Our objectives were to test whether or not prostaglandin E2 (PGE2) relaxes equine isthmic circular smooth muscle in vitro, and to determine the effects of surgery, time postexcision, and source of tissue on isthmic responsiveness to PGE2. Oviducts were surgically collected after general anesthesia from four research herd mares on Day 5 postovulation, after stunning and exsanguination from four research herd mares on Day 5, and from seven slaughterhouse mares during diestrus. Circular smooth muscle contractions were measured in vitro with an isometric force transducer. Tissues were precontracted with 2 μM histamine and then were treated with PGE2 (0, 0.1, 1, 10, 100, or 1000 nM). Histamine treatment significantly (p < 0.001) increased tissue tension (500.0 ± 0.0 to 818.9 ± 47.9 mg) and contractile frequency (13.3 ± 0.4 to 29.8 ± 1.6 contractions/min), and decreased contractile amplitude (69.7 ± 11.9 to 23.7 ± 4.2 mg). PGE2 treatment significantly (p < 0.001) decreased tissue tension and contractile frequency and significantly (p < 0.001) increased contractile amplitude of histamine-precontracted tissue in a dose-dependent manner. Tissue obtained from mares anesthetized on Day 5 was significantly (p < 0.05) less responsive to PGE2 than tissue obtained from mares killed on Day 5. Responsiveness to PGE2 of tissue from mares killed on Day 5 did not decrease significantly after 12 h of storage at 4°C. These results support the hypothesis that PGE2 relaxes isthmic circular smooth muscle, and indicate that anesthetic agents reduce isthmic responsiveness to PGE2.
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Weber et al. (1995) studied this question.
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