Effect of maitotoxin analogues on calcium influx and phosphoinositide breakdown in cultured cells

Maitotoxin (MTX) and the analogues, bis-desulfated-MTX (didesulfo-MTX), mono-desulfated-MTX (monodesulfo-MTX), and hydrogenated-MTX (H-MTX) were examined on 45Ca2+ influx and phosphoinositide breakdown with hamster insulinoma HIT cells and rat glioma C6 cells. The activity of MTX was greatly reduced either by desulfation or by hydrogenation. Didesulfo-MTX weakly stimulated calcium influx in HIT cells, but had no stimulatory effect on either calcium influx or phosphoinositide breakdown in C6 cells. All the analogues inhibited MTX-induced calcium influx in either HIT or C6 cells. Didesulfo-MTX inhibited the calcium influx elicited by 3 ng/ml MTX in C6 cells with an IC50 of 7.0 +/- 0.7 ng/ml. The data suggest that the sulfate groups in MTX are important for stimulation of calcium influx and phosphoinositide breakdown, but are not essential for binding to a receptor-site on cell membranes. Although catalytic reduction of double bonds in MTX reduced activity by nearly 100-fold, a tritiated H-MTX still represents a potential radioligand for identification of MTX-binding sites.