Calcium-dependent effects of maitotoxin on phosphoinositide breakdown and on cyclic AMP accumulation in PC12 and NCB-20 cells.

The marine dinoflagellate toxin maitotoxin (MTX) stimulates phosphoinositide breakdown in pheochromocytoma PC12 cells and in neuroblastoma hybrid NCB-20 cells. In both cell lines, the stimulation of phosphoinositide breakdown by MTX is dependent on extracellular calcium, but it is not reduced by organic or inorganic calcium channel blockers. In PC12 cells, the maximal stimulation of phosphoinositide breakdown occurs at 1.5 mM [Ca2+]o, whereas in NCB-20 cells the maximal stimulation is observed at 2.5-4.5 mM [Ca2+]o. Phosphoinositide breakdown is known to lead to formation of both inositol phosphates and diacylglycerols. The latter, through stimulation of protein kinase C, would, like phorbol esters, be expected to augment cyclic AMP accumulation in PC12 cells and to inhibit receptor-mediated cyclic AMP accumulation in NCB-20 cells. MTX does potentiate forskolin-induced accumulation of cyclic AMP in PC12 cells and does inhibit prostaglandin E2-induced accumulation of cyclic AMP in NCB-20 cells. The effects of MTX on accumulation of cyclic AMP are calcium dependent and the concentrations of calcium required for maximal responses are the same as the ones required for maximal stimulation of phosphoinositide breakdown. MTX increases intracellular calcium in both cell lines, as measured by calcium-quin2 fluorescence. But the effects of MTX on forskolin- and prostaglandin E2-mediated cyclic AMP accumulation are not mimicked by a calcium ionophore and are not blocked by nifedipine, a calcium channel blocker. Translocation of protein kinase C occurs after treatment with MTX in both cell lines; the protein kinase C activity and content are reduced in the cytosol and increased in membranes after exposure to either MTX or a phorbol ester. The results confirm previous studies on the heterogeneous input of protein kinase C to cyclic AMP-generating systems performed with phorbol esters and demonstrate the utility of MTX as a unique tool for studies of systems that involve second messengers generated through stimulation of phosphoinositide breakdown.