Ex vivo and in vivo implications for PGE₂ modulation of cAMP platelet content. (a) Wild-type platelets collected on sodium citrate exposed to 8 × 10^–5 M PGE₂, then treated with 5 μM ADP did not display a full aggregation (intermediate trace). The PGE₂ concentration used was similar to that used for studies described in Figure 4b. In the same experimental conditions, Ep3^–/– platelets, which have higher intracellular cAMP levels, do not aggregate (upper trace). By contrast, platelets lacking the receptor for prostacyclin and thus containing lower cAMP levels aggregated maximally (lower trace). Similar results were obtained in two consecutive experiments. (b) Photomicrographs of venous thrombosis in vivo, induced by periadventitial application of arachidonic acid. Shown are the effect of vehicle (ethanol, EtOH), and the effect induced by arachidonic acid in EP3-deficient, wild-type, and IP-deficient mice. ×15. (c) Thrombotic scores in EP3-deficient, wild-type, IP-deficient, and IP- and TP-deficient mice. Thrombosis was scored as follows: 0, no apparent thrombus; 1, small and isolated thrombus; 2, mural thrombi; 3, partially occlusive thrombi; 4, occlusive thrombi. The scores were 0 in wild-type veins treated with vehicle (n = 6) or in Ip^–/– × Tp^–/– veins (n = 10). Errors bars = SEM. Data were analyzed using a Kruskal-Wallis test followed by Dunn’s tests. This showed significant differences between Ep3^–/– mice (0.60 ± 0.34; n = 14), and control (3.00 ± 0.32; n = 18) or Ip^–/– mice (3.75 ± 0.17; n = 16). ^AP < 0.01. ^BP < 0.001. (d) Proposed model for the role of cAMP in platelets exposed to prostaglandins. PGE₂ preferentially stimulates the EP3 receptor, resulting in a decrease in adenylate cyclase activity and opposing the stimulatory effect induced by the IP receptor. The resulting cAMP level in platelets affects both calcium mobilization and aggregation induced by these agents.