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

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Abstract

  • 1This study describes properties of monovalent cation currents through ECaC, a recently cloned epithelial Ca2+-permeable channel from rabbit.
  • 2The 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 (δ= 0.31) and a voltage-dependent binding constant (at 0 mV 3.1 mm Ca2+, obtained from a Woodhull type analysis).
  • 3Currents were only stable in the absence of divalent cations and showed under these conditions a small time- and voltage-dependent component of activation.
  • 4Single 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.
  • 5The permeation sequence for monovalent cations through ECaC was Na+ > Li+ > K+ > Cs+ > NMDG+ which is identical to the Eisenmann sequence X for a strong field-strength binding site.
  • 6It 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.

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