The mechanism of calcium-independent catecholamine depleting action of monensin from clonal rat pheochromocytoma cells.

Monensin, a monovalent cation ionophore, induced profound release of radiolabeled materials from clonal rat pheochromocytoma cells (PC12h) preloaded with [3H]norepinephrine (NE). The release was suppressed in the absence of external Na+, but was not affected at all in the absence of external Ca2+. Cytosolic free Ca2+ concentration ([Ca2+]i), that was monitored by means of a fluorescent Ca2+ indicator, Quin 2, was temporarily increased upon a depolarizing stimulus of high-K+, which induced the Ca2+-dependent release of [3H]NE from PC12h cells. On the other hand, monensin induced only a slight increase in [Ca2+]i. The radiolabeled materials released by high-K+ treatment were mainly [3H]NE, whereas those by monensin were mainly the metabolites of [3H]NE. Pargyline, a monoamine oxidase inhibitor, suppressed both the degradation of [3H]NE stored in PC12h cells and the monensin-induced release of radiolabeled compounds from them. Monensin decreased the content of [3H]NE in storage granules of pargyline-treated cells. Thus, it is likely that monensin expels NE from the storage vesicles to cytosol and then its metabolites by monoamine oxidase are released in a non-exocytotic manner.