Activation of rabbit platelets by Ca²⁺ influx and thromboxane A₂ release in an external Ca²⁺‐dependent manner by zooxanthellatoxin‐A, a novel polyol

Abstract

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 μM), accompanied by an increase in cytosolic Ca2+ concentration ([Ca2+]i). 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 μM), a receptor operated Ca2+ channel antagonist, and mefenamic acid (0.1–10 μM), a non‐specific divalent cation channel antagonist, inhibited platelet aggregation and the increase in [Ca2+], induced by ZT‐A. Indomethacin (0.1–10 μM), a cyclo‐oxygenase inhibitor, and SQ‐29548 (0.1–10 μM), a thromboxane A2 (TXA2) receptor antagonist, inhibited platelet aggregation and the increase in [Ca2+]i induced by ZT‐A. Methysergide (0.01‐1 μM), a 5‐HT2 receptor antagonist, inhibited ZT‐A‐induced platelet aggregation but did not affect the increase in [Ca2+]i induced by ZT‐A. Tetrodotoxin (1 μM), a Na+ channel blocker and chlorpheniramine (1 μM), a H1‐histamine receptor antagonist, neither affected ZT‐A‐induced platelet aggregation nor the increase in [Ca2+]i induced by ZT‐A. Genistein (1–100 μM), a protein tyrosine kinase inhibitor, and staurosporine (0.01‐1 μM), a protein kinase C inhibitor, also inhibited ZT‐A‐induced platelet aggregation. 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 TXA2 from platelets. Furthermore, the response to ZT‐A may be associated with tyrosine phosphorylation.

Bioactive secondary metabolites from symbiotic marine dinoflagellates: symbiodinolide and durinskiols

“…Vasoconstrictive activity was only seen with the macrolactone congeners ZTs and ZAD‐Cs, which indicated that the huge macrolactone structures were important for their activity. Furthermore, ZT‐A ( 23 ) caused aggregation in rabbit washed platelets, accompanied by an increase in the cytosolic Ca 2+ concentration 49. Thus, this potent voltage‐dependent Ca 2+ channel‐opening activity may be a common feature of symbiodinolide ( 1 ) and ZTs, and may be intimately involved in such constrictive activity.…”

Platelet abnormalities in diabetes mellitus

“…This leads to a cascade of reactions involving the release of calcium, the activation of calmodulin, protein kinase C (PKC) and phospholipase A 2 , ultimately leading to TXA 2 biosynthesis [58,59], the release of agranules [33] and surface glycoprotein expression.…”

Alterations in platelet Ca 2+ signalling in diabetic patients is due to increased formation of superoxide anions and reduced nitric oxide production

“…In addition, the depletion of intracellular Ca 2+ stores initiates activation of a so called capacitative Ca 2+ entry [7,8]. Both, intracellular Ca 2+ release and capacitative Ca 2+ entry increase free intracellular Ca 2+ concentration [Ca 2+ ]; and induce platelet aggregation by stimulation of the thromboxane A 2 biosynthesis [9]. In platelets from diabetic patients, increased intracellular Ca 2+ response was reported [10].…”