Vol. 289, Issue 1, 477-485, April 1999

Mechanisms of -Hexachlorocyclohexane Toxicity: I. Relationship Between Altered Ventricular Myocyte Contractility and Ryanodine Receptor Function¹

Edmond D. Buck, Wilhelm G. Lachnit and Isaac N. Pessah

Department of Molecular Biosciences, School of Veterinary Medicine, University of California, Davis, California

Several isomers of hexachlorocyclohexanes (HCHs) have been shown to be toxic to mammals. Previous studies have revealed that the  isomer (-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  and  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 - but not -HCH depressed the peak of intracellular Ca²⁺ transients and prolonged recovery time. These effects were correlated with the ability of -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 -HCH. Results from these studies revealed that -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.