Spinach saponin-enriched fraction inhibits platelet aggregation in cAMP- and cGMP-dependent manner by decreasing TXA₂ production and blood coagulation

In this study, we investigated the effect of spinach saponin-enriched fraction (SSEF) on collagen (10 μg/ml)-stimulated platelet aggregation. SSEF inhibited collagen-induced platelet aggregation, and which was involved in the inhibition of thromboxane A₂ (TXA₂) production, an intracellular Ca²⁺-agonist as an aggregation-inducing autacoidal molecule. In addition, SSEF significantly increased the formation of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), intracellular Ca²⁺-antagonists as aggregation-inhibiting molecules, in collagen-stimulated platelets. These results suggest that SSEF might inhibit Ca²⁺-elevation and TXA₂ formation by increasing the production of Ca²⁺-antagonistic molecules cAMP and cGMP. These mean that SSEF is a potent inhibitor of collagen-stimulated platelet aggregation. On the other hand, prothrombin time (PT) and activated partial thromboplastin time (APTT) were potently prolonged by SSEF. These findings suggest that SSEF prolongs the internal time between the conversion of fibrinogen to fibrin. Accordingly, our data demonstrate that SSEF may be a crucial tool for a negative regulator during platelet activation and blood coagulation on thrombotic diseases.