Practical usage concentrations of monensin have non-specific actions other than as a sodium ionophore in rat parotid acinar cells.
Monensin is used as a sodium ionophore to examine the effect of Na+ on cellular function in a variety of cell types. In the present study, we investigated the effects of different concentrations of monensin on the signal transduction system in exocrine parotid acinar cells. Monensin increased cytosolic free Na+ concentration, measured by the Na+ indicator sodium-binding benzofuran isophthalate in a concentration-dependent manner (0.01 to 100 microM). Likewise, monensin concentration-dependently increased amylase release and intracellular Ca2+ concentration in the presence and the absence of extracellular Ca2+. Low concentrations (0.01 to 1 microM) of monensin did not release Ca2+ from non-mitochondrial intracellular pools in permeabilized cells with saponin but high concentrations (10 and 100 microM) of monensin which are of practical usage did. Monensin itself did not change the cyclic AMP accumulation, whereas high concentrations (10 and 100 microM) but not low concentrations (0.01 to 1 microM) of monensin inhibited cyclic AMP accumulation elevated by isoproterenol in the presence and absence of extracellular Na+. These results indicate that high concentrations of monensin, which are practically used, have nonspecific actions in rat parotid acinar cells, and lower concentrations of monensin are recommended for use as a sodium ionophore.