GnRH agonist stimulation of the pituitarygonadal axis in children: age and sex differences in circulating inhibin-B and activin-A.

BACKGROUND: Inhibin-B decreases and activin increases FSH secretion in adults. We investigated whether an FSHinhibin/activin feedback loop exists before or during puberty. METHODS: FSH secretion was stimulated with 10?g/kg leuprolide acetate (GnRH agonist) in 18 girls, ages 1.013.2 years, and 11 boys, ages 8.915.2 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 24?h 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 8?h and in boys by 20?h (P<0.05), but did not affect activin-A. Inhibin-B increases were seen only in girls older than 5 years. CONCLUSIONS: An inhibin-BFSH 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 concentrations. [ABSTRACT FROM AUTHOR]