GnRH agonist stimulation of the pituitary - Gonadal axis in children: Age and sex differences in circulating inhibin-B and activin-A

Abstract

Background: Inhibin-B decreases and activin increases FSH secretion in adults. We investigated whether an FSH-inhibin/activin feedback loop exists before or during puberty. Methods: FSH secretion was stimulated with 10 μg/kg leuprolide acetate (GnRH agonist) in 18 girls, ages 1.0-13.2 years, and 11 boys, ages 8.9-15.2 years, with variations in pubertal development, and in five normal 9- to 10-year-old girls. Blood, obtained at 0, 0.5, 1, 2, 4, 8, 12, 16, 20 and 24h after GnRH agonist, was analysed for LH, FSH, activin-A, inhibin-A, inhibin-B, follistatin 288 and estradiol/testosterone. Results: FSH increased within 30 min of GnRH agonist administration with a peak greater in girls than boys (P = 0.0006). Baseline inhibin-B was greater in boys than girls (P = 0.01), while baseline activin-A concentrations were greater in girls. GnRH agonist-stimulated FSH increased inhibin-B in girls by 8h and in boys by 20h (P < 0.05), but did not affect activin-A. Inhibin-B increases were seen only in girls older than 5 years. Conclusions: An inhibin-B-FSH feedback loop exists prior to the onset of puberty in girls older than 5 years. Sex differences in activin-A and inhibin-B concentrations may be responsible for sex differences in serum FSH concentration. © European Society of Human Reproduction and Embryology 2004; all rights reserved.