Methylmercury induces Ca2+-dependent hyperpolarization of mouse thymocytes: a flow cytometric study using fluorescent dyes

The effect of methylmercury on mouse thymocytes was examined using fluorescent dyes for membrane potential and intracellular Ca²⁺. Methylmercury at concentrations of 1 ωM or higher (up to 30 ωM) produced hyperpolarization in a dose-dependent fashion. Charybdotoxin and quinine, but not 4-aminopyridine and tetraethylammonium, greatly suppressed methylmercury-induced hyperpolarization. Removal of external Ca²⁺ reduced the degree of hyperpolarization. Pretreatment of thymocytes with A23187 under Ca²⁺-free conditions abolished the hyperpolarization induced by methylmercury. Under both normal and Ca²⁺-free conditions methylmercury increased the intracellular concentration of Ca²⁺. The results suggest that the increase in intracellular Ca²⁺ is mediated through a Ca²⁺ release from intracellular stores as well as through influx of external Ca²⁺. Therefore, it is likely that methylmercury increases the intracellular concentration of Ca²⁺, resulting in activation of Ca²⁺-dependent K⁺ conductance of mouse thymocytes.