TY - JOUR T1 - Mechanisms of δ-Hexachlorocyclohexane Toxicity: II. Evidence for Ca<sup>2+</sup>-Dependent K<sup>+</sup>-Selective Ionophore Activity JF - Journal of Pharmacology and Experimental Therapeutics JO - J Pharmacol Exp Ther SP - 486 LP - 493 VL - 289 IS - 1 AU - Edmond D. Buck AU - Isaac N. Pessah Y1 - 1999/04/01 UR - http://jpet.aspetjournals.org/content/289/1/486.abstract N2 - 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 sulfoxide 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+ &gt; 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. The American Society for Pharmacology and Experimental Therapeutics ER -