Regulation of Growth Hormone and Somatomedin-C Secretion in Postmenopausal Women: Effect of Physiological Estrogen Replacement*

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Abstract
To determine the effects of estrogen deficiency and replacement on GH secretion, we measured the 22-h GH secretory pattern and response to 1 h of light exercise in 16 normal postmenopausal women before and after treatment replacement with ethinyl estradiol (20 μg/day for 15 days). To determine whether the changes found were due to pituitary sensitization by estrogen, the response to synthetic GH-releasing hormone (GHRH; 1.0 μg/kg, iv) was measured. To assess the biological effectiveness of GH in estrogen-treated women, somatomedin- C (Sm-C) responses to GHRH were measured. Pre- and postestrogen GH secretion rates, expressed as mean areas circumscribed by plasma GH values, were as follows: 22-h study, 1.4 ± 0.1 (±SEM) VS. 2.0 ± 0.3 ng/ml-h (P = 0.04; n = 5); during 1 h of exercise, 2.3 ± 0.4 vs. 3.2 ± 0.4 ng/ml h (P = 0.03; n = 16); after GHRH-(l–40), 6.7 ± 1.7 vs. 8.5 ± 1.5 ng/ml-h (P = 0.12; n = 16). There also was a modest but significant increase in resting plasma GH (1.5 ± 0.2 vs. 2.3 ± 0.5 ng/ml (P = 0.039). Pre- and postestrogen plasma Sm-C concentrations were 0.56 ± 0.08 and 0.32 ± 0.03 U/ml, respectively (P = 0.006; n = 16). Thus, estrogen therapy increased spontaneous and exerciseinduced GH secretion in postmenopausal women and reduced Sm-C levels. The mechanisms of GH elevation by estrogen may include both central effects and a negative feedback linkage to reduced plasma Sm activity.

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