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.