The determination of how testosterone controls the secretion of growth hormone (GH) by temporarily withdrawing testosterone. Masculating substances (androgens) participate critically in amplifying the GH axis in puberty, and the joint action between GH and testosterone contributes to the remarkable increase in muscle mass, bone mineral density, and stature in adolescence. In adults, androgen concentrations gradually decline with aging, which occurs at the same time as fall-off in growth hormone secretion and plasma IGF-1 concentrations. This dual decrease likely contributes to loss of muscle mass, gain in fat tissue of organs, loss of skeletal strength, decrease in sexual function, and relative loss of endurance in the aging male population. However, how androgens and growth hormone interact in the healthy adult is largely unknown. The hypothesis of this study is that the androgens control the pituitary secretory response to critical proteins which normally supervise the release of growth hormones. The study will examine the ability of testosterone to amplify the ability of the growth hormone GHRH alone, GHRP-2 alone, and a combination of the two, to stimulate pituitary growth hormone secretion. A total of 60 male subjects, 30 older (60-80), and 30 younger (20-40) men will take part in the randomized study. Subjects will receive either placebo or testosterone enanthate. A single injection of the medication will lower the testosterone approximately three weeks later and will keep the value low for about four weeks. There will also be weekly injections (4 in all) of either placebo or testosterone. There will be six blood sampling sessions during days 25 and 38. One session will take place overnight and there will be five morning sessions. Drawing of blood samples before and after the injections will tell whether the pituitary responds more or less when testosterone is present or withdrawn in young and older volunteers.
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