IN VITRO TESTOSTERONE SECRETION BY TESTICULAR TISSUE FROM YOUNG BULLS AND THE EFFECTS OF CHRONIC AND ACUTE EXPOSURE TO ESTRADIOL- 17β

The possibility that estradiol-17/3 may directly influence testicular steroidogenesis in bulls was investigated in vitro. Testicular tissues were incubated for 4 h and regression coefficients (b, ng·ml-1·h-1) based on the increase in testosterone in the medium were used to describe testosterone secretion rates. In the first experiment, testicular tissues from control bulls and bulls chronically implanted with estradiol were incubated in the absence (basal conditions) or presence of 10 mlU/ml human chorionic gonadotropin (hCG). Under basal conditions, testosterone secretion rates were similar for tissues from control (b = 24.1 ± 6.0) and implanted (b = 34.7 ± 6.0) bulls. Testosterone secretion rates were increased approximately fourfold during incubation with hCG; tissues from implanted animals secreted testosterone at a higher rate (

Validation of a Direct Radioimmunoassay for Testosterone in Unextracted Serum from Five Species: Application to Study of the Hypothalamic-Pituitary-Gonadal Axis in Males

A method for the radioimmunoassay of serum testosterone that does not require extraction or chromatography is described. Utilization of a highly specific antiserum, tritiumlabeled ligand, and double antibody precipitation makes the direct radioimmunoassay feasible. The direct radioimmunoassay is valid based on the criteria of sensitivity, specificity, accuracy, and precision. Because extraction, chromatography, and transfer are eliminated in this procedure, recovery of serum testosterone is always 100%. Interference by serum binding components is lacking, and values obtained for ram, bull, dog, rat, and man by direct radioimmunoassay are in agreement with those obtained by a conventional extraction assay. Direct testosterone radioimmunoassay has been applied to the study of hypothalamic control of testosterone secretion in bull, ram, and rat. The intravenous injection of synthetic luteinizing hormone releasing hormone (LHRH) increases blood testosterone and can be administered to mimic the pulsatile nature of testosterone secretion characteristic of intact males. On the other hand, immunoneutralization of LHRH by active or passive immunization results in substantially reduced serum testosterone and failure of testosterone secretion in response to exogenous LHRH. These investigations lead us to conclude that testosterone secretion by the mammalian testis is regulated largely by inputs from the hypothalamus and that synthetic LHRH and LHRH antisera provide useful tools for studying the hypothalamopituitary- gonadal axis. The direct radioimmunoassay provides a convenient and inexpensive method to study testosterone secretion by the testis

Hypothalamic control of the post-castration rise in serum LH concentration in rams

Sexually mature rams were left intact, castrated (wethers), castrated and implanted with testosterone, or castrated, implanted with testosterone and pulse-infused every hour with LHRH. Serum concentrations of LH increased rapidly during the first week after castration and at 14 days had reached values of 13·1 ± 2·2ng/ml (mean ± s.e.m.) and were characterized by a rhythmic, pulsatile pattern of secretion (1·6 ± 0·1pulses/h). Testosterone prevented the post-castration rise in serum LH in wethers (1·0 ± 0·5ng/ml; 0 pulses/h), but a castrate-type secretory pattern of LH was obtained when LHRH and testosterone were administered concurrently (10·7 ± 0·8 ng/ml; 1·0 pulse/h). We conclude that the hypothalamus (rather than the pituitary) is a principal site for the negative feedback of androgen in rams and that an increased frequency of LHRH discharge into the hypothalamo-hypophysialportal system contributes significantly to the post-castration rise in serum LH

Testosterone feedback on FSH secretion in male sheep

Testosterone-filled Silastic capsules were implanted into mature rams at the time of castration (wethers). After 6 weeks, a tonic pattern of FSH secretion was observed in rams, wethers and wethers implanted with testosterone. Castration caused serum concentrations of FSH to increase 4-5-fold. Relatively low serum concentrations of testosterone (25-50% of intact ram values) did not significantly affect FSH secretion in wethers, but wethers exposed to concentrations of testosterone equivalent to those of intact rams had serum concentrations of FSH similar to those of intact rams. We suggest that testosterone feedback can account for the gross differences in FSH observed between intact and castrated male sheep

PULSATILE LUTEINIZING HORMONE SECRETION IN THE CASTRATE MALE BOVINE: EFFECTS OF TESTOSTERONE OR ESTRADIOL REPLACEMENT THERAPY

Luteinizing hormone (LH) secretory profiles have been determined for the male bovine following castration and steroid replacement therapy. Serum LH concentrations increased approximately threefold during the first week following castration and thereafter remained elevated (6.6 ± .7 ng/ml). Castrates not receiving steroid replacement showed a rhythmic pattern of LH release that was of high frequency (mean pulse interval; 85 ± 5 min) and high amplitude (mean peak concentration, 11.2 ± 1.4 ng/ml). Chronic administration of estradiol-17β via subdermal Silastic implants reduced mean serum LH concentrations (2.1 ± .3 ng/ml) and blocked the pulsatile pattern of LH release in all steers. Similar administration of testosterone suppressed mean serum LH and blocked pulsatile LH release in two of four animals. The number of implants used in this study provided physiological concentrations of estradiol (9.8 ± 1.5 pg/ml) and testosterone (4.1 + .2 ng/ml) in systemic blood for the two respective treatment groups. Differences in the LH secretory profiles among testosterone;implanted steers may have been related, in part, to differences in the amounts of steroid not bound to serum proteins. These findings demonstrate that estradiol is a particularly potent inhibitor of pulsatile LH secretion in the male bovine and suggest that gonadal steroid feedback on LH secretion may, in part, be imposed at the level of the hypothalamus. The mechanism for pulsatile LH release is discussed relative to a centrally-located luteinizing hormone releasing hormone pulse generator

Comparative Response of Rams and Bulls to Long-Term Treatment with Gonadotropin-Releasing Hormone Analogs

The objective was to compare the relative response between rams and bulls in characteristics of LH, FSH and testosterone (T) secretion, during and after long-term treatment with GnRH analogs. Animals were treated with GnRH agonist, GnRH antagonist, or vehicle (Control) for 28 days. Serial blood samples were collected on day 21 of treatment, and at several intervals after treatment. Injections of natural sequence GnRH were used to evaluate the capacity of the pituitary to release gonadotropins during and after treatment. Treatment with GnRH agonist increased basal LH and T concentrations in both rams and bulls, with a greater relative increase in bulls. Endogenous LH pulses and LH release after administration of GnRH were suppressed during treatment with GnRH agonist. Treatment with GnRH antagonist decreased mean hormone concentrations, LH and T pulse frequency, and the release of LH and T after exogenous GnRH, with greater relative effects in bulls. Rams previously treated with antagonist had a greater release of LH after administration of GnRH compared with control rams, while rams previously treated with agonist showed a reduced LH response. Bulls previously treated with agonist had reduced FSH concentrations and LH pulse amplitudes compared with control bulls while bulls previously treated with antagonist had greater T concentrations and pulse frequency. The present study was the first direct comparison between domestic species of the response in males to treatment with GnRH analogs. The findings demonstrated that differences do occur between rams and bulls in LH, FSH and testosterone secretion during and after treatment. Also, the consequences of treatment with either GnRH analog can persist for a considerable time after discontinuation of treatment