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Effects of Ovarian Secretions and Dopamine on Secretion of Luteinizing Hormone and Prolactin in Ewes¹

West Virginia University, Morgantown 26506²

Abstract

In the present study, effects of dopamine (DA) and ovarian secretions on release of luteinizing hormone (LH) and prolactin (PRL) were examined using two methods of withdrawal of progesterone. Eight to 10 d after estrus, 36 ewes were assigned to one of six treatments: 1) lutec tomy; 2) lutectomy and DA; 3) ovariectomy; 4) ovariectomy and DA; 5) ovariectomy and estradiol-17)β or 6) ovariectomy, estradiol-17β and DA. Estradiol-17β was administered by Silastic implants inserted during surgery. Immediately after surgery, ewes were infused iv with DA (6.6 µg•kg^–1 •min^–1) or saline for 48 h. Infusion of DA did not affect secretion of LH after surgical removal of the corpus luteum. In lutectomized ewes, concentrations of LH in serum increased between 0 and 12 h from .5 to 3.3 ng/ml, declined between 12 and 16 h and then plateaued at 1.1 to 1.5 ng/ml. In contrast, concentrations of LH in ovariectomized ewes increased between 0 and 12 h and then remained at 3 to 5 ng/ml. Surges of LH occurred between 40 and 50 h postsurgery in eight of 12 lutectomized ewes, 11 of 12 ovariectomized ewes with implants of estradiol-17β and zero of 12 ovariectomized ewes. The magnitude of the surge of LH was higher in lutectomized ewes than in ovariectomized ewes with implants of estradiol-17β. Infusion of DA inhibited release of PRL, but there was no effect of method of progesterone withdrawal. Lutectomized ewes, but not ovariectomized ewes with implants of estradiol-17β, had a surge of PRL associated with the surge of LH. Based on these results, it appears that: 1) DA at a dose of 6.6 µg•kg^–1•min^–1 does not affect secretion of LH after surgical removal of the corpus luteum, but alters concentrations of PRL and 2) ovarian secretions in addition to estradiol-17β and progesterone influence tonic and phasic secretion of LH and PRL.

¹ Published with the approval of the Director of the West Virginia Agr. and Forestry Exp. Sta. as Scientific Paper No. 1794 . Supported by Hatch Project 224 (NE-72). The authors thank Dr. R.L. Butcher for the antisera for progesterone and estradiol-17β, Dr. G. D. Niswender for the ovine LH antisera (GDN #15), Dr. L. Reichert for the purified ovine LH (LER 1374-A) used for iodinations, and the National Institute of Arthritis, Metabolism and Digestive Diseases for the NIAMDD-oPRL-1-1 and NIAMDD-anti-oPRL-I.

² Division of Anim. and Vet.Sci.