Testosterone regulation of luteinizing hormone and follicle stimulating hormone secretion in young male lambs.

Polydimethylsiloxane (Silastic) capsules containing crystalline testosterone were implanted subcutaneously in castrate lambs and evaluated for their ability to (1) provide stable, physiological concentrations of serum testosterone and (2) regulate secretion of LH and FSH. Testosterone was nondectable (< .2 ng/ml) between 12 and 24 weeks of age in the serum of castrate lambs implanted with empty capsules but was increased at those ages in lambs implanted with 20 cm (1.2 +/- .1 and .5 +/- .1 ng/ml), 60 cm (2.8 +/- .1 and .9 +/- .1 ng/ml) and 180 cm (6.6 +/- .3 and 1.9 +/- .2 ng/ml) of testosterone-filed Silastic tubing. Serum concentrations of LH and FSH were suppressed by exogenous testosterone and did not exceed concentrations in intact control rams as long as serum testosterone concentrations were greater than 1 ng/ml. In conclusion, simultaneous suppression of serum LH and FSH by concentrations of testosterone that cause a dose-dependent stimulatory effect on accessory sex gland weights indicates that testosterone is a physiological regulator of gonadotropin secretion in ram lambs. Although testosterone per se may not be directly responsible for negative feedback on LH and FSH secretion, the results presented herein question the need for a nonsteroidal "inhibin-like" testicular hormone to regulate FSH in lambs.