GHRP-2 pulsatile GH secretion driven by testosterone, not DHT or estradiol, in older men
Background
Age-related decline in pulsatile GH secretion is well documented but its hormonal drivers remain poorly defined. Testosterone (Te) is aromatized to estradiol (E2) and 5alpha-reduced to dihydrotestosterone (DHT), yet whether these distinct metabolic fates differentially regulate the GH axis has never been cleanly dissected in humans. Standard approaches cannot separate Te's androgenic from its estrogenic effects in vivo. Understanding which steroid metabolite gates somatostatin tone, GHRH sensitivity, or GH secretagogue responsiveness is essential for designing rational androgen-replacement or GH-augmentation protocols in aging men.
Study Design
Forty-two healthy older men (ages 50–79 yr) were enrolled at an academic medical center. Investigators imposed a pharmacological testosterone clamp and layered in the 5alpha-reductase inhibitor dutasteride or the aromatase inhibitor anastrozole to selectively suppress DHT or E2 formation. Sequential infusions of somatostatin (SS), GHRH, GHRP-2, and a triple stimulus (L-arginine/GHRH/GHRP-2) were administered to probe distinct nodes of GH regulation. Primary endpoints were deconvolution-estimated basal and pulsatile GH secretion rates across all treatment arms.
Results
Te/placebo elevated Te 2.8-fold, DHT 2.6-fold, and E2 1.9-fold above placebo/placebo. Te/dutasteride and Te/anastrozole selectively reduced stimulated DHT and E2 by 89% and 86%, respectively, confirming effective enzymatic blockade. Stepwise forward-selection regression showed Te positively predicted mean GH (p = 0.017), peak GH (p < 0.001), basal GH secretion (p = 0.015), and GHRP-2-stimulated pulsatile GH secretion (p < 0.001).
GHRP-2-stimulated pulsatile GH secretion was independently and positively predicted by testosterone alone (p < 0.001), establishing Te — not its metabolites — as the primary driver of GH secretagogue responsiveness.
For the triple-stimulus response, Te and E2 were joint predictors: Te acted positively (p = 0.006) while E2 acted negatively (p = 0.031), suggesting opposing modulation at the combined secretagogue level. DHT correlated positively with pulsatile GH secretion during SS infusion (p = 0.011), indicating a selective DHT role in somatostatin-regulated GH release. All effects persisted after inclusion of abdominal visceral fat in the regression model.
Key Findings
- Te/placebo raised Te 2.8-fold, DHT 2.6-fold, and E2 1.9-fold vs. placebo/placebo
- Dutasteride and anastrozole reduced stimulated DHT and E2 by 89% and 86%, respectively
- GHRP-2-stimulated pulsatile GH secretion positively predicted by Te alone (p < 0.001)
- Triple-stimulus GH: Te positive (p = 0.006), E2 negative (p = 0.031) — opposing effects
- DHT positively correlated with pulsatile GH during somatostatin infusion (p = 0.011)
Why It Matters
Clinicians and peptide users co-administering testosterone with GHRP-2 should recognize that testosterone itself — not its aromatized or 5alpha-reduced metabolites — is the primary amplifier of GHRP-2-driven pulsatile GH output. This means aromatase inhibitors taken to control estrogen during a Te protocol will not blunt GHRP-2 efficacy, but may paradoxically enhance the triple-stimulus GH response by removing E2's negative contribution. Conversely, DHT levels appear to modulate somatostatin-gated GH pulses, so agents that suppress 5alpha-reduction (finasteride, dutasteride) could dampen GH release under conditions of high somatostatin tone. The findings push toward individualized GH-secretagogue dosing anchored to steroid metabolite profiling and visceral adiposity rather than total testosterone alone.
