Methylmercury elicits intracellular Zn2+ release in rat thymocytes: Its relation to methylmercury-induced decrease in cellular thiol content

We have previously revealed that thimerosal, an organomercurial preservative, increases intracellular Zn2+ concentration in rat thymocytes. Because thimerosal contains ethylmercury that confers the toxicity, it is a possibility that methylmercury (MetHg), an environmental pollutant, also increases intracellular Zn2+ concentration. This possibility was tested by measuring intracellular Zn2+ level with FluoZin-3, a fluorescent probe for intracellular Zn2+. MetHg at concentrations ranging from 100 nM to 1 μM significantly increased the intensity of FluoZin-3 fluorescence, an indicator for intracellular Zn2+ concentration, under external Ca2+- and Zn2+-free condition in a concentration-dependent manner. TPEN, a chelator for intracellular Zn2+, completely diminished the MetHg-induced augmentation of FluoZin-3 fluorescence. MetHg at 100 nM or more significantly decreased the intensity of 5-chlormethylfluorescein fluorescence, an indicator for cellular thiol content. Such MetHg-induced changes in the fluorescence were correlated with a coefficient of −0.917. Taken together, it is suggested that submicromolar MetHg releases Zn2+ from intracellular thiol, resulting in the increase in intracellular Zn2+ concentration. However, it is unlikely that MetHg at critical maternal blood concentration (27 nM) affects intracellular Zn2+ homeostasis.