α-Bulnesene, a novel PAF receptor antagonist isolated from Pogostemon cablin

α-Bulnesene is a sesquiterpenoid isolated from the water extract of Pogostemon cablin. It showed a potent and concentration-dependent inhibitory effect on platelet-activating factor (PAF) and arachidonic acid (AA) induced rabbit platelet aggregation. In a radioligand binding assay for the PAF receptor, α-bulnesene competitively inhibited [3H]PAF binding to the PAF receptor with an IC50 value of 17.62 ± 5.68 μM. α-Bulnesene also dose-dependently inhibited PAF-induced intracellular Ca2+ increase in fluo-3/AM-loaded platelets (IC50 values of 19.62 ± 1.32 μM). Furthermore, α-bulnesene inhibited AA-induced thromboxane B2 (TXB2) formation and prostaglandin E2 (PGE2) formation. These results indicate that the inhibitory effect of α-bulnesene on platelet aggregation was due to a dual activity; specifically the chemical blocked PAF-induced intracellular signal transduction and interfered with cyclooxygenase activity, which resulted in a decrease in thromboxane formation. This study is the first to demonstrate that α-bulnesene is a PAF receptor antagonist as well as an anti-platelet aggregation agent.