Inhibitory effects of total saponin from Korean red ginseng on [ca²⁺]_i mobilization through phosphorylation of cyclic adenosine monophosphate-dependent protein kinase catalytic subunit and inositol 1,4,5-trisphosphate receptor type I in human platelets

Background: Intracellular ca²⁺([ca²⁺]_i) is a platelet aggregation-inducing molecule. Therefore, understanding the inhibitory mechanism of [ca²⁺]_i mobilization is very important to evaluate the antiplatelet effect of a substance. This study was carried out to understand the ca²⁺-antagonistic effect of total saponin from Korean Red Ginseng (KRG-TS). Methods: We investigated the ca²⁺-antagonistic effect of KRG-TS on cyclic nucleotides-associated phosphorylation of inositol 1,4,5-trisphosphate receptor type I (IP₃RI) and cyclic adenosine monophosphate (cAMP)-dependent protein kinase (PKA) in thrombin (0.05 U/mL)-stimulated human platelet aggregation. Results: The inhibition of [ca²⁺]_i mobilization by KRG-TS was increased by a PKA inhibitor (Rp-8-BrcAMPS), which was more stronger than the inhibition by a cyclic guanosine monophosphate (cGMP)dependent protein kinase (PKG) inhibitor (Rp-8-Br-cGMPS). In addition, Rp-8-Br-cAMPS inhibited phosphorylation of PKA catalytic subunit (PKAc) (Thr¹⁹⁷) by KRG-TS. The phosphorylation of IP₃RI (Ser¹⁷⁵⁶) by KRG-TS was very strongly inhibited by Rp-8-Br-cAMPS compared with that by Rp-8-BrcGMPS. These results suggest that the inhibitory effect of [ca²⁺]_i mobilization by KRG-TS is more strongly dependent on a cAMP/PKA pathway than a cGMP/PKG pathway. KRG-TS also inhibited the release of adenosine triphosphate and serotonin. In addition, only G-Rg3 of protopanaxadiol in KRG-TS inhibited thrombin-induced platelet aggregation. Conclusion: These results strongly indicate that KRG-TS is a potent beneficial compound that inhibits [ca²⁺]_i mobilization in thrombineplatelet interactions, which may result in the prevention of platelet aggregation-mediated thrombotic disease.