Schwemmer M, Sommer O, Bassenge E
Institute of Applied Physiology, Albert-Ludwigs University, Freiburg, Germany.
Cardiovasc Drugs Ther. 2001 Jul;15(4):301-7. doi: 10.1023/a:1012750430056.
Enhanced platelet activity and platelet endothelial interaction are hallmarks of different vascular and metabolic diseases with subsequent thrombus formation. In atherosclerosis, coronary artery disease, congestive heart failure, nitrate tolerance, chronic inflammation, or diabetic states, platelet activation may in part be due to a stimulation of the renin-angiotensin-aldosteron system, which also contributes to enhanced oxidant stress in these conditions.
We examined the putative role of the angiotensin receptor (AT1) and of phospholipase A2 (PLA2) in mediating platelet activation under defined in vitro conditions using the AT1 receptor antagonists losartan, EXP 3174, candesartan, and the PLA2 inhibitor arachidonyltrifluoromethyl ketone (AACOCF3), respectively.
In washed human or canine platelet suspensions, losartan (10(-4)-10(-6) mol/L) dose-dependently suppressed thrombin-induced calcium transients as well as thromboxane (TxA2) release. In both species, aggregation of washed platelets in response to thrombin or ADP was substantially diminished by different doses of losartan. This inhibition of platelet aggregation was even maintained in ADP-stimulated platelet-rich plasma. While the PLA2 inhibitor AACOCF3 effectively inhibited thrombin-induced TxA2 release from washed human or canine platelets (similar to the effects observed with losartan), the AT1 agonist angiotensin II elicited platelet TxA2 release only at high supra-physiological doses (e.g., at 10(-4) mol/L). The AT1 specific antagonist candesartan did not diminish stimulated platelet aggregation, TxA2 formation, or calcium transients. By contrast, the active losartan metabolite EXP 3174 dose-dependently inhibited stimulated platelet calcium transients as well as TxA2 release at 1-100 micromol/L.
Losartan significantly counteracts ex vivo platelet activation, probably via the blockade of TxA2 receptor-dependent signaling (e.g. implying activation of phospholipase A2) rather than acting at the AT1 receptor itself. This implies that the TxA2 signaling pathway plays a significant role during platelet activation, which may be successfully antagonized in vivo under different pathological states with enhanced thrombus formation or platelet-endothelium interactions.
血小板活性增强以及血小板与内皮细胞的相互作用是不同血管和代谢性疾病的特征，随后会形成血栓。在动脉粥样硬化、冠状动脉疾病、充血性心力衰竭、硝酸盐耐受性、慢性炎症或糖尿病状态下，血小板活化部分可能归因于肾素 - 血管紧张素 - 醛固酮系统的刺激，该系统在这些情况下也会导致氧化应激增强。
我们分别使用血管紧张素受体1（AT1）拮抗剂氯沙坦、EXP 3174、坎地沙坦以及磷脂酶A2（PLA2）抑制剂花生四烯酰三氟甲基酮（AACOCF3），研究了在特定体外条件下，AT1和PLA2在介导血小板活化中的假定作用。
在洗涤后的人或犬血小板悬液中，氯沙坦（10⁻⁴ - 10⁻⁶ mol/L）剂量依赖性地抑制凝血酶诱导的钙瞬变以及血栓素（TxA2）释放。在这两个物种中，不同剂量的氯沙坦均显著减少了洗涤后血小板对凝血酶或二磷酸腺苷（ADP）的聚集反应。在ADP刺激的富含血小板血浆中，这种对血小板聚集的抑制作用仍然存在。虽然PLA2抑制剂AACOCF3有效抑制了凝血酶诱导的洗涤后人或犬血小板TxA2释放（类似于氯沙坦观察到的效果），但AT1激动剂血管紧张素II仅在高于生理剂量时（例如10⁻⁴ mol/L）才引起血小板TxA2释放。AT1特异性拮抗剂坎地沙坦并未减少刺激后的血小板聚集、TxA2形成或钙瞬变。相比之下，活性氯沙坦代谢物EXP 3174在1 - 100 μmol/L剂量依赖性地抑制刺激后的血小板钙瞬变以及TxA2释放。
氯沙坦可能通过阻断TxA2受体依赖性信号传导（例如意味着磷脂酶A2的激活）而非作用于AT1受体本身，显著对抗体外血小板活化。这意味着TxA2信号通路在血小板活化过程中起重要作用，在体内不同病理状态下，当血栓形成或血小板 - 内皮细胞相互作用增强时，可以成功拮抗该通路。
