Changes in the suppressive effects of recombinant inhibin A on FSH secretion in ram lambs during sexual maturation: evidence for alterations in the clearance rate of inhibin

We have investigated the effectiveness of human recombinant inhibin A (hr-inhibin A) to suppress the secretion of follicle-stimulating hormone (FSH) in ram lambs from 1 to 18 months of age. Seventeen rams (nine castrated, eight intact) were used. At 1, 3, 6, 9, 12 and 18 months of age the rams were given an i.v. injection of either vehicle or hr-inhibin A (0.64 mu g/kg). Blood samples were taken over 24 h. Plasma concentrations of FSH were suppressed in castrated and intact rams following injection of hr-inhibin A with maximal suppression occurring 6 h after injection. Vehicle injection had no effect. At 12 months of age the suppression in FSH was most rapid in castrated rams and was maximal in intact rams. The clearance rate of inhibin was greatest at older ages but during the period of seasonally induced testicular activity, there was a significant decrease in the inhibin clearance. The testicular weight was reduced in rams treated with hr-inhibin A, indicating the importance of FSH for testicular development during the pubertal period. There was no effect of hr-inhibin A on plasma concentrations of luteinizing hormone. We conclude that inhibin is capable of suppressing the secretion of FSH in rams from 1 month of age onwards and that the pituitary gland becomes maximally responsive to the actions of inhibin by the age of puberty.