Methylmercury increases intracellular concentrations of Ca++ and heavy metals in NG108-15 cells

To determine if methylmercury (MeHg) increased [Ca++]i in intact neuron-like cells, we initiated studies using fluorescence microscopy of single NG108-15 cells preloaded with fura-2. Whereas at 0.5 microM, MeHg had no effect on the ratio of fura-2 fluorescence at 340/380 nm, at 2 and 5 microM it produced a biphasic increase in this ratio. The initial phase increase was sustained; its time to onset was concentration-dependent whereas its maximum increase was not. This phase likely consists of both intra- and extracellular components inasmuch as removal of extracellular Ca++ reduced but did not eliminate the increase. Continued exposure to MeHg resulted in a further pronounced increase in fluorescence ratio, but only in the presence of extracellular Ca++. The time to onset of this second phase was also concentration-dependent. In Ca(++)-containing, but not Ca(++)-deficient medium, the second phase increase in fluorescence ratio was followed by loss of fura-2 from the cells. Both 2 and 5 microM, but not 0.5 microM MeHg, depolarized the mitochondrial membrane potential (psi m) as measured by loss of preloaded rhodamine 123 from the mitochondria. The latency of this effect was concentration-dependent, but the maximum amplitude was not. Removal of extracellular Ca++ had no effect on the initial changes in rhodamine 123 fluorescence produced by MeHg, but did retard subsequent loss of dye from the cells. The onset as well as peak amplitude of the initial MeHg-induced increase in fura-2 fluorescence ratio occurred before changes in psi m. In the absence of MeHg, depolarization of psi m by the combination of sodium azide and oligomycin failed to elicit a significant increase in [Ca++]i, but did reduce the initial increase in fura-2 fluorescence ratio produced by 2 microM MeHg independent of extracellular Ca++. MeHg increased fura-2 fluorescence measured at the Ca(++)-insensitive excitation wavelength of 360 nm. This effect did not coincide with alterations in rhodamine 123 fluorescence and was inhibited by the cell-permeant heavy metal chelator N,N,N',N'-tetrakis-(2-pyridylmethyl)ethylenediamine, but not the cell-impermeant chelator diethylenetriaminepentaacetic acid. This suggests that the initial phase, extracellular Ca(++)-independent changes in fura-2 fluorescence were due to increases in the intracellular concentration of endogenous cations other than Ca++. Thus, MeHg altered fura-2 fluorescence in these cells in a concentration- and time-dependent fashion. The initial effect involved alterations in intracellular cation buffering as well as increased permeability of the plasma membrane to Ca++.(ABSTRACT TRUNCATED AT 400 WORDS)