiveness as measured with psychometric scales. A total of 82 healthy male volunteers (age 18-26 years) exercised on a bicycle ergometer with incremental load to achieve their maximal performance. Before exercise, subjects filled in approbated versions of Beck Depression Inventory (BDI), Spielberger's State-Trait Anxiety Scale, Cohens Perceived Stress Scale, and Schwartzers Self-Efficacy Scale. Blood samples were collected before and after exercise, and growth hormone, cortisol, and testosterone were measured by chemiluminescence immunoassay. Median perceived stress score of the subjects was identical to our population-based database median value, but the subjects had higher self-efficacy and lower depressiveness as shown by median values. In the majority of subjects, physical exercise induced remarkable increases in blood levels of the hormones. Cortisol and testosterone levels were not associated with the scores of psychometric scales. However, growth hormone response was virtually absent in high scorers (above median population score, n = 24) in BDI total score and the negative attitude subcomponent. Hence, this study demonstrates that growth hormone response to physiological stimuli is reduced in psychometrically measured depressiveness.

Growth hormone and exercise

This review summarizes the interactions between GH and exercise. Not only does exercise have profound effects upon the GH/IGF-I axis per se, but there is increasing evidence that such physiological perturbations might be influential in the performance responses to repeated training. However, the effects of systemic administration of rGH in restoring exercise capability and muscle strength in GH deficient adults and the ergogenic benefits of GH doping amongst athletes remain unproven. What is certain is that these issues will be of increasing relevance to clinical endocrinologists.

Effect of the enhancement of the cholinergic tone by pyridostigmine on the exercise-induced growth hormone release in man

We have evaluated the effect of pyridostigmine (PD), a cholinergic agonist, on the growth hormone (GH) response to physical exercise (EXC) in nine healthy volunteers. PD administration and EXC caused a similar increase of GH secretion to mean (+/- SE) peak values of 5.3 +/- 0.9 and 6.5 +/- 1.2 micrograms/l, respectively. Pretreatment with PD caused a significant augmentation of the EXC-induced GH release evaluated both as maximum peak (13.5 +/- 2.1 micrograms/l, p < 0.01 vs EXC) and as area under the secretory curve (EXC = 292.6 +/- 41.9 micrograms.min.l; PD + EXC = 587.3 +/- 68.9 micrograms.min.l, p < 0.005). The action of PD on GH secretion was additive to that of EXC since the sum of the GH responses to PD and EXC was not significantly different from the response obtained during PD + EXC. Whether PD and EXC act through a common final pathway, i.e. inhibition of endogenous somatostatin release, or the EXC-induced GH secretion involves stimulation of endogenous GHRH remains matter of investigation.