INHIBITION OF MAITOTOXIN-INDUCED CA2+ INFLUX IN RAT GLIOMA C6 CELLS BY BREVETOXINS AND SYNTHETIC FRAGMENTS OF MAITOTOXIN

Ca-45(2+) 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 mu M, respectively. This inhibit ion 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 exclude s the involvement of voltage-sensitive sodium channels. The depolarizi ng effects of these brevetoxins were also not a likely cause of this i nhibition, because melittin, a channel-forming peptide, did not signif icantly block MTX-induced Ca-45(2+) influx. Instead, this inhibition w as thought to be mediated by blockade of an MTX-binding site by the br evetoxins, based on the fact that these toxins, particularly PbTx2, cl osely mimic the partial structure of MTX. Synthetic fragments of MTX c orresponding to the hydrophilic EF-GH rings (200 mu M) and LM-NO rings (500 mu M) of MTX significantly reduced MTX-elicited Ca2+ influx. The observation that the effects of MTX were inhibited by structural mimi cs of both its hydrophobic and hydrophilic portions implies that both portions of the MTX molecule recognize its target.