Elucidation of the mechanism of excitatory action of marine natural products on platelets and muscle cells

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In our screening program for natural products from marine organisms, useful not only for basic researches but also for the treatment of a variety of diseases, we have studies the mechanism of action of these compounds. The new monogalactopyranosylacylglyceroI was isolated from the cultured marine alga and caused an inhibition of platelet aggregation. Theonezolide-A (TZ-A), a novel polyketide macrolide caused a marked platelet shape change. Our pharmacological studies on this compound suggest that TZ-A is a useful chemical tool for clarifying the signal transduction mechanism. Zooxanthellatoxin-A (ZT-A) from a symbiotic dinoflagellate caused Ca^<2+>-or thromboxane A_2-dependent and genestin-sensitive aggregation of platelets. It is suggested that ZT-A primarily activate a protein tyrosine kinase and that the activated protein tyrosine kinase subsequently stimulates the activation of mitogen-activated protein kinase probably via PLC-gamma2 and PKC.On the other hand, ptilomycalin A has been showm to interacts competitively with ATP at the ATP binding site of Na^+, K^+-ATPase or Ca^<2+>-ATPaase, resulting in inhibition of both the enzymes. Recently, we have found that 9-methyl-7-bromo-eudistomin D,the most powerful caffeine like substances not but caffeine failed to induce a contraction of skinned smooth muscles. These data suggest that intracellular Ca^<2+> stores consist of two functional distinct types. These studies lead to the conclusion that these natural products are useful not only as pharmacological tools but also as leading compounds for new drug development.