Maitotoxin Activates Cation Channels Distinct from the Receptor-activated Non-selective Cation Channels of HL-60 Cells

We investigated whether maitotoxin activates non-selective cation channels, as was recently proposed [Soergel, Yasumoto, Daly and Gusovsky (1992) Mol. Pharmacol. 41, 487-493]. Stimulation of dibutyryl cyclic AMP-differentiated HL-60 cells with the chemotactic peptide N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP; 0.1 microM), the Ca(2+)-ATPase inhibitor thapsigargin (0.1 microM) or maitotoxin (25 ng/ml) resulted in an increase in cytoplasmic free calcium concentration ([Ca2+]i). Unlike fMLP and thapsigargin, maitotoxin produced no increase in [Ca2+]i in the absence of extracellular Ca2+. The increase in [Ca2+]i induced by fMLP was blocked by pretreatment with pertussis toxin (100 ng/ml for 24 h) but not that induced by maitotoxin. Similarly, the increase in [Ca2+]i produced by fMLP but not that produced by maitotoxin was inhibited by pretreatment with phorbol myristate acetate (100 ng/ml). Both fMLP- and maitotoxin-induced increases in [Ca2+]i were blocked by 1-(beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenylethyl)-1H-imid azole hydrochloride (SKF 96365) in a concentration-dependent manner. However, the maitotoxin-induced increase in [Ca2+]i was more sensitive to inhibition by SKF 96365 than the fMLP-induced increase. fMLP-induced increases in [Ca2+]i were blocked by cations with Gd3+ being more effective than Cd2+, whereas for maitotoxin Cd2+ was more effective than Gd3+. Both fMLP and thapsigargin stimulated quenching of Fura-2 fluorescence in the presence of extracellular Mn2+, whereas maitotoxin produced no Mn2+ quenching. Taken together these results suggest that maitotoxin does not stimulate the nonselective cation channel activated by fMLP, but instead activates Ca2+ influx by a different mechanism.