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Ca(2+)-dependent aggregation of rabbit platelets induced by maitotoxin, a potent marine toxin, isolated from a
A Watanabe1, Y Ishida, H Honda
1Department of Pharmaceutical Molecular Biology, Tohoku University, Miyagi, Japan.
Abstract:
1. Administration of maitotoxin (MTX), a dinoflagellate toxin, caused aggregation of rabbit washed platelets. The cytosolic Ca2+ concentration ([Ca2+]i), measured by fura-2 fluorescence technique, was also increased by the presence of MTX. Rates of aggregation response and [Ca2+]i-increase were dependent on tested concentrations (3-100 ng ml-1) of the toxin. 2. The MTX-induced platelet aggregation and [Ca2+]i-increase were totally abolished in a Ca(2+)-free solution. The successive administration of Ca2+ in the presence of MTX elicited the aggregation and increase in [Ca2+]i. 3. Ba2+ was capable of substituting for Ca2+ in the MTX-induced platelet aggregation. In the presence of external Ca2+, transition metals, Co2+, Cd2+ and Ni2+, inhibited the aggregation response to MTX. 4. Organic calcium antagonists (verapamil and nifedipine) as well as a cyclo-oxygenase-inhibitor (aspirin) did not apparently inhibit the aggregation response to MTX, except for a high concentration (10(-5) M) of verapamil, while procaine (10 mM) reduced the rate of platelet aggregation. 5. MTX also elicited a release of ATP from platelets, which was abolished in the absence of external Ca2+. 6. In contrast, thrombin 0.5 unit ml-1 could elicit platelet shape change, [Ca2+]i-increase and ATP-release in the absence of external Ca2+. 7. These results suggest that the MTX-induced platelet activation is caused by an enhanced Ca(2+)-influx presumably through voltage-independent Ca2+ channels on the plasma membrane.
Insights
Maitotoxin (MTX) causes platelet aggregation and calcium influx, dependent on extracellular calcium. This activation mechanism differs from thrombin, suggesting MTX opens specific calcium channels.
Area of Science:
- Pharmacology
- Toxicology
- Cell Biology
Background:
- Maitotoxin (MTX) is a potent marine toxin known to affect cellular processes.
- Platelet activation plays a crucial role in hemostasis and thrombosis.
- Understanding the mechanisms of MTX-induced platelet activation is important for its toxicological assessment.
Purpose of the Study:
- To investigate the mechanism by which maitotoxin (MTX) induces platelet aggregation and intracellular calcium increase.
- To compare the effects of MTX on platelets with those of thrombin.
- To elucidate the role of extracellular calcium and specific ion channels in MTX-induced platelet activation.
Main Methods:
- Washed rabbit platelets were used to study aggregation and intracellular calcium concentration ([Ca2+]i) changes.
- Fura-2 fluorescence was employed to measure [Ca2+]i.
- Experiments were conducted in Ca(2+)-free solutions and with various ions (Ba2+, Co2+, Cd2+, Ni2+) and inhibitors (verapamil, nifedipine, aspirin, procaine).
- ATP release was also measured.
Main Results:
- MTX induced concentration-dependent platelet aggregation and increased [Ca2+]i.
- These effects were abolished in Ca(2+)-free solution but restored by Ca2+ addition.
- Barium ions (Ba2+) could substitute for calcium, while transition metals inhibited the response.
- Organic calcium antagonists and aspirin showed minimal inhibition, except for high-concentration verapamil.
- MTX-induced ATP release was calcium-dependent.
- Thrombin induced platelet activation independently of extracellular calcium.
Conclusions:
- MTX-induced platelet activation relies on extracellular calcium influx.
- The mechanism appears to involve the opening of voltage-independent calcium channels.
- MTX-induced platelet activation differs significantly from thrombin-mediated activation.