Pituitary androgen receptors and the resistance of long-term castrated rams to the androgenic control of luteinizing hormone (LH) secretion

Abstract

Luteinizing hormone (LH) responses of short-term (24 h) and long-term (6 mo) castrated rams to testosterone replacement therapy were investigated. Testosterone filled Silastic capsules which maintained physiological concentrations of testosterone in blood (∼3.2ng/ml) prevented the postcastration rise in serum LH of short-term castrated rams but failed to effectively reduce the established elevated serum LH levels in long-term castrated rams. The LH response to exogenous LHRH was also suppressed in the short-term castrated rams, whereas the response was increased in direct proportion to basal LH levels in long-term castrates. The possibility that a change in pituitary androgen receptors may explain the resistance of long-term castrates to testosterone feedback was examined. For this purpose, a cytosolic androgen receptor binding assay was developed using tritiated methyltrienolone (R1881) as the labeled ligand and dextran-charcoal to separate free and protein-bound steroid. Stable, high affinity ( K d = 0.3–1.5 nM), saturable binding of R1881 was demonstrated in pituitary cytosol from both intact and castrate rams. Relative binding specificities in intact rams, however, suggested partial binding to a progestin-binding component, whereas receptor binding in long-term castrates was androgen specific. For this reason, androgen receptor binding was studied after addition of triamcinolone acetonide to occupy apparent progestin binding sites. We observed no differences in either androgen receptor concentration (4.10 ± 0.70 versus 3.23 ± 0.45fmol/mg protein; Mean ± SD) nor ligand affinity at 0°C ( K d = 0.66 ± 0.34 versus 0.40 ± 0.08 nM) in 6 mo compared to 24-h castrate rams. These data indicate that the inability of androgens to suppress gonadotropin secretion in long-term castrate rams is unlikely to be related to specific changes in pituitary cytosol androgen receptors.