title: Fe²⁺ block and permeation of CaV3.1 (α1G) T-type calcium channels: candidate mechanism for non-transferrin-mediated Fe²⁺ influx.
abstract: Iron is a biologically essential metal, but excess iron can cause damage to the cardiovascular and nervous systems. We examined the effects of extracellular Fe²⁺ on permeation and gating of Ca(V)3.1 channels stably transfected in HEK293 cells, by using whole-cell recording. Precautions were taken to maintain iron in the Fe²⁺ state (e.g., use of extracellular ascorbate). With the use of instantaneous I-V currents (measured after strong depolarization) to isolate the effects on permeation, extracellular Fe²⁺ rapidly blocked currents with 2 mM extracellular Ca²⁺ in a voltage-dependent manner, as described by a Woodhull model with K(D) = 2.5 mM at 0 mV and apparent electrical distance δ = 0.17. Extracellular Fe²⁺ also shifted activation to more-depolarized voltages (by ∼10 mV with 1.8 mM extracellular Fe²⁺) somewhat more strongly than did extracellular Ca²⁺ or Mg²⁺, which is consistent with a Gouy-Chapman-Stern model with surface charge density σ = 1 e(-)/98 Å² and K(Fe) = 4.5 M⁻¹ for extracellular Fe²⁺. In the absence of extracellular Ca²⁺ (and with extracellular Na⁺ replaced by TEA), Fe²⁺ carried detectable, whole-cell, inward currents at millimolar concentrations (73 ± 7 pA at -60 mV with 10 mM extracellular Fe²⁺). With a two-site/three-barrier Eyring model for permeation of Ca(V)3.1 channels, we estimated a transport rate for Fe²⁺ of ∼20 ions/s for each open channel at -60 mV and pH 7.2, with 1 μM extracellular Fe²⁺ (with 2 mM extracellular Ca²⁺). Because Ca(V)3.1 channels exhibit a significant "window current" at that voltage (open probability, ∼1%), Ca(V)3.1 channels represent a likely pathway for Fe²⁺ entry into cells with clinically relevant concentrations of extracellular Fe²⁺.
authors.affiliation: Department of Physiology and Biophysics, Case Western Reserve University, Cleveland, OH 44106, USA.
authors.forename: Kyle V
authors.initials: KV
authors.lastname: Lopin
authors.forename: I Patrick
authors.initials: IP
authors.lastname: Gray
authors.forename: Carlos A
authors.initials: CA
authors.lastname: Obejero-Paz
authors.forename: Frank
authors.initials: F
authors.lastname: Thévenod
authors.forename: Stephen W
authors.initials: SW
authors.lastname: Jones
mesh_terms: D001464:Barium / Q000378:metabolism; D002118:Calcium / Q000378:metabolism*; D020747:Calcium Channels, T-Type / Q000378:metabolism*; D002460:Cell Line; D005296:Ferrous Compounds / Q000378:metabolism* / Q000494:pharmacology*; D057809:HEK293 Cells; D006801:Humans; D015640:Ion Channel Gating / Q000187:drug effects; D008274:Magnesium / Q000378:metabolism; D008564:Membrane Potentials / Q000187:drug effects; D018408:Patch-Clamp Techniques / Q000379:methods; D014168:Transferrin / Q000378:metabolism*
chemical_list: C121855:CACNA1G protein, human; D020747:Calcium Channels, T-Type; D005296:Ferrous Compounds; D014168:Transferrin; D001464:Barium; D008274:Magnesium; D002118:Calcium