Abstract

Several isomers of hexachlorocyclohexanes (HCHs) have been shown to be toxic to mammals. Previous studies have revealed that the δ isomer (δ-HCH) was particularly potent toward disrupting Ca²⁺homeostasis in a variety of excitable and nonexcitable cells and altering contractility of cardiac muscle. The effects of the δ and γ isomers of HCH were further investigated on isolated ventricular myocytes from guinea pig and on single cardiac ryanodine receptor (RyR2) Ca²⁺-release channels from cardiac SR vesicles. Intracellular Ca²⁺ transients were examined in electrically stimulated cells using the fluorescent dye indo-1, and twitch contractions of myocytes were analyzed using a video-based edge motion detection system. Exposure of myocytes to δ- but not γ-HCH depressed the peak of intracellular Ca²⁺ transients and prolonged recovery time. These effects were correlated with the ability of δ-HCH to inhibit the binding of [³H]ryanodine, a conformationally sensitive probe for RyR2 function, to SR preparations (IC₅₀ = 2 and 18 μM for high- and low-affinity interactions, respectively). Measurements of single-channel gating kinetics under voltage-clamp provided direct evidence of a potent isoform-selective activation of RyR2 by δ-HCH. Results from these studies revealed that δ-HCH alters Ca²⁺ homeostasis and contractility in cardiac myocytes and that the mechanism can be ascribed, at least in part, to a direct interaction with the RyR2 channel complex.