Antiluteolytic Effect of the Conceptus and of PGE2 in Ewes1

Abstract

The responses of pregnant and nonpregnant ewes on day 13 post-estrus to a dose of prostaglandin (PG)F₂α known to be luteolytic in nonpregnant ewes were compared (experiment I). Either 270 µg PGF₂α THAM salt (eight nonmated and 11 mated ewes) or saline (eight nonmated and 10 mated ewes) was injected into the largest follicle on each ovary bearing a corpus luteum. Jugular plasma samples taken just prior to surgery and 4, 8, 12, 16, 20, 24, 28, 32 and 36 hr after injection were analyzed for progesterone, estradiol-17ß and luteinizing hormone (LH) by radioimmunoassay. Treatment with PGF₂α shortened the estrous cycle by 1.5 days in nonmated ewes and mated ewes without embryos (P<.05). Mated ewes with embryos did not return to estrus. The linear regression of progesterone over time varied with treatment (P<.01), pregnancy status (P<.01) and treatment × pregnancy status (P<.09). Progesterone did not reach as low a level (1.3 vs .4 ng/ml; P<.05) in pregnant as in nonmated ewes. Both LH and estradiol-17ß increased in treated ewes after progesterone had decreased to approximately 50% of pre-treatment levels; the increase in estradiol was transient in the pregnant ewes. The pattern of LH did not differ with pregnancy status. It is concluded that pregnancy overcame the luteolytic effects of PGF₂α (i.e., had an antiluteo-lytic effect). Experiment II was designed to ascertain whether PGE₂ has antiluteolytic properties. A catheter was inserted into each uterine horn that was ipsilateral to a corpus luteum in nonmated ewes on day 11 postestrus. Intrauterine injections of 0, 300, 600 or 900 µg of PGE₂ were begun on the morning of day 12 and continued every 8 hr until estrus or day 20. Jugular plasma samples were taken just before the first injection and every 24 hr thereafter in all ewes and at .25, .5, 1, 2, 4 and 8 hr from several ewes in each group and were assayed for progesterone. The estrous cycle was lengthened and the decline in progesterone delayed by 2 days by PGE₂ (P<.05). Frequent samples on day 12 did not differ in progesterone levels among treatments or over time. Prostaglandin E₂ had antiluteolytic activity in ewes but no in vivo steroidogenic effect was observed. Copyright © 1977. American Society of Animal Science . Copyright 1977 by American Society of Animal Science.