Mechanisms of δ-hexachlorocyclohexane toxicity: II. Evidence for Ca²⁺-dependent K⁺-selective ionophore activity

δ-Hexachlorocyclohexane (δ-HCH) interacts with cardiac ryanodine- sensitive Ca²⁺ channels (RyR2), accounting in part for altered Ca²⁺ transients and contractility (reported in companion report). Analysis of channel gating kinetics in the presence of δ-HCH also revealed a nonfluctuating membrane current that remained even after RyR2 channels were blocked. We further elucidated the nature of a direct interaction between δ- HCH and biological membranes by meaSUring ionic currents across planar lipid bilayers made from defined lipids lacking cellular protein using voltage- clamp. Dimethyl sulfoxide, in the presence or absence of 50 μM γ-HCH (lindane) or δ-HCH, produced negligible steady-state current with symmetric 100 mM CsCl in the range of ±50 mV. However, the addition of 50 μM Ca²⁺ to the bilayer chamber in the presence of δ-HCH induced a profound increase in ionic permeability that was not seen in the presence of γ-HCH or dimethyl surfoxide control. Significantly, the permeability increase 1) was proportional with increasing Ca²⁺ to ~600 μM and saturated between 1 and 2 mM Ca²⁺ regardless of holding potential, 2) occurred only when δ-HCH and Ca²⁺ were added to the same side of the membrane, and 3) was independent of the order of addition or of the side of the membrane to which δ-HCH and Ca²⁺ was added. The Ca²⁺-dependent current produced by δ-HCH was highly selective for monovalent cations (K⁺ >> Cs⁺ > Na⁺), with negligible conductance for Ca²⁺ or Cl^-. In symmetric 100 mM K⁺, the conductance induced with 50 μM concentration each of δ-HCH and Ca²⁺ was 4.25 pA/mV. The results show that δ-HCH increases the ionic permeability of phospholipid membranes by two distinct Ca²⁺-dependent mechanisms: one mediated through RyR and the other mediated by a unique ionophore activity.