Testosterone supplementation in older men restrains insulin-like growth factor's dose-dependent feedback inhibition of pulsatile growth hormone secretion

Background: Pulsatile GH secretion declines in older men. The causal mechanisms are unknown. Candidates include deficient feedforward (stimulation) by endogenous secretagogues and excessive feedback (inhibition) by GH or IGF-I due to age and/or relative hypoandrogenemia. Hypothesis: Testosterone (T) supplementation in healthy older men will restrain negative feedback by systemic concentrations of IGF-I. Subjects: Twenty-four healthy men(ages, 50 to 75 yr; body mass index, 24 to 30 kg/m²) participated in the study. Methods: We performed a prospectively randomized, double-blind, placebo-controlled assessment of the impact of pharmacological T supplementation on GH responses to randomly ordered separate-day injections of recombinant human IGF-I doses of 0, 1.0, 1.5, and 2.0 mg/m². Analysis: Deconvolution and approximate entropy analyses of pulsatile, basal, and entropic (pattern-sensitive) modes of GH secretion were conducted. Results: Recombinant human IGF-I injections 1) elevated mean and peak serum IGF-I concentrations dose-dependently (both P < 0.001); 2) suppressed pulsatile GH secretion (P = 0.003), burst mass (P=0.025), burst number (P=0.005), interpulse variability (P=0.032), and basal GH secretion (P=0.009); and 3) increased secretory pattern regularity (P=0.020). T administration did not alter experimentally controlled IGF-I concentrations, but it elevated mean GH concentrations (P=0.015) and stimulated pulsatile GH secretion (frequency P = 0.037, mass per burst P = 0.038). Compared with placebo, T attenuated exogenous IGF-I's inhibition of GH secretory-burst mass (P < 0.038) without restoring pulse number, basal secretion, or pattern regularity. Conclusion: The capability of systemic T to mute IGF-I feedback on pulsatile GH secretion suggests a novel mechanism for augmenting GH production.