PbCl₂-induced hyperpolarization of rat thymocytes: Involvement of charybdotoxin-sensitive K⁺ channels

The effect of PbCl₂ on membrane potential and intracellular divalent metal cation concentrations of rat thymocytes was examined by flow cytometry. PbCl₂ at concentrations of 0.3 μM or higher (up to 10 μM) produced persistent, dose-dependent hyperpolarization (decrease in the intensity of di-BA-C₄ fluorescence). Removal of external Ca²⁺ did not significantly affect the PbCl₂-induced hyperpolarization. Charybdotoxin, a specific antagonist of Ca²⁺-dependent K⁺ conductance, greatly attenuated the PbCl₂-induced hyperpolarization. PbCl₂ increased the intensity of fluo-3 fluorescence under both normal Ca²⁺ and nominally Ca²⁺-free conditions. These results suggest that Pb²⁺ enters thymocytes, causing an increase in fluo-3 fluorescence, and activates Ca²⁺-dependent K⁺ channels, resulting in hyperpolarization. The persistent activation of K⁺ channels by Pb²⁺, leading to persistent hyperpolarization, may be one mechanism whereby Pb²⁺ alters immune function, as membrane potential changes influence physiological functions of lymphocytes.