Gonadotroph responsiveness in orchidectomized sheep: IV. Effect of estradiol infusion during the breeding and anestrous seasons.

The interaction between estradiol (E2) and season on gonadotroph function was assessed in orchidectomized sheep (wethers). Wethers in the breeding (November) or anestrous (May) season received E2 (in 20% ethanol-saline [vehicle]) by continuous infusion at rates of 0.31, 1.25, 5, or 20 micrograms/h (n = 6 wethers/group). Controls received vehicle alone. Gonadotroph responsiveness (LH release induced by 500 ng GnRH, i.v.) was assessed at the end of the 48-h infusion period. Mean serum concentrations of E2 at the end of infusion were < 0.6 pg/ml in animals receiving vehicle, and 2.5 +/- 0.3, 10.7 +/- 0.4, 35.1 +/- 0.9, and 86.1 +/- 4.1 pg/ml in animals receiving 0.31, 1.25, 5, and 20 micrograms E2/h, respectively. Mean serum concentrations of LH and FSH, the amplitude and frequency of LH secretion, and gonadotroph responsiveness did not differ (p > 0.05) with season in controls. Infusion of E2 at 5 or 20 micrograms/h decreased (p < 0.05) serum concentrations of LH and FSH and eliminated episodic secretion of LH, regardless of season. Conversely, E2 infusion at 0.31 microgram/h resulted in significant suppression of gonadotropin secretion in May, but not November. Similarly, the reduction in serum concentrations of LH and FSH induced by E2 infusion at 1.25 micrograms/h was more pronounced (p < 0.05) in May than November. Compared to gonadotroph responsiveness in controls, responsiveness in the breeding, but not the anestrous, season increased (p < 0.05) after infusion of 0.31 microgram E2/h.(ABSTRACT TRUNCATED AT 250 WORDS)