ACTIVATION OF RABBIT PLATELETS BY CA2+ INFLUX AND THROMBOXANE A(2) RELEASE IN AN EXTERNAL CA2+-DEPENDENT MANNER BY ZOOXANTHELLATOXIN-A, A NOVEL POLYOL

1 Zooxanthellatoxin-A (ZT-A), a novel polyhydroxylated long chain compound, isolated from a symbiotic marine alga Simbiodinium sp., caused aggregation in rabbit washed platelets in a concentration-dependent manner (1-4 mu M), accompanied by an increase in cytosolic Ca2+ concentration ([Ca2+](i)). 2 ZT-A did not cause platelet aggregation or increase [Ca2+](i) in a Ca2+-free solution, and Cd2+ (0.1- 1 mM), Co2+ (1-10 mM) and Mn2+ (1-10 mM) inhibited ZT-A-induced aggregation. SK&F96365 (1-100 mu M), a receptor operated Ca2+ channel antagonist, and mefenamic acid (0.1-10 mu M), a non-specific divalent cation channel antagonist, inhibited platelet aggregation and the increase in [Ca2+](i) induced by ZT-A. 3 Indomethacin (0.1-10 mu M), a cyclo-oxygenase inhibitor, and SQ-29548 (0.1-10 mu M), a thromboxane A(2) (TXA(2)) receptor antagonist, inhibited platelet aggregation and the increase in [Ca2+](i) induced by ZT-A. 4 Methysergide (0.01-1 mu M), a 5-HT2 receptor antagonist, inhibited ZT-A-induced platelet aggregation but did not affect the increase in [Ca2+](i) induced by ZT-A. 5 Tetrodotoxin (1 mu M), a Na+ channel blocker and chlorpheniramine (1 mu M), a H-1-histamine receptor antagonist, neither affected ZT-A-induced platelet aggregation nor the increase in [Ca2+](i) induced by ZT-A. 6 Genistein (1-100 mu M), a protein tyrosine kinase inhibitor, and staurosporine (0.01-1 mu M), a protein kinase C inhibitor, also inhibited ZT-A-induced platelet aggregation. 7 The present results suggest that ZT-A elicits Ca2+-influx from platelet plasma membranes. The resulting increase in [Ca2+](i) subsequently stimulates the secondary release of TXA(2) from platelets. Furthermore, the response to ZT-A may be associated with tyrosine phosphorylation.