Ca(2+)-dependent aggregation of rabbit platelets induced by maitotoxin, a potent marine toxin, isolated from a dinoflagellate

The unique contribution of ion channels to platelet and megakaryocyte function

Ion channels are transmembrane proteins that play ubiquitous roles in cellular homeostasis and activation. In addition to their recognized role in the regulation of ionic permeability and thus membrane potential, some channel proteins possess intrinsic kinase activity, directly interact with integrins or are permeable to molecules up to ≈1000 Da. The small size and anuclear nature of the platelet has often hindered progress in understanding the role of specific ion channels in hemostasis, thrombosis and other platelet-dependent events. However, with the aid of transgenic mice and 'surrogate' patch clamp recordings from primary megakaryocytes, important unique contributions to platelet function have been identified for several classes of ion channel. Examples include ATP-gated P2X1 channels, Orai1 store-operated Ca2+ channels, voltage-gated Kv1.3 channels, AMPA and kainate glutamate receptors and connexin gap junction channels. Furthermore, evidence exists that some ion channels, such as NMDA glutamate receptors, contribute to megakaryocyte development. This review examines the evidence for expression of a range of ion channels in the platelet and its progenitor cell, and highlights the distinct roles that these proteins may play in health and disease.

Protective effects of total flavones of rhododendra against global cerebral ischemia reperfusion injury

The purpose of this research was to demonstrate the protective effects and possible mechanisms of total flavones of rhododendra (TFR) against global cerebral ischemia reperfusion injury in rats. Global cerebral ischemia/reperfusion injury was caused by four vessel occlusion (bilateral vertebral arteries and bilateral carotid arteries, 4-VO). The electroencephalographic (EEG) changes were recorded. The EEG, brain water content, levels of malondialdehyde (MDA) and lactate dehydrogenase (LDH) activity in plasma, aggregation of platelets induced by ADP, and the resting and CaCl(2)-induced increase of free intracellular calcium concentration ([Ca(2+)](i)), were also evaluated. TFR dramatically elevated EEG amplitude, reduced the brain water content and the resting cytoplasmic free calcium concentration, inhibited the increase of [Ca(2+)](i) induced by CaCl(2) and had an inhibitory effect on platelet aggregation. The LDH activity and the MDA content in plasma were also decreased. These results indicate that TFR has protective effects against cerebral injury in rats, which might be associated with its antioxidant properties, antiplatelet effects and possible inhibition of Cal(2+) influx to reduce [Ca(2+)](i).

Ca2+ channel activating action of maitotoxin in cultured brainstem neurons

The actions of maitotoxin were studied using cultured brainstem cells and adrenal chromaffin cells. Maitotoxin induced a profound increase in the Ca2+ influx into cultured brainstem cells after a brief lag period. The maitotoxin-induced Ca2+ influx was suppressed by various voltage-dependent Ca2+ channel blockers such as Co2+, Mn2+, verapamil and diltiazem. Maitotoxin-catecholamine release in brainstem cells initiated to increase after a lag period of about 1 min and the increase continued even at 4 min after treatment, while in the adrenal chromaffin cells the release started after an about 1-min lag period to attain a maximum within first 2-min and gradually decrease thereafter. These results suggest that maitotoxin acts on Ca2+ channels to increase the Ca2+ influx, accompanied by enhancement of catecholamine release in the brainstem cells with a different temporal profile from that in the adrenal chromaffin cells.

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

Stably transfected PC12D cell lines overexpressing a catecholamine biosynthesis regulatory protein, V-1, were used to examine the functional role of V-1 in catecholamine secretion. High K(+)-induced dopamine secretion in V-1 overexpressing clones was shown to be markedly potentiated compared with control clones carried with a vector alone. As assayed intracellular calcium concentration ([Ca(2+)](i)) using fura-PE3, V-1 overexpression was observed to enhance high K(+)-elicited [Ca(2+)](i) elevation. Electron microscopic analysis revealed an increase in dense-cored vesicle formation by V-1 overexpression. These results suggest that the enhancement of high K(+)-induced dopamine secretion by V-1 overexpression results from the potentiation of high K(+)-induced [Ca(2+)](i) elevation and the increase in the number of dense-cored vesicles.

