Whole-cell and single channel monovalent cation currents through the novel rabbit epithelial Ca2+ channel ECaC

1. This study describes properties of monovalent cation currents through ECaC, a recently cloned epithelial Ca2+-permeable channel from rabbit. 2. The kinetics of currents through ECaC was strongly modulated by divalent cations. Currents were inhibited in the presence of extracellular Ca2+. They showed an initial voltage-dependent decay in the presence of 1 mM Mg2+ at hyperpolarizing steps in Ca2+-free solutions, which represents a voltage-dependent Mg2+ block through binding of Mg2+ to a site localized in the electrical field of the membrane (delta = 0.31) and a voltage-dependent binding constant (at 0 mV, 3.1 mM Ca2+, obtained from a Woodhull type analysis). 3. Currents were only stable in the absence of divalent cations and showed under these conditions a small time- and voltage-dependent component of activation. 4. Single channel currents in cell-attached and inside-out patches had a conductance of 77.5 +/- 4.9 pS (n = 11) and reversed at +14.8 +/- 1.6 mV (n = 9) in the absence of divalent cations. 5. The permeation sequence for monovalent cations through ECaC was Na+ > Li+ > Ki(+) > Cs+ > NMDG(+) which is identical to the Eisenmann sequence X for a strong field-strength binding site. 6. It is concluded that the permeation profile of ECaC for monovalent cations suggests a strong field-strength binding site that may be involved in Ca2+ permeation and Mg2+ block.