Resonator led me to Testosterone Blunts Feedback Inhibition of Growth Hormone Secretion by Experimentally Elevated Insulin-Like Growth Factor-I Concentration, Johannes D. Veldhuis, Stacey M. Anderson, Ali Iranmanesh and Cyril Y. Bowers, The Journal of Clinical Endocrinology & Metabolism Vol. 90, No. 3 1613-1617, 2005, where they found:
“…supplementation of a high dose of Te in middle-aged and older men attenuates IGF-I feedback-dependent inhibition of nadir and peak GH secretion.”
The results of this study were confirmed in a recent study published this month:
Testosterone Supplementation in Older Men Restrains Insulin-Like Growth Factor’s Dose-Dependent Feedback Inhibition of Pulsatile Growth Hormone Secretion,Johannes D. Veldhuis, Daniel M. Keenan, Joy N. Bailey, Adenborduin Adeniji, John M. Miles, Remberto Paulo, Mihaela Cosma and Cacia Soares-Welch,The Journal of Clinical Endocrinology & Metabolism Vol. 94, No. 1 246-254, 2009
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.