Effects of the amphiphilic peptides melittin and mastoparan on calcium influx, phosphoinositide breakdown and arachidonic acid release in rat pheochromocytoma PC12 cells

Two amphiphilic peptides from hymenopterid insects, melittin and mastoparan, stimulate secretion in a variety of cell types. In PC12 cells, both peptides stimulate calcium influx with melittin some 20-fold more potently than mastoparan. Melittin stimulates both breakdown of phosphoinositides (PI) by phospholipase C to yield inositol phosphates and hydrolysis of phospholipids by phospholipase A₂ to release arachidonic acid (AA). Mastoparan stimulates PI breakdown, but has no effect on AA release. Maximal stimulation of PI breakdown occurs at 1 to 2.5 μg/ml melittin and 30 μg/ml mastoparan, whereas maximal stimulation of AA release occurs at 2 to 5 μg/ml melittin. Organic calcium channel blockers (nifedipine, verapamil, diltiazem) have little or no effect on responses to the peptides. The influx of calcium elicited by melittin or mastoparan is completely or nearly completely blocked by inorganic calcium channel blockers (Co⁺⁺, Mn⁺⁺, Cd⁺⁺). Mn⁺⁺ and Cd⁺⁺ inhibit melittin-induced PI breakdown and AA release and mastoparan-induced PI breakdown. Co⁺⁺ has no effect on melittin-induced PI breakdown and potentiates mastoparan-induced PI breakdown. Pertussis toxin has no effect on the PI breakdown induced by either peptide. The responses to melittin and mastoparan in PC12 cells are compared to those reported for maitotoxin.