Maitotoxin activates a nonselective cation channel and a P2Z/P2X7-like cytolytic pore in human skin fibroblasts

Maitotoxin (MTX),a potent cytolytic agent, activatesCa²⁺ entry via nonselective cationchannels in virtually all types of cells. The identity of the channelsinvolved and the biochemical events leading to cell lysis remainunknown. In the present study, the effect of MTX on plasmalemmalpermeability of human skin fibroblasts was examined. MTX produced atime- and concentration-dependent increase in cytosolic freeCa²⁺ concentration that dependedon extracellular Ca²⁺ and wasrelatively insensitive to blockade by extracellular lanthanides. MTXalso produced a time- and concentration-dependent increase inplasmalemma 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 activatesCa²⁺-permeable cation channels andlater induces the formation of large pores. These effects of MTX onplasmalemmal permeability are similar to those seen on activation ofP2Z/P2X₇ receptors ina variety of cell types, raising the intriguing possibility that MTXand P2Z/P2X₇ receptor stimulationactivate a common cytolytic pore.