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Testosterone (Te) concentrations fall gradually in healthy aging men. Postulated mechanisms include relative failure of gonadotropin-releasing hormone (GnRH), luteinizing hormone (LH), and/or gonadal Te secretion. Available methods to test Leydig cell Te production include pharmacological stimulation with human chorionic gonadotropin (hCG). We reasoned that physiological lutropic signaling could be mimicked by pulsatile infusion of recombinant human (rh) LH during acute suppression of LH secretion. To this end, we studied eight young (ages 19-30 yr) and seven older (ages 61-73 yr) men in an experimental paradigm comprising 1) inhibition of overnight LH secretion with a potent selective GnRH-receptor antagonist (ganirelix, 2 mg sc), 2) intravenous infusion of consecutive pulses of rh LH (50 IU every 2 h), and 3) chemiluminometric assay of LH and Te concentrations sampled every 10 min for 26 h. Statistical analyses revealed that 1) ganirelix suppressed LH and Te equally (> 75% median inhibition) in young and older men, 2) infused LH pulse profiles did not differ by age, and 3) successive intravenous pulses of rh LH increased concentrations of free Te (ng/dl) to 4.6 +/- 0.38 (young) and 2.1 +/- 0.14 (older; P < 0.001) and bioavailable Te (ng/dl) to 337 +/- 20 (young) and 209 +/- 16 (older; P = 0.002). Thus controlled pulsatile rh LH drive that emulates physiological LH pulses unmasks significant impairment of short-term Leydig cell steroidogenesis in aging men. Whether more prolonged pulsatile LH stimulation would normalize this inferred defect is unknown.
Veldhuis et al. (2004) studied this question.