The vaso-contractile action of zooxanthellatoxin-B from a marine dinoflagellate is mediated via Ca2+influx in the rabbit aorta

We previously showed that zooxanthellatoxin-B, isolated from dinoflagellate, caused a sustained contraction of the aorta in an external Ca2+-dependent manner. To clarify the role of Ca2+in this action, we examined the effects of zooxanthellatoxin-B as well as a depolarizing stimulus (60 mM KCl), using the simultaneous recording for cytosolic Ca2+level (fura-2) and developed tension in the rabbit aorta. KCl (60 mM) elicited a rapid cytosolic Ca2+elevation followed by a pronounced contraction, and time required for half-maximum contraction was 2 min. Zooxanthellatoxin-B caused an increase in cytosolic Ca2+followed by a gradual contraction, with a time for half-maximum contraction of 5–10 min in a concentration-dependent manner. We found a strong correlation between Ca2+elevation and the contraction in zooxanthellatoxin-B action. In a Ca2+-free solution, zooxanthellatoxin-B caused neither the contraction nor the increase in cytosolic Ca2+. Furthermore, both pre- and post-treatment with verapamil, a voltage-operated Ca2+-channel blocker, partially suppressed both an increase in cytosolic Ca2+and the contraction by zooxanthellatoxin-B. Zooxanthellatoxin-B-induced contraction was also inhibited by other voltage-operated Ca2+-channel blockers: nifedipine or diltiazem. These results suggest that zooxanthellatoxin-B-elicited contraction is caused by a Ca2+influx into the smooth muscle cells, partially via voltage-operated Ca2+channels.Key words: zooxanthellatoxin, Ca2+imaging, rabbit aorta, contraction, voltage-operated Ca2+-channels.

Maitotoxin-induced calcium entry in human lymphocytes: modulation by yessotoxin, Ca(2+) channel blockers and kinases

We have studied the effect of the ciguatera-related toxin maitotoxin (MTX) on the cytosolic free calcium concentration ([Ca(2+)]i) of human peripheral blood lymphocytes loaded with the fluorescent probe Fura2 and the regulation of MTX action by different drugs known to interfere in cellular Ca(2+) signalling mechanisms and by the marine phycotoxin yessotoxin (YTX). MTX produced a concentration-dependent elevation of [Ca(2+)]i in a Ca(2+)-containing medium. This effect was stimulated by pretreatment with YTX 1 microM and NiCl(2) 15 microM. The voltage-independent Ca(2+) channel antagonist 1-[beta-[3-(4-methoxyphenyl)propoxyl]-4-methoxyphenyl]-1H-imidazole hydrochloride (SKF96365) blocked the MTX-induced [Ca(2+)]i elevation, while the L-type channel blocker nifedipine had no effect. Pretreatment with NiCl(2) or nifedipine did not modify YTX-induced potentiation of MTX effect, and SKF96365-induced inhibition was reduced in the presence of YTX, which suggest different pathways to act on [Ca(2+)]i. Preincubation with N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide.2HCl (H-89) or genistein (10 microM) also had no effect on the MTX-induced [Ca(2+)]i increment. In contrast, the PKC inhibitor bisindolilmaleimide I (GF109203X 1 microM) potentiated the MTX effect, whereas phosphatidylinositol (PI) 3-kinase inhibition with wortmannin (10 nM) reduced the MTX-elicited Ca(2+) entry. In summary, MTX produced Ca(2+) influx into human lymphocytes through a SKF96365-sensitive, nifedipine-insensitive pathway. The MTX-induced [Ca(2+)]i elevation was stimulated by the marine toxin YTX through a mechanism insensitive to SKF96365, nifedipine or NiCl(2). It was also stimulated by the divalent cation Ni(2+) and PKC inhibition and was partially inhibited by PI 3-kinase inhibition.

