Inhibition of platelet aggregation by rat trophoblasts.

OBJECTIVE To determine the effect of rat trophoblast cell suspensions on adenosine diphosphate (ADP)-induced platelet aggregation. MATERIALS AND METHODS The trophoblasts were isolated from ectoplacental cones (a preplacental tissue very rich in trophoblasts) developed in rat embryos on day 12 of pregnancy (normal period of gestation: 22-23 days). Platelet-rich plasma (PRP) was obtained from adult male rats. The trophoblasts were preincubated (37 degrees C, 30 min), suspended in the medium, and then re-incubated with PRP for 3-5 min. RESULTS 5 x 10(4) and 7 x 10(4) trophoblasts inhibited ADP-induced platelet aggregation by 10 and 18%, respectively. When the trophoblast cell concentration was increased to 1-2 x 10(5) cells, a proaggregatory phenomenon was observed, even in the absence of ADP. However, there was no inhibition of aggregation or promotion of aggregation when fixed trophoblasts or live endometrial stromal cells were incubated with PRP. CONCLUSION The results indicate that the aggregation inhibition response was cell specific and concentration dependent. A 68-kD protein was detected in the medium when it was conditioned with 5-7 x 10(4) but not with 1-2 x 10(5) trophoblasts. However, the inhibitory or stimulatory effect does not seem to be dependent on the presence of this protein.