Inhibition of maitotoxin-induced Ca²⁺ influx in rat glioma C6 cells by brevetoxins and synthetic fragments of maitotoxin

⁴⁵Ca²⁺ influx in rat glioma C6 cells induced by 0.3 nM maitotoxin (MTX) was markedly inhibited by brevetoxin A (PbTx1) and brevetoxin B (PbTx2), with EC₅₀ values of 16 and 13 μM, respectively. This inhibition was observed immediately after addition of MTX when monitored with fura-2, which suggests that PbTx2 directly blocks the action of MTX. This blockade by PbTx2 was not affected by tetrodotoxin, which excludes the involvement of voltage-sensitive sodium channels. The depolarizing effects of these bravetoxins were also not a likely cause of this inhibition, because melittin, a channel-forming peptide, did not significantly block MTX-induced ⁴⁵Ca²⁺ influx. Instead, this inhibition was thought to be mediated by blockade of an MTX-binding site by the brevetoxins, based on the fact that these toxins, particularly PbTx2 closely mimic the partial structure of MTX. Synthetic fragments of MTX corresponding to the hydrophilic EF-GH rings (200 μM) and LM-NO rings (500 μM) of MTX significantly reduced MTX-elicited Ca²⁺ influx. The observation that the effects of MTX were inhibited by structural mimics of both its hydrophobic and hydrophilic portions implies that both portions of the MTX molecule recognize its target.