Comparison of maitotoxin with thromboxane A2 in rabbit platelet activation

Maitotoxin (MTX), a Ca2+ channel-activating marine toxin, caused shape change followed by aggregation in rabbit platelets, like U46619, a thromboxane A2 analogue. Although both drugs failed to cause aggregation in the absence of external Ca2+, U46619, but not maitotoxin, elicited shape change in the absence of external Ca2+. The observations of platelets with a scanning electron microscope showed that both drugs caused contraction of platelets and extension of pseudopodia (shape change) followed by aggregation with a clot in the presence of Ca2+. It is noteworthy that long term exposure to MTX caused the lysis of platelets in the presence of Ca2+. While U46619 transiently increased the internal Ca2+ concentration ([Ca2+]i), maitotoxin slowly but irreversibly increased [Ca2+]i in an external Ca2(+)-dependent manner. MTX-induced phosphoinositide hydrolysis was totally dependent on the presence of external Ca2+, but U46619-induced phosphoinositide hydrolysis was still observed in the absence of external Ca2+. MTX-induced phosphoinositide hydrolysis was partly inhibited by SK&F96365, a voltage-independent Ca2+ channel antagonist, or by genistein, a tyrosine kinase inhibitor. MTX caused phosphorylation of tyrosine residues of several proteins, like U46619. Thus, MTX is similar to U46619 in functions of Ca2+ mobilization, phosphoinositide hydrolysis and tyrosine phosphorylation, but MTX-induced actions are strictly dependent on the presence of external Ca2+.

Maitotoxin activates a nonselective cation channel and a P2Z/P2X(7)-like cytolytic pore in human skin fibroblasts

Maitotoxin (MTX), a potent cytolytic agent, activates Ca(2+) entry via nonselective cation channels in virtually all types of cells. The identity of the channels involved and the biochemical events leading to cell lysis remain unknown. In the present study, the effect of MTX on plasmalemmal permeability of human skin fibroblasts was examined. MTX produced a time- and concentration-dependent increase in cytosolic free Ca(2+) concentration that depended on extracellular Ca(2+) and was relatively insensitive to blockade by extracellular lanthanides. MTX also produced a time- and concentration-dependent increase in plasmalemma permeability to larger molecules as indicated by 1) uptake of ethidium (314 Da), 2) uptake of YO-PRO-1 (375 Da), 3) release of intracellular fura 2 (636 Da), 4) uptake of POPO-3 (715 Da), and, ultimately, 5) release of lactate dehydrogenase (relative molecular weight of 140,000). At the single cell level, uptake of YO-PRO-1 correlated in time with the appearance of large MTX-induced membrane currents carried by the organic cation, N-methyl-D-glucamine (167 Da). Thus MTX initially activates Ca(2+)-permeable cation channels and later induces the formation of large pores. These effects of MTX on plasmalemmal permeability are similar to those seen on activation of P2Z/P2X(7) receptors in a variety of cell types, raising the intriguing possibility that MTX and P2Z/P2X(7) receptor stimulation activate a common cytolytic pore.

Vasoconstriction induced by zooxanthellatoxin-B, a polyoxygenated long-chain product from a marine alga

We found that zooxanthellatoxin-B from a symbiotic marine alga, Symbiodinium sp., caused a concentration-dependent contraction of the rabbit isolated aorta at concentrations of 10(-7)-10(-5) M. Verapamil (10(-6) M) and mefenamic acid (10(-5) M) significantly attenuated the contractile response to zooxanthellatoxin-B at lower concentrations (10(-7)-10(-6) M) but not at higher concentrations (3 X 10(-6)-10(-5) M). The response to zooxanthellatoxin-B was partly inhibited by phentolamine (10(-6) M), whereas it was potentiated by ouabain (10(-5) M). Tetrodotoxin (10(-6) M), methysergide (10(-6) M), chlorpheniramine (10(-6) M) or indomethacin (3 X 10(-6) M), however, did not affect it. The zooxanthellatoxin-B-induced contraction was abolished by incubation in Ca2+-free solution. The contractile response increased in a concentration-dependent fashion with Ca2+ (0.03 and 10 mM) or Sr2 + (0.10 and 10 mM). After treatment with verapamil (10(-6) or 5 X 10(-6) M), the concentration-contractile response curves for Ca2+ and Sr2+ in the presence of zooxanthellatoxin-B were shifted to the right in parallel. MgCl2 (10 mM) shifted the concentration-response curve for Ca2+ more markedly than did verapamil. Zooxanthellatoxin-B increased tissue Na+ and reduced tissue K+ contents in the aorta, suggesting that zooxanthellatoxin-B increases Na+ and K+ permeability across the plasma membrane. These results suggest that the zooxanthellatoxin-B-induced contraction of the aorta is caused mainly by a direct action on smooth muscle, i.e., an increase in Ca2+ permeability that occurs at least partly through voltage-sensitive Ca2+ channels as well as through nonselective cation channels in the cell membrane of smooth muscle.

