Abstract
The mechanism of suppression of gonadotrophins and testosterone during prolonged exercise was studied. Fourteen healthy males were injected immediately before an exhaustive bicycle run for 4 h, and before a control period without exercise, with one of the following substances: (1) 1 ml of saline, (2) 100 micrograms of a GnRH agonist (buserelin), and (3) 50 IU/kg of hCG. Each test was repeated for each subject 2 weeks apart, in a randomized single-blind fashion. Blood samples were taken before the injections (sample A), 6 h after the injections (sample B), and the following morning (sample C). During placebo treatment testosterone concentration decreased from A samples to B samples more in the exercise trial than in rest trial (31 vs 8%, P = 0.02). This exercise-associated decrease was reversed by both the GnRH agonist and hCG treatments. Serum gonadotrophin concentrations responded identically to GnRH agonist injections during exercise and rest trials. hCG resulted in a greater increase of serum testosterone from A to C samples at rest than during the exercise trial (52 vs 33%, P = 0.04). In conclusion, the exercise-induced suppression of serum testosterone is associated with two effects: suppressed endogenous GnRH stimulation of gonadotrophin release during exercise, and decreased testicular capacity to secrete testosterone during recovery period.
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