Mechanism by Which Progesterone Shortens the Estrous Cycle of the Ewe

Abstract

Progesterone administered early in the estrous cycle of ewes decreases the average life of corpora lutea. The effects of exogenous progesterone during estrus on patterns of concentrations of luteinizing hormone (LH) and progesterone in peripheral plasma and of prostaglandin (PGF_2α) in utero-ovarian venous plasma were studied. In experiment 1, eight ewes received 40 mg of progesterone (i.m.) in corn oil and eight were given only corn oil 8 h after the onset of estrus (Day 0). Mean cycle length in progesterone-treated ewes was less than in control ewes (12.5 ± 1.7 vs 16.3 ± 0.2 days: P2α in uterine venous plasma on Day 5 proestrus did not differ significantly between groups. In experiment 2, ewes were injected (i.m.) with corn oil containing 0 (n = 6) or 40 mg (n = 6) of progesterone 8 and 32 h after the onset of estrus. Beginning on Day 5, blood samples were collected via a catheter in one utero-ovarian vein every 8 h until the onset of the next estrus for quantification of PGF_2α. Five of six progesterone-treated ewes had short cycles (11.0 ± 1.0 days), while cycles averaged 17.0 ± 0.3 days for the six control ewes. Concentrations of LH were lower 8 h after the second injection of progesterone but reached control levels by 32 h. Concentrations of progesterone in peripheral plasma of progesterone-treated and control ewes did not differ on Days 4 to 7. The first peak of PGF_2α occurred earlier in progesterone-treated (Day 8.0 ± 0.3) than in control ewes (Day 12.3 ± 1.0; P2α on estradiol-17β revealed that plasma concentrations of estradiol at the same hour and at 16 and 24 h previous to PGF_2α could account for some of the variation in PGF_2α. Decreased concentrations of LH immediately following exogenous progesterone did not interfere with the ability of the young corpus luteum to produce progesterone. Earlier peaks of PGF_2α following exogenous progesterone during estrus may be more important than reduced LH in causing earlier regression of corpora lutea.