Ca²⁺‐dependent aggregation of rabbit platelets induced by maitotoxin, a potent marine toxin, isolated from a dinoflagellate

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"Ji-increase were dependent on tested concentrations (3-100ng ml-') of the toxin.2 The MTX-induced platelet aggregation and [Ca2+]i-increase were totally abolished in a Ca2+-free solution. The successive administration of Ca2" in the presence of MTX elicited the aggregation and increase in [Ca2+]i.3 Ba" 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-'M) of verapamil, while procaine (10mM) 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 unitml-' could elicit platelet shape change, [Ca2+]i-increase and ATPrelease in the absence of external Ca2 . 7 These results suggest that the MTX-induced platelet activation is caused by an enhanced Ca2+-influx presumably through voltage-independent Ca2+ channels on the plasma membrane.

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

“…ZT-Ainduced platelet aggregation is similar to MTX-induced platelet aggregation, but ZT-A has different characteristics from MTX. MTX-induced platelet aggregation was inhibited by the presence of Co2" (0.3 mM) or Cd2`(0.3 mM) (Watanabe et al, 1993). Although ZT-A-induced platelet aggregation was inhibited by Cd2", Co2+ and Mn21, effective concentration of Co2+ was different from the effective concentration on MTXinduced aggregation ( Figure 6).…”

The unique contribution of ion channels to platelet and megakaryocyte function

“…This results from a modification of the gating mechanism of the Na + /K + ‐ATPase, converting the exchanger into a non‐selective cation ion channel [136,137]. Another marine toxin maitotoxin induces an extremely large Ca 2+ influx in rabbit platelets, which results in shape change, phosphoinositide hydrolysis and aggregation [138,139]. Studies in other cell types suggest that this action of maitotoxin results from conversion of the plasma membrane Ca 2+ ‐ATPase into a Ca 2+ ‐permeable non‐selective cation channel [140,141].…”

“…Platelets are hyperactive in cystic fibrosis, but the role of CFTR in the enhanced responses remains unclear as the only suggestedrolesforthisproteininplateletsareNOproductionand platelet-leukocyte interactions [133,134]. Palytoxin is a potent marine toxin that has been reported to induce a non-selective cation channel inmurinemegakaryocytes [135].This resultsfrom amodificationofthegatingmechanismoftheNa + /K + -ATPase, converting the exchanger into a non-selective cation ion channel [136,137].Anothermarinetoxinmaitotoxininducesanextremely large Ca 2+ influx in rabbit platelets, which results in shape change, phosphoinositide hydrolysis and aggregation [138,139]. Studies in other cell types suggest that this action of maitotoxin results from conversion of the plasma membrane Ca 2+ -ATPase into a Ca 2+ -permeable non-selective cation channel [140,141].…”

V‐1, a catecholamine biosynthesis regulatory protein, positively controls catecholamine secretion in PC12D cells

“…Cells were plated at a density of 5×10 5 cells per 35‐mm dish and cultured overnight, and subjected to the measurements of [Ca 2+ ] i according to the previously reported method [9] except for using fura‐PE3/AM. For loading fura‐PE3, cells were washed with Hanks’ HEPES buffer and incubated in 2 ml of Hanks’ HEPES buffer containing 10 μM fura‐PE3/AM for 1 h at 37°C.…”