New insights into maitotoxin action.

Maitotoxin (3 ng/mol) induced a massive uptake of 45Ca2+ into BC3H1 cells. This effect exhibits a lag phase of 3 min. Inositol diphosphate formation occurred concomittantly with the 45Ca2+ uptake but inositol monophosphate formation was found only after a 5-min delay following toxin addition. Maitotoxin-induced 45Ca2+ influxes could not be blocked by either 1 microM verapamil, 1 microM nifedipine or 1 mM La3+ but was blocked by Zn2+ (IC50 = 41 microM). In addition to inositol phosphate formation and 45Ca2+ uptake, maitotoxin stimulated a large uptake of Na+ and a great loss of K+ in BC3H1 cells. In the absence of Ca2+ (1 mM EGTA) none of the four maitotoxin effects could be detected. After restoration of Ca2+, the maitotoxin effects reappeared even when the toxin itself was no longer present. The divalent cation, Co2+ (1 mM), inhibited ion movements induced by maitotoxin and also digitonin (8.1 microM). The toxin action showed a very pronounced pH dependence. At low pH, maitotoxin was inactive. The dose-response curves for H+ ion inhibition of maitotoxin-induced Ca2+ uptake showed a shift to the right when determined in the absence of HCO3- and HCO3-/Cl- ions. It was concluded that the primary action of maitotoxin in BC3H1 cells was a pore-forming or channel-forming activity of a non-classical type. Some properties of maitotoxin resemble those of alpha-latrotoxin, others those of pore-forming agents such as melittin or alpha-toxin of Staphylococcus aureus.