Abstract
BACKGROUND: Why pulsatile GH secretion declines in estrogen-deficient postmenopausal individuals remains unknown. One possibility is that estrogen not only enhances stimulation by secretagogues but also attenuates negative feedback by systemic IGF-I. SITE: The study took place at an academic medical center. SUBJECTS: Subjects were healthy postmenopausal women (n=25). METHODS: The study included randomized assignment to estradiol (n=13) or placebo (n=12) administration for 16 d and randomly ordered administration of 0, 1.0, 1.5, and 2.0 mg/m2 recombinant human IGF-I sc on separate days fasting. ANALYSIS: Deconvolution analysis of pulsatile and basal GH secretion and approximate entropy (pattern-regularity) analysis were done to quantify feedback effects of IGF-I. OUTCOMES: Recombinant human IGF-I injections increased mean and peak serum IGF-I concentrations dose dependently (P<0.001) and suppressed mean GH concentrations (P<0.001), pulsatile GH secretion (P=0.001), and approximate entropy (P<0.001). Decreased GH secretion was due to reduced secretory-burst mass (P=0.005) and frequency (P<0.001) but not basal GH release (P=0.52). Estradiol supplementation lowered endogenous, but did not alter infused, IGF-I concentrations while elevating mean GH concentrations (P=0.012) and stimulating pulsatile (P=0.008) and basal (P<0.001) GH secretion. Estrogen attenuated IGF-I's inhibition of pulsatile GH secretion (P=0.042) but was unable to restore physiological GH pulse frequency or normalize approximate entropy. CONCLUSION: Short-term estrogen replacement in postmenopausal women selectively mutes IGF-I-mediated feedback on pulsatile GH secretion. Disinhibition of negative feedback thus confers a novel mechanism by which estrogen may obviate hyposomatotropism.