Antiplatelet and calcium inhibitory properties of eugenol and sodium eugenol acetate

1. Eugenol (3-methoxy-4-hydroxy-propenylbenzene) or sodium eugenol acetate (4-O-acetic acid sodium-3-methoxy-1-propenylbenzene) (0.25, 0.5, 1 mM) concentration-dependently inhibited arachidonic acid (AA)-, collagen-, epinephrine- and ADP-induced platelet aggregation. 2. Eugenol or sodium eugenol acetate inhibited collagen-induced aggregation of washed rabbit platelets synergistically with creatine phosphate/creatine phosphokinase (CP/CPK, 5 mM/10 U/ml) or p-bromophenacyl bromide (p-BPB, 10 microM), and they also potentiated the inhibitory action of imidazole (0.5 mM) on AA-induced aggregation. 3. Eugenol or sodium eugenol acetate (0.25, 0.5, 1 mM) concentration-dependently inhibited AA-induced thromboxane B2 and prostaglandin E2 formation. 4. The rise of intracellular Ca2+ caused by collagen, epinephrine, ADP, and AA were inhibited by eugenol or sodium eugenol acetate (1 mM).

A novel monogalactosylacylglycerol with inhibitory effect on platelet aggregation from the cyanophyceae Oscillatoria rosea

(2S)-1-O-Palmitoyl-3-O-beta-D-galactopyranosylglycerol, isolated from the marine alga Oscillatoria rosea, inhibited platelet aggregation induced by U46619, a thromboxane A2 analogue. Its structure was elucidated by spectroscopic analysis and chemical evidence.

Activation of rabbit platelets by Ca2+ influx and thromboxane A2 release in an external Ca(2+)-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 microM), accompanied by an increase in cytosolic Ca2+ concentration ([Ca2+]i). 2. ZT-A did not cause platelet aggregation or increase [Ca2+]i in a Ca(2+)-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 microM), a receptor operated Ca2+ channel antagonist, and mefenamic acid (0.1-10 microM), 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 microM), a cyclo-oxygenase inhibitor, and SQ-29548 (0.1-10 microM), a thromboxane A2 (TXA2) receptor antagonist, inhibited platelet aggregation and the increase in [Ca2+]i induced by ZT-A. 4. Methysergide (0.01-1 microM), 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 microM), a Na+ channel blocker and chlorpheniramine (1 microM), a H1-histamine receptor antagonist, neither affected ZT-A-induced platelet aggregation nor the increase in [Ca2+]i induced by ZT-A. 6. Genistein (1-100 microM), a protein tyrosine kinase inhibitor, and staurosporine (0.01-1 microM), a protein kinase C inhibitor, also inhibited ZT-A-induced platelet aggregation. 7. The present results suggest that ZT-A elicits Ca(2+)-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.

