Growth Hormone Released During Exercise - Somatostatin & GH Levels

A group of Finnish researchers initially wanted to test the effects of exercise-induced growth-hormone secretion, but instead discovered a new pathway of testosterone secretion in the body.

The scientists tested 12 healthy men during two exercise regimens. The first involved a short workout of 20 minutes; the second, a prolonged training session of more than two hours. To test the effects of growth hormone during exercise, they injected the subjects with either a somatostatin (ST) analog (Sandostatin) or a saline placebo injection.

During this study, somatostatin blocked the release of growth hormone during short-term exercise, but not during the longer session. This confirms past studies that found that prolonged exercise is a potent releaser of growth hormone. But an unexpected finding concerned testosterone release after injecting ST: Higher doses increased serum-testosterone levels by 18-25%. during both exercise and rest.

While testosterone is normally stimulated by a pituitary hormone called luteinizing hormone (LH), levels of LH didn't increase in this study. The same held true for other steroid-based hormones, such as cortisol. This indicates that injected ST had a direct effect in the testes, where the majority of testosterone is synthesized.

Another indication that the testes were the scene of action was the speed of the increased testosterone release. Even under the influence of LH from the pituitary, it normally takes one to two hours for testosterone synthesis to increase. But in this study, the effect occurred almost immediately. The magnitude of the effect rivals that of the drug human chornic gonadotropin (HCG), the structure of which resembles LH. A 5,000-lu injection of HCG increases serum testosterone by 32% in two hours.

What remains a mystery is exactly how ST suddenly raises blood-testosterone levels. The researchers suggest that ST blocks the action of a substance in the testes that prevents testosterone release.