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Bovine Ovarian and Pituitary Responses to PMS and GnRH Administered during Metestrus^{1 ,2,}

Oregon State University, Corvallis 97331

Abstract

The effects of pregnant mare serum gonadotropin (PMS) and gonadotropin releasing hormone (GnRH) on metestrus ovarian follicular development, ovulation and serum concentrations of luteinizing hormone (LH), progesterone (P₄) and estradiol-17 (E₂) were investigated. Eight Hereford heifers were assigned randomly in equal numbers to two groups and treated as follows: 1) PMS administered intramuscularly at 12 (1,000 IU) and 36 hr (2,000 IU) after detected estrus followed at 55 hr by an intravenous (IV) injection of 100 µg GnRH, and 2) sterile water (vehicle) at 12 and 36 hr followed at 5 5 hr by GnRH. Group assignments for each heifer were reversed during each of three consecutive estrous cycles. During the first and second estrous cycles, blood samples were collected by jugular venipuncture and the sera were analyzed for LH, P₄ and E₂ by use of radioimmunoassay. Heifers were sacrificed 10 days after detected estrus during the third estrous cycle and follicular characteristics were measured. Treatment of heifers with PMS failed to stimulate follicular growth during metestrus, as determined by palpation, but increased follicular growth during the remainder of the cycle and prolonged the cycle (P<.025). Treatment with GnRH increased serum LH levels (P<.01) from 15 to 120 min following injection, but failed to cause ovulation. During the first cycle, LH released after injection of GnRH was lower (P<.01) in heifers treated with PMS than in vehicle-injected heifers. Daily serum levels of LH and P₄ were increased (P<.01) in heifers treated with PMS during the first cycle compared to controls, and heifers (PMS during the first cycle) receiving vehicle during the second cycle. Serum concentrations of E₂ in heifers treated with PMS were increased (P<.01) compared to levels of this estrogen in heifers injected with vehicle.

In a corollary study, IV injection of 100 µg GnRH into three heifers at 55 hr after detected estrus resulted in a significant reduction in serum progesterone concentration during the luteal phase of the cycle.

¹ Technical Paper No. 4660, Oregon Agricultural Experiment Station.

² The authors express their appreciation to Dr. L. V. Swanson, Oregon State University, for the supply of GnRH which was generously provided by Dr. Russ Mauer, Abbott Labs.

³ Present address: U.S. Meat Animal Research Center, Agricultural Research Service, U.S. Department of Agriculture, P.O. Box 166, Clay Center, NB 68933.

⁴ Department of Animal Science.