Impact of angiotensin-converting enzyme inhibition on platelet Tissue Factor expression in stroke-prone rats

ABSTRACT Objective: Hypertension is a well-known risk factor for thrombotic events such as myocardial infarction and stroke. Platelets express tissue factor (TF), the key activator of blood coagulation and thrombus formation. The number of TF-positive platelets increases in pathological conditions characterized by thrombotic complications but whether this occurs in hypertension is unknown. Here we investigated whether: 1) platelet TF expression is increased in a hypertensive status through a mechanism acting on megakaryocytes; 2) the phenomenon could be modulated by anti-hypertensive drug as captopril; 3) angiotensin influences platelet TF expression. Methods: SHRSP rats received standard diet (StD) or a high-salt permissive diet (JpD). After 3 weeks, JpD animals were randomized to receive captopril or vehicle. Normotensive Wistar Kyoto (WKY) rats were used as controls. Cell-associated TF expression and activity were analyzed by flow cytometry and Calibrated Automated Thrombogram, respectively. Results: Hypertensive StD-SHRSP showed an increased number of TF-positive platelet compared to normotensive WKY. After JpD administration, SHRSP developed severe hypertension and renal damage; the number of TF-positive megakaryocytes significantly increased compared to StD-SHRSP resulting in a higher number of TF-positive platelets with a faster kinetic of thrombin generation. These effects were reverted by captopril. Ex vivo stimulation of platelets, isolated from normotensive WKY and from healthy subjects, with angiotensin induced a concentration-dependent increase of surface-associated TF expression. Conclusion: This study shows for the first time that in hypertension the number of TF-positive megakaryocytes increases thus releasing in the circulation more platelets carrying a functionally active TF. Angiotensin stimulates platelets to express TF.

摘要
研究目的：高血压是心肌梗死、脑卒中这类血栓事件的明确危险因素。血小板可表达组织因子（Tissue Factor, TF），后者是凝血激活与血栓形成的关键启动因子。在以血栓并发症为特征的病理状态下，TF阳性血小板的数量会升高，但高血压状态下是否也存在这一现象尚不明确。本研究旨在探讨以下三个问题：1）高血压状态下是否通过作用于巨核细胞的机制，使血小板TF表达水平升高；2）该现象是否可被卡托普利等抗高血压药物调控；3）血管紧张素是否会影响血小板TF表达。
方法：本研究将自发性高血压脑卒中易感性（SHRSP）大鼠分为标准饮食（StD）组与高盐诱导饮食（JpD）组。干预3周后，将JpD组大鼠随机分为卡托普利给药组与赋形剂对照组。以正常血压Wistar京都（WKY）大鼠作为对照。分别采用流式细胞术与校准自动化血栓图（Calibrated Automated Thrombogram）检测细胞相关TF表达水平与凝血活性。
结果：与正常血压WKY大鼠相比，高血压StD组SHRSP大鼠的TF阳性血小板数量显著升高。给予JpD饮食后，SHRSP大鼠出现重度高血压与肾损伤，其TF阳性巨核细胞数量较StD组SHRSP大鼠显著升高，进而导致循环中TF阳性血小板数量增加，且凝血酶生成动力学更快。卡托普利可逆转上述变化。对从正常血压WKY大鼠及健康受试者体内分离得到的血小板进行体外血管紧张素刺激实验，结果显示血小板表面TF表达水平呈浓度依赖性升高。
结论：本研究首次证实，高血压状态下TF阳性巨核细胞数量升高，进而释放更多携带功能性活性TF的血小板进入循环。血管紧张素可刺激血小板表达TF。