Abstract
1. Maitotoxin (MTX) was an extraordinarily potent stimulant of phosphoinositide breakdown in the neuroblastoma hybrid NCB-20 cells. 2. Maximal responses were obtained at 0.25-0.5 ng MTX/ml, and resulted in increased formation of [3H]inositol mono-, bis-, and trisphosphates. Increased formation of [3H]inositol bis- and trisphosphate was observed as early as 15 sec after the addition of MTX. 3. MTX-induced phosphoinositide breakdown in NCB-20 cells was not antagonized by organic (nifedipine, methoxyverapamil) or inorganic (Mn2+, Co2+, Cd2+) calcium channel blockers. However, the response on phosphoinositide breakdown was completely eliminated in the absence of extracellular calcium. 4. The results suggest that MTX either directly stimulates phosphoinositide breakdown in a calcium-dependent manner or acts indirectly through calcium channels insensitive to organic/inorganic calcium channel blockers.