Endocrine and ovulatory responses of the gilt to exogenous gonadotropins and estradiol during sexual maturation.

Three studies were conducted to investigate the endocrine and ovulatory responses of the prepubertal gilt to exogenous estradiol and gonadotropins. In Study One, prepubertal gilts of 190 days of age were injected s.c. with pregnant mare's serum gonadotropin (PMSG) or physiological saline (SAL). Following PMSG injection, circulating levels of estradiol-17 beta (E2) increased. This increase was followed by a surge of luteinizing hormone (LH), estrus, a rise in progesterone (P4) levels, and ovulation. None of the gilts given SAL had increased levels of E2, LH or P4, and none ovulated. In Study Two, prepubertal gilts of 165 days of age were treated with varying doses of PMSG. A positive correlation was observed between dose of PMSG and peak levels of E2 (r = 0.83, P less than 0.001) and between dose of PMSG and number of corpora lutea (r = 0.96, P less than 0.001). In Study Three, gilts were treated at ages of 70 to 190 days with estradiol benzoate (EB), PMSG, or corn oil plus saline (CO/SAL) followed in 72 to 96 h by human chorionic gonadotropin (hCG) or SAL. All gilts treated with EB at 100 to 175 days of age had two surges of LH at an approximately 24-h interval. Gilts responding to EB at 70 and 190 days had only one surge of LH. Gilts of 100 days of age or older responded to PMSG with a single surge or two surges of LH. Ovulation in response to treatment was observed in gilts of 100 days of age or greater but not at 70 days. The conclusions drawn from these studies are that 1) PMSG-induced ovulation is preceded by an increase in circulating levels of E2 and in some gilts by a surge of LH, and 2) prepubertal gilts are able to respond to exogenous endocrine stimulation with either a single surge or multiple surges of LH at 70 to 190 days but are unable to ovulate in response to exogenous gonadotropins until 100 days of age.