MAITOTOXIN (MTX) ACTIVATES A NONSELECTIVE CATION CHANNEL IN XENOPUS-LAEVIS OOCYTES

Maitotoxin (MTX) may exert its toxic effect by activating ion conducta nces and has been shown to elicit a fertilization-like response in Xen opus laevis oocytes. In the present study we investigated the electrop hysiological response of stage V-VI Xenopus oocytes to MTX using the t wo-microelectrode voltage-clamp technique. Membrane voltage (V-m) meas urements demonstrated that MTX (50 pM to 1 nM) depolarized the oocytes from -49+/-7 to -14+/-1 mV. Subsequent replacement of bath Na+ by the impermeant cation NMDG (N-methyl-D-glucamine) shifted V-m from -14+/- 1 to -53+/-5 mV (n=29). This indicates that MTX activates a cation con ductance. Indeed, current measurements at a holding potential of -60 o r -100 mV showed that within 10 s of MTX application an inward current component developed which was largely abolished by extracellular Naremoval. After a 1-min application of 1 nM MTX the NMDG-sensitive curr ent increased more than 100-fold from 0.14+/-0.03 mu A to a peak value of 21+/-3 mu A (n=11). The effect of MTX was concentration dependent with an EC50 of about 250 pM but only slowly reversible. Ion substitut ion experiments indicated that the stimulated conductance was nonselec tive for monovalent cations with a slight preference for NH4+ (2.1) > K+ (1.5) > Na+ (1.0) > Li+ (0.7). Regarding divalent cations, a comple x biphasic response to extracellular Na+ replacement by Ca2+ was obser ved, which suggests that the stimulated channels may have a small Ca2 permeability but that exposure to high extracellular Ca2+ enhances re covery from MTX stimulation. No significant conductance for Mn2+ was o bserved. Application of 1 mM benzamil, 1 mM amiloride, or 100 mu M yph enyl)-propoxy]-4-methoxyphenethyl)-1H-imidazole hydrochloride (SK&F 96 365) reduced the MTX-stimulated inward current by 81%, 62%, or 65%, re spectively. Gd3+ had an inhibitory effect of 29% and 38% at concentrat ions of 10 mu M or 100 mu M, respectively. Flufenamic acid, niflumic a cid, RS)-(3,4-dihydro-6,7-dimethoxyisoquinoline-1-gamma ,N-di-[2-(2,3, 4-trimethoxyphenyl)-ethyl]-acetamide (LOE908), and 3',5'-dichlorodiphe nylamine-2-carboxylic acid (DCDPC), known blockers of other nonselecti ve cation channels, had no significant effect. We conclude that MTX ac tivates a nonselective cation conductance in Xenopus oocytes. The unde rlying channels may be involved in changes in V-m that occur during th e early stages of fertilization.