RT Journal Article SR Electronic T1 Fe2+ Block and Permeation of CaV3.1 (α1G) T-Type Calcium Channels: Candidate Mechanism for Non–Transferrin-Mediated Fe2+ Influx JF Molecular Pharmacology JO Mol Pharmacol FD American Society for Pharmacology and Experimental Therapeutics SP 1194 OP 1204 DO 10.1124/mol.112.080184 VO 82 IS 6 A1 Kyle V. Lopin A1 I. Patrick Gray A1 Carlos A. Obejero-Paz A1 Frank Thévenod A1 Stephen W. Jones YR 2012 UL http://molpharm.aspetjournals.org/content/82/6/1194.abstract AB 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 CaV3.1 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 KD = 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 KFe = 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 CaV3.1 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 CaV3.1 channels exhibit a significant “window current” at that voltage (open probability, ∼1%), CaV3.1 channels represent a likely pathway for Fe2+ entry into cells with clinically relevant concentrations of extracellular Fe2+.