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

Abstract

To study the relationship between serum inhibin B and sex steroid concentrations and pituitary FSH responsiveness to GnRH in boys in early puberty, and to examine serum inhibin B levels in prepubertal boys with different timing of the onset of gonadotropin deficiency (GD). Twenty-five boys with constitutional delay of puberty (CDP; 20 in Tanner stage G2 and 5 in G3; age range, 13. 5-16.8 years) and eight prepubertal boys (G1P1) with GD (age range, 10.0-13.2 years) were clinically examined, and serum inhibin B, testosterone and estradiol concentrations were measured from sera obtained immediately before the administration of GnRH (Relefact; 3.5 microgram/kg, maximum 100 microgram i.v.). Thereafter, FSH levels were measured at 30min intervals up to 90min. In the boys with CDP, basal inhibin B and FSH levels did not correlate. 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 serum FSH levels were inversely related (r(S)= -0.63, n=25, P<0.005). This relationship remained significant in boys at stage G2 (r(S)= -0.66, n=20, P<0.005). Basal testosterone concentrations and GnRH-induced FSH levels did not correlate. Estradiol levels were too low (64% of the boys had estradiol levels below the assay sensitivity) to allow correlation analysis. The boys with GD had low inhibin B concentrations (range, <15.6-53pg/ml); the lowest levels were observed in boys with presumably congenital onset of the disease. Serum inhibin B levels and testis volumes correlated positively (r(S)=0.70, n=8, P=0.07). These results suggest that, in boys, the reciprocal regulation between inhibin B and FSH is in operation before mid-puberty. Moreover, autonomous inhibin B secretion by the prepubertal human testis is likely to reflect the number of Sertoli cells.