Estradiol Influences on Pattern of Gonadotropin Secretion in Bovine Males during the Period of Changed Responses to Estradiol Feedback in Age-Matched Females1

Abstract

When ovaries are removed prior to puberty, administration of exogenous 17.beta.-extradiol (E2) decreases concentrations of luteinizing hormone (LH) below that of ovariectomized heifers receiving no E2. Subsequent to the time age-matched intact heifers reach puberty, exogenous E2 increases secretion of LH in ovariectomized heifers above that of ovariectomized heifers receiving no E2. The hypothesis that E2 would inhibit gonadotropin secretion in bovine males during th e time E2 no longer inhibited gonadotropin secretion in age-matched bovine females was tested. Males (n = 12) and female (n = 12) were gonadectomized at 241 .+-. 3 days of age, and half of each sex (6 males and 6 females) were administered at 27-cm E2 implant. An additional group of males (n = 6) and females (n = 6) remained intact and served as controls. Blood samples were collected (to quantify LH and follicle-stimulating hormone [FSH]) from all animals at 15-min intervals for 24 h at 1, 7, 13, 17, 21, 25, 29, 33, 37, and 43 wk after gonadectomy. Additional blood samples were collected twice weekly from control females to monitor progesterone and onset of corpus luteum function (451 days of age). E2 inhibited frequency of pulses of LH (p< 0.01) and decreased mean concentration of LH and FSH (p<0.01) at Week 1 in gonadectomized males treated with E2 compared to gonadectomized males not administered E2. This suppression of LH of FSH in gonadectomized males administered E2 did not continue to occur throughout the study. By Week 43, gonadectomized males implanted with E2 had similar concentrations of FSH, increased concentrations of LH (p<0.01), and an increased frequency of pulses of LH (p<0.10) compared to that of gonadectomized males receiving no E2. A similar suppression during Week 1 and lack of inhibition of LH and FSH during Week 43 was observed in the gonadectomized females administered E2. Thus, a decrease in sensitivity in E2-negative feedback on secretion of LH and FSH occurred in the bovine male during the period when age-matched females initiated estrous cycles. This timing of the change in E2 feedback on secretion of LH and FSH was similar to that observed in the bovine female.