G45E hemichannels exhibit increased permeability to Ca²⁺. (A) Representative currents elicited by a series of voltage steps applied to oocytes expressing WT Cx26, A40V, and G45E. Oocytes were voltage clamped to −20 mV. The applied voltage step protocol consisted of 10-s steps from 60 to −100 mV in intervals of 10 mV followed by a 5-s step to −110 mV. Each series was repeated in extracellular solutions containing 0.2 mM Ca²⁺, 1.8 mM Ca²⁺, and 1.8 mM Ba²⁺. Currents from oocytes expressing any one of the three hemichannel types were similar in low Ca²⁺ (0.2 mM). Upon raising Ca²⁺ to 1.8 mM, a large transient inward current developed in oocytes expressing G45E when the voltage was stepped to −110 mV after depolarizing steps that activated these hemichannels. Equimolar substitution of Ca²⁺ with Ba²⁺ abolished the large transient inward current. Also, the hemichannel currents in Ba²⁺ resembled those in low Ca²⁺. (B and C) The transient current observed in oocytes expressing G45E represents a Ca²⁺-activated chloride current. (B) A representative recording of the reversal potential of the transient current for an oocyte bathed in 100 mM NaCl and 0.2 mM Ca²⁺. At various voltages (as indicated), oocytes were briefly exposed to 1.8 mM Ca²⁺ (indicated by the black boxes). The transient current that developed (filled in gray) reversed between −20 and −30 mV and was followed by a decay in the hemichannel current that reversed upon returning to 0.2 mM Ca²⁺. This protocol was repeated in an external solution in which 100 mM NaCl was replaced with a 50:50 NaCl/NaAsp. (C) Plot of peak transient current after exposure to 1.8 mM Ca²⁺ in 100 mM NaCl and 50:50 NaCl/NaAsp. Reversal potential shifted in accordance with the change in the extracellular Cl concentration.