Both ADP and thrombin regulate arteriolar thrombus stabilization and embolization, but are not involved in initial hemostasis as induced by micropuncture.

OBJECTIVE: Thrombosis and embolization are main causes of morbidity and mortality. Up to now, the relative importance of mediators involved is only partly known. It was the aim of this study to investigate the involvement of ADP and thrombin in subsequent phases of arteriolar hemostasis and thromboembolism in vivo. METHODS: Rabbit mesenteric arterioles were punctured, which induced bleeding, hemostasis, and subsequent thromboembolism. This reaction as well as the activation state of platelets involved ([Ca(2+)](i)), was monitored in real time by intravital (fluorescence) microscopy. RESULTS: Neither inhibition of thrombin formation or thrombin activity nor blockade of platelet ADP receptors P2Y(1) and P2Y(12) influenced the initial hemostatic reaction: in all experiments initial bleeding was stopped by a primary thrombus within 2-3 s. On the other hand, both thrombin inhibition and P2Y(1) blockade increased rebleeding frequency, which indicates reduced thrombus stability in the long term. Finally, inhibition of either thrombin or ADP (via both receptors) reduced aggregate formation during the embolization phase by at least 90%. While most participating platelets exhibited a transient increase in [Ca(2+)](i) during embolization, an increased percentage of platelets showed no calcium response at all during P2Y(1) blockade, which was accompanied by reduced platelet-platelet interaction strength. CONCLUSIONS: Whereas thrombin and ADP are not involved in the initial hemostatic reaction, both substances appear to be essential to prevent rebleedings in the long term. During subsequent embolization, ADP (via both receptors) and small amounts of thrombin are involved in platelet activation.

Tumor cell plasticity in Ewing sarcoma, an alternative circulatory system stimulated by hypoxia.

A striking feature of Ewing sarcoma is the presence of blood lakes lined by tumor cells. The significance of these structures, if any, is unknown. Here, we report that the extent of blood lakes correlates with poor clinical outcomes, whereas variables of angiogenesis do not. We also show that Ewing sarcoma cells form vessel-like tubes in vitro and express genes associated with vasculogenic mimicry. In tumor models, we show that there is blood flow through the blood lakes, suggesting that these structures in Ewing sarcoma contribute to the circulation. Furthermore, we present evidence that reduced oxygen tension may be instrumental in tube formation by plastic tumor cells. The abundant presence of these vasculogenic structures, in contrast to other tumor types, makes Ewing sarcoma the ideal model system to study these phenomena. The results suggest that optimal tumor treatment may require targeting of these structures in combination with prevention of angiogenesis.