benzofurans and maitotoxin

In this study, the effects of the marine toxin maitotoxin on cytosolic Ca2+ levels and membrane potential in rat brain synaptosomes were evaluated. Maitotoxin (10 ng/ml) caused a remarkable increase of intrasynaptosomal Ca2+ levels monitored by the fluorescent probe fura-2. This increase was prevented by the removal of external Ca2+ ions. Tetrodotoxin, as well as the removal of extracellular Na+ ions, failed to affect maitotoxin-induced increase of intrasynaptosomal Ca2+ levels. Also the complete removal of all monovalent and divalent cations, except Ca2+ ions, from the incubation medium (0.32 M sucrose substitution), was unable to prevent the effect of maitotoxin on intrasynaptosomal Ca2+ levels. Maitotoxin (0.3-10 ng/ml), produced a dose-dependent depolarization of synaptosomal membranes, which required the presence of extracellular Ca2+ ions. The substitution of extracellular Na+ with choline or the removal of all cations from the incubation medium and their replacement with an isotonic concentration of sucrose (0.32 M), did not prevent the depolarizing effect exerted by maitotoxin. Also under these two ionic conditions, the effect of maitotoxin on membrane potential was critically dependent on the presence of 1 mM extracellular Ca2+. The depolarizing effect exerted by maitotoxin on synaptosomal membrane potential was also observed when extracellular Ca2+ ions were substituted with an equimolar concentration of Ba2+ or Sr2+ ions. In summary, these results appear to suggest that, in presence of 1 mM extracellular Ca2+ ions, maitotoxin depolarizes synaptosomal plasmamembrane by promoting the influx of extracellular Ca2+ ions. This enhanced influx of Ca2+ causes an increase of intrasynaptosomal Ca2+ levels.

Topics: Animals; Barium; Benzofurans; Brain; Calcium; Cations, Divalent; Cytosol; Fluorescent Dyes; Fura-2; Male; Marine Toxins; Membrane Potentials; Oxocins; Rats; Rats, Inbred Strains; Sodium; Strontium; Synaptic Membranes; Synaptosomes; Tetrodotoxin; Thiobarbiturates

Maitotoxin, a potent marine toxin isolated from toxic tropical dinoflagellates and poisonous fishes induces contraction of different smooth muscle preparations. Actions of maitotoxin on phosphoinositides and calcium metabolism were studied using a primary culture of aortic smooth muscle cells. Maitotoxin induced a very large increase of cytosolic calcium concentration as evaluated by fura-2 acetoxymethyl ester fluorescence. This increase was concomitant with stimulation of inositol-phosphate accumulation and loss of viability of aortic smooth muscle cells. These responses to maitotoxin were abolished in Ca2+-free medium, and were mimicked by saponin. Calcium ionophores or K+ depolarisation did not induce inositol-phosphate formation. These results suggest that maitotoxin acts by altering smooth muscle cells permeability allowing a sustained calcium influx which is able to activate inositol-phosphate formation and which is lethal for the cells.

Topics: Angiotensin II; Animals; Aorta, Thoracic; Benzofurans; Calcium; Cell Survival; Cells, Cultured; Fluorescent Dyes; Fura-2; Male; Marine Toxins; Muscle, Smooth, Vascular; Oxocins; Phosphatidylinositols; Potassium; Rats; Rats, Inbred Strains; Saponins; Spectrometry, Fluorescence