Signaling between the pituitary gland and the testes: inverse relationshipbetween serum FSH and inhibin B concentrations in boys in early puberty

Objectives: To study the relationship between serum inhibin B and sex stero id concentrations and pituitary FSH responsiveness to GnRH in boys in early puberty and to examine serum inhibin B levels in prepubertal boys with dif ferent timing of the onset of gonadotropin deficiency (GD). Design: Twenty-five boys with constitutional delay of puberty (CDP; 20 in T anner stage G2 and 5 in G3: age range, 13.5-16.8 years) and eight prepubert al boys (G1P1) with GD (age range, 10.0-13.2 years) were clinically examine d, and serum inhibin B, testosterone and estradiol concentrations were meas ured from sera obtained immediately before the administration of GnRH (Rele fact; 3.5 mu g/kg. maximum 100 mu g i.v.). Thereafter, FSH levels were meas ured at 30 min intervals up to 90 min, Results: In the boys with CDP, basal inhibin B and FSH levels did not corre late. however, inhibin B and GnRH-stimulated FSH concentrations (r(S) = -0. 43 to -0.45, n = 25, P < 0.05) and the difference between basal and peak se rum FSH levels were inversely related (r(S) = -0.63, n = 25, P < 0.005), Th is relationship remained significant in boys at stage G2 (r(S) = -0.66, n = 20, P < 0.005). Basal testosterone concentrations and GnRH-induced FSH lev els did not correlate. Estradiol levels were too low (64% of the boys had e stradiol levels below the assay sensitivity) to allow correlation analysis. The boys with GD had low inhibin B concentrations (range, < 15.6-53 pg/ml) ; the lowest levels were observed in boys with presumably congenital onset of the disease, Serum inhibin B levels and testis volumes correlated positi vely (r(S) = 0.70, n = 8, P = 0.07). Conclusions: These results suggest that, in boys, the reciprocal regulation between inhibin B and FSH is in operation before mid-puberty. Moreover, au tonomous inhibin B secretion by the prepubertal human testis is likely to r eflect the number of Sertoli cells.