The effect of estrogen, progesterone and prostaglandin F 2α on uterine contractions in seasonally anovulatory mares

Abstract

Uterine contractions were studied in two experiments utilizing ultrasonography and seasonally anovulatory mares. A one-minute ultrasound scan was done to produce longitudinal real-time images of the uterine body and an overall uterine contractile activity score (0 = no or minimal activity to 4 = maximal activity) was assigned to each scan. In experiment 1, a two-hour uterine activity trial (one score every 10 minutes) was done in mares given a single injection of prostaglandin F 2α (PGF 2α group; n=4) and control mares (n=4). There was no difference between the two groups over the two-hour trial (mean activity score averaged over the two-hour trial: PGF 2α group, 0.2; control group, 0.1). In experiment 2, 16 mares were randomly assigned to one of four groups: 1) controls (corn oil vehicle), 2) 1 mg estradiol 17β on days 0 to 9 and 100 mg progesterone on days 10 to 20 (E 2-->P 4 group), 3) 100 mg progesterone on days 0 to 20 (P 4 group), and 4) 100 mg progesterone on days 0 to 9 and 1 mg estradiol 17ß + 100 mg progesterone on days 10 to 20 (P 4-->E 2 + P 4 group). Uterine activity was assessed for each mare daily. The day by group interaction was significant. Scores for the E 2-->P 4 group were greater on days 4 to 11 (P<.05) than for the other three groups. From day 14 to 21, scores did not differ among the three steroid-treated groups (except on day 15), but the scores averaged over each steroid-treated group were greater for each day (P < .1 or .05) than for the controls (except on day 17). A single injection of PGF 2α on day 10 did not increase uterine activity in the controls and E 2-->P 4 groups, but did increase scores (P < .05) within 30 minutes after injection in both P 4-treated groups (means before PGF 2α and 30 minutes after PGF 2α, respectively; P 4 group, 0.4, 1.0; P 4-->E 2±P 4 group, 0.2, 1.2). Results demonstrated that in anovulatory-season mares with minimal ovarian follicular activity, daily administration of estradiol alone or progesterone alone resulted in elevated uterine contractile activity in four and 14 days, respectively. The addition of estradiol to the progesterone regime after 10 days of progesterone treatment did not alter the level of uterine contractile activity from that obtained with progesterone alone. A PGF 2α injection after 10 days of steroid treatment increased the activity within 30 minutes in progesterone-treated mares, but did not further increase the high level of activity in estradiol-treated mares.