Effect of platelet-generated thromboxane on contractions of the pregnant rat uterus

Recent evidence indicates that thromboxanes are produced by uterine tissues. The effect of platelet-generated thromboxane A2 (TXA2) was therefore studied for its effect on uterine contractions. An incubation tube consisting of washed platelets (WP) was incubated with sodium arachidonate (AA). An aliquot was transferred to an aggregation assay cuvette containing platelet rich plasma (PRP) to test for TXA2 activity. Immediately thereafter, a second aliquot from the incubation tube was transferred to an in vitro preparation of pregnant rat uterus and contractions were monitored. Both assay systems (i.e., aggregation and uterine contraction) contained indomethacin to prevent endogenous prostanoid production from platelets and uteri, respectively. Incubation media from human, rat or rabbit WP incubated with AA for 1 min elicited both platelet aggregation and uterine contractions when transferred to these assay systems. Indomethacin or imidazole added to the incubation tube blocked both responses. Delaying transfer of media until 10 min after addition of AA did not elicit either aggregation or contractile response, consistent with the short production interval and half-life of TXA2. Addition of an antiaggregatory dose of prostacyclin (PGI2) to the incubation tube did not affect TXA2 production or the ability of transferred media to promote uterine contractions. These experiments provide evidence that TXA2 is capable of promoting contractions in the pregnant rat uterus and may be important in the labor mechanism.