Modulation of ovine myometrial activity by estradiol-17β. The possible involvement of prostaglandins

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Abstract
Administration of estradiol-17β (E2; 50 μg s.c. per day) to nonpregnant ewes results in a cyclic pattern of uterine activity; periods of high activity (6- to 8-h duration) alternating with periods of low activity or quiescence (2–3 h). To examine the role of prostaglandins (PG) in this pattern five nonpregnant, bilaterally ovariectomized ewes were treated in turn with corn oil injected s.c. (control), E2 (50 μg s.c), and E2 (50 μg s.c.) plus 600 μg/min meclofenamic acid (MFA) i.v. Intrauterine pressure (IUP) and uterine electromyographic (EMG) activity were recorded continuously. Samples of blood from the inferior vena cava and abdominal aorta were collected every 30 min for one period of 16 h during each treatment. Following corn oil injection, uterine activity remained at the low level and there was no correlation between PG concentrations and uterine activity. After administration of E2, plasma E2 concentrations rose fivefold from basal values by 3.5 h after injection and returned to basal levels by 13 h. Administration of E2 resulted in the appearance of a cyclical pattern of uterine activity. Vena cava concentrations of 13,14-dihydro-15-keto PGF2α were positively correlated to the frequency (five out of five ewes), and the maximum amplitude (four out of five ewes) of IUP cycles, and were significantly elevated during high activity compared with low activity periods. Venous PGE concentrations did not show a consistent relation with uterine activity parameters. MFA reduced plasma PG concentrations below other treatments and also abolished the cyclical pattern of activity. These data suggest that PG's may play a role in the cycles of myometrial activity resultant upon E2 treatment of the ewe.