Serum levels of luteinizing hormone and gonadotropin-releasing hormone in cycling, castrated and anestrous ewes.

Abstract

Blood samples were collected at 4-min intervals for 4 hr from 2 ewes on the day of estrus and 2 ewes castrated 90 and 120 days previously. Serum levels of luteinizing hormone (LH) and gonadotropin-releasing hormone (Gn-RH) were determined by double antibody radioimmunoassays. Samples were collected during a portion of the preovulatory LH surge in each ewe with LH levels ranging from <5 ng/ml to=300 ng/ml. Gn-RH levels ranged from 15 to 96 pg/ml and were not significantly elevated when LH levels were high. LH in castrated ewes ranged from 2.7 to 13.3 ng/ml while Gn-RH ranged from 119 to 368 pg/ml during the 4-hr sampling period. There was no apparent correlation between serum levels of LH and Gn-RH. Both cycling ewes and castrated ewes demonstrated rhythmic oscillations in serum levels of Gn-RH with peaks occurring at a frequency of 15.4 ± 4.0 min (SD). Four anestrousewes were given an intramuscular injection of 100μ g 17β-estradiol and blood samples were collected at 10-min intervals from 1 hr pre-injection until10 hr post-injection, at 5-min intervals from 10 to 16 hr and at 10-min intervals from 16 to 24 hr post-injection. LH, Gn-RH and follicle-stimulating hormone (FSH) were quantified in these samples. Peak levels of LH (mean = 184 ng/ml) occurred in 3 of the 4 ewes simultaneously with peak levels of FSH (mean = 621 ng/ml). Gn-RH levels did not change significantly throughout the sampling period in those ewes that responded to 17β-estradiol with elevated LH levels, nor in the ewe which did not respond. It was concluded that increased levels of Gn-RH in the peripheral circulation were not responsible for LH release during estrus in cycling ewes, in castrated ewes, or in anestrous ewes treated with 17β-estradiol. (Endocrinology94: 713, 1974)