Response of the ewe to synthetic gonadotropin releasing hormone.

Abstract

Several experiments were performed to evaluate the response of the ewe to synthetic gonadotropin releasing hormone, pGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH2 (GnRH). The intramuscular injection of GnRH during early- or mid-anestrus effected serum LH changes which were both quantitatively and qualitatively similar. Peak serum concentrations occurred at 120 min. and were directly related to the dose of GnRH. The LH response in cycling ewes following GnRH (days 3 and 10) was also dose related, but not influenced by the stages of the estrous cycle. The peak levels of serum LH were 2.6 ± 0.3, 8.1 ± 1.8, 14.6 ± 2.9 and 41.2 ± 8.9 ng/ml, respectively, for GnRH doses of 0.2, 1, 5 and 25 micrograms. The times of the LH peaks after injection were 7.5, 15, 60 and 90 min., respectively. In another group of mid-luteal phase ewes (day 10), doses of 25 and 100 µg GnRH induced ovulation in four and six of six ewes, respectively. In contrast, GnRH did not consistently ovulate the anestrous ewe although the release of LH was as great or greater than during the estrous season. The LH response in the ovariectomized ewe following GnRH was more abrupt than in intact ewes. Maximal serum concentrations of LH occurred within 20 to 40 min. and tended to remain elevated for a longer period. Moreover, the normal endogenous episodic releases of LH had not resumed within 9 hr. of GnRH treatment. In contrast, LH-responsiveness of the early postpartum ewe to GnRH was considerably less than that observed in nonlactating animals. Peak serum concentrations of LH were 13 to 17 ng/ml compared to mean levels of 40 to 90 ng/ml in cycling ewes following a dose of 25 µg GnRH. In the postpartum ewe serum levels of LH were maximized within 20 to 45 min. and returned to baseline within 4 hours. In each of the experiments in this report, the time required for LH to return to baseline following injection of GnRH was greater than expected based on the half-disappearance time for LH in ovine serum. This may reflect a continual release of LH from the pituitary, but at a decreasing rate.