Objective: Thrombin generated during the active clotting of blood appears to be a potent uterotonic agonist; however, the mechanism underlying this effect on uterine smooth muscle is not well understood. We performed studies to confirm the uterotonic effects of thrombin and to determine whether prostaglandin production plays a role during the uterotonic effects of thrombin or clotting blood.
Methods: Uterine contraction studies were performed using adult nonpregnant and near-term pregnant rats. The in vitro isometric contraction studies used uterine strips pretreated with indomethacin or vehicle (ethanol), which were then stimulated with thrombin. For the in vivo contraction studies, rats were pretreated with intraperitoneal injections of indomethacin or vehicle (ethanol) then stimulated by intraluminal injection of fresh rat blood or thrombin into the uterus. The contraction data were acquired using isometric force transducers, were computer digitized, normalized for spontaneous activity, and statistically analyzed. Prostaglandin (PG) F2alpha was measured using an enzyme-linked immunoassay.
Results: The in vitro contraction studies demonstrated that both thrombin and actively clotting blood produce a significant increase in the frequency and intensity of uterine contractions. Thrombin stimulation was associated with a 54% increase in PGF2alpha concentration in vitro; indomethacin (1 microM) pretreatment completely inhibited that increase in PGF2alpha production. Despite the suppression of PGF2alpha production, pretreatment with indomethacin had no inhibitory effect on thrombin-stimulated contractile activity. In vivo contraction studies further confirmed that indomethacin (2 mg/kg) pretreatment had no effect on blood- or thrombin-stimulated contractile activity.
Conclusions: We confirmed that thrombin and thrombin produced by actively clotting blood had a robust uterotonic effect in the rat and that prostaglandin production did not play a significant role in thrombin-stimulated contractions.
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