The release of luteinizing hormone (LH) in ewes deprived of prolactin during lactation

Author: B.P. Louw, A.W. Lishman, W.A. Botha, P.A.R. Arangie, B.G. Poultney & M.J. Gunter
Year: 1976
Issue: 2
Volume: 6
Page: 87 - 91

In a study aimed to establish whether prolactin secretion in lactating ewes influences the release of LH, and the response of the ovary to gonadotropin stimulation, 20 Merino ewes which lambed over an eight-day period were divided into two comparable groups of 10 animals. Between day 10 and 20 post-partum the lactating ewes in the one group received 12-hourly injections (subcutaneous) of 1,2 mg ergocornine hydrogen maleinate (EC), an effective inhibitor of prolactin secretion. Using the same schedule the ewes in the second group were injected with 1,0 cm3 of the suspension vehicle, and served as controls. On day 21 post-partum the EC-treated and control ewes were each injected (intramuscularly) with 50 μg of gonadotropin-releasing hormone (Gn-RH). During the ensuing six hours blood samples were obtained from each ewe at intervals of 15 minutes. The ewes were laparotomized on day 24, and the occurrence of ovulations noted. Thereafter, and until day 60 post-partum, vasectomized rams were joined with the ewes twice daily to detect overt oestrus. All the experimental ewes exhibited an LH surge in response to Gn-RH administration. The magnitude and duration of the surges varied considerably between individual animals. The area under the LH curve (an indication of the total quantity of LH released) In the EC-treated ewes (246,9 ± 37,9 mm2) did not differ significantly from that measured in the controls (208,5 ±66,0 mm2). Although three EC-treated, and four control ewes ovulated, no signs of oestrus were observed in any ewes between day 21 and 60 post-partum. After pooling the results obtained in the EC-treated and control ewes, it was evident that the area under the LH curve in the animals which ovulated in response to Gn-RH (303,8 ±90,9 mm2) did not differ significantly from that measured in the ewes which failed to ovulate (186,7 ±33,0 mm2). Six of the nine ewes (66,7%) which gained in mass between day 1 and 21 ovulated In response to Gn-RH, whereas ovulation occurred In only one of the eight ewes (12,5%) which lost mass during this period. In view of the variation encountered in the LH response of the ewes it was not possible to conclude whether or not the suppression of prolactin secretion in lactating ewes results in an enhanced release of LH, and an improved ovulatory reponse to gonadotropin stimulation.

Read article