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

delta-Hexachlorocyclohexane (delta-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 delta-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 delta-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 microM gamma-HCH (lindane) or delta-HCH, produced negligible steady-state current with symmetric 100 mM CsCl in the range of +/-50 mV. However, the addition of 50 microM Ca2+ to the bilayer chamber in the presence of delta-HCH induced a profound increase in ionic permeability that was not seen in the presence of gamma-HCH or dimethyl sulfoxide control. Significantly, the permeability increase 1) was proportional with increasing Ca2+ to approximately 600 microM and saturated between 1 and 2 mM Ca2+ regardless of holding potential, 2) occurred only when delta-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 delta-HCH and Ca2+ was added. The Ca2+-dependent current produced by delta-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 microM concentration each of delta-HCH and Ca2+ was 4.25 pA/mV. The results show that delta-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.