Intracerebral microdialysis combined with recording of extracellular field potential: a novel method for investigation of depolarizing drugs in vivo

1. The purpose of this study was to examine whether depolarizations evoked by excitatory amino acids can be recorded quantitatively, in vivo, with a microelectrode incorporated within a microdialysis probe. 2. Microdialysis probes incorporating a chlorided silver wire were implanted in the striatum of anaesthetized rats and perfused with artificial cerebrospinal fluid (ACSF). Increasing concentrations of excitatory amino acids were applied for 2 min via the microdialysis probe, and the extracellular direct current (d.c.) potential was recorded between the microdialysis electrode and a reference electrode placed under the scalp. 3. N-methyl-D-aspartate (NMDA, 25-500 microM), alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA, 5-1000 microM), kainate (5-500 microM), and glutamate (0.25-100 mM) evoked concentration-dependent depolarizations with maxima ranging from 7 to 10 mV, i.e. 3 to 10 times larger than those recorded from brain slices in vitro. Depolarizations evoked by glutamate receptor agonists applied by microdialysis shared several features with those recorded from brain slices. The most characteristic were: steep onset and recovery of NMDA and glutamate responses; marked post-depolarization hyperpolarization with NMDA; and very slow recovery after kainate application. At high concentrations (500 microM), NMDA occasionally initiated spreading depression. The relative potency of glutamate and NMDA was of the same order of magnitude to that obtained with the cortical wedge and hippocampal slices, glutamate being 100 to 400 times less potent than NMDA. 4. Two consecutive series of NMDA-stimuli within the same procedure evoked comparable depolarizations, indicating that reliable quantitative analysis of drug action can be performed, with each animal serving as its own control. This is relevant to the study of drugs acting on glutamate receptors especially antagonists. The remarkable inter-animal reproducibility is also a valuable feature.5. Pretreatment with dizocilpine maleate (MK-801, 2mgkg'1, i.p.) reduced by 65% the responses evoked by NMDA (500 fM). The non-NMDA antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX,100 1M) applied via the microdialysis probe reduced by around 78% the responses to AMPA and kainate (250 micro M). The fact that drugs, especially antagonists, can be administered either systemically, or directly through the dialysis probe to by-pass the blood-brain barrier or avoid peripheral effects, is especially relevant for neuropharmacological studies.6. Intracerebral microdialysis combined with in vivo recording of extracellular field potential is a novel and valuable method for the quantitative analysis of the action of drugs acting on glutamate receptors.This method should prove especially useful for comparing the sensitivity of specific brain structures to selective glutamate receptor agonists under normal conditions and when the neuronal micro environment is altered. It should also be useful for investigating the action of other depolarizing agents, such as veratridine, and their antagonists.

Maitotoxin activates cation channels distinct from the receptor-activated non-selective cation channels of HL-60 cells

We investigated whether maitotoxin activates non-selective cation channels, as was recently proposed [Soergel, Yasumoto, Daly and Gusovsky (1992) Mol. Pharmacol. 41, 487-493]. Stimulation of dibutyryl cyclic AMP-differentiated HL-60 cells with the chemotactic peptide N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP; 0.1 microM), the Ca(2+)-ATPase inhibitor thapsigargin (0.1 microM) or maitotoxin (25 ng/ml) resulted in an increase in cytoplasmic free calcium concentration ([Ca2+]i). Unlike fMLP and thapsigargin, maitotoxin produced no increase in [Ca2+]i in the absence of extracellular Ca2+. The increase in [Ca2+]i induced by fMLP was blocked by pretreatment with pertussis toxin (100 ng/ml for 24 h) but not that induced by maitotoxin. Similarly, the increase in [Ca2+]i produced by fMLP but not that produced by maitotoxin was inhibited by pretreatment with phorbol myristate acetate (100 ng/ml). Both fMLP- and maitotoxin-induced increases in [Ca2+]i were blocked by 1-(beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenylethyl)-1H-imid azole hydrochloride (SKF 96365) in a concentration-dependent manner. However, the maitotoxin-induced increase in [Ca2+]i was more sensitive to inhibition by SKF 96365 than the fMLP-induced increase. fMLP-induced increases in [Ca2+]i were blocked by cations with Gd3+ being more effective than Cd2+, whereas for maitotoxin Cd2+ was more effective than Gd3+. Both fMLP and thapsigargin stimulated quenching of Fura-2 fluorescence in the presence of extracellular Mn2+, whereas maitotoxin produced no Mn2+ quenching. Taken together these results suggest that maitotoxin does not stimulate the nonselective cation channel activated by fMLP, but instead activates Ca2+ influx by a different mechanism.