TY - JOUR T1 - Fe<sup>2+</sup> Block and Permeation in CaV3.1 (α1G) T-type Calcium Channels. A Candidate Mechanism for Non-Tranferrin-mediated Fe<sup>2+</sup> Influx JF - Molecular Pharmacology JO - Mol Pharmacol DO - 10.1124/mol.112.080184 SP - mol.112.080184 AU - Kyle V. Lopin AU - I. Patrick Gray AU - Carlos A. Obejero-Paz AU - Frank Thevenod AU - Stephen W. Jones Y1 - 2012/01/01 UR - http://molpharm.aspetjournals.org/content/early/2012/09/12/mol.112.080184.abstract N2 - Iron is a biologically essential metal, but excess iron can cause damage to the cardiovascular and nevous systems. We have examined the effects of Fe2+o on permeation and gating in CaV3.1 channels, stably transfected in HEK 293 cells, using whole-cell recording. Precautions were taken to maintain iron in the Fe2+ state (e. g., extracellular ascorbate). Using instantaneous I-V relations (measured following strong depolarization) to isolate effects on permeation, Fe2+o rapidly blocked currents with 2 mM Ca2+o in a voltage-dependent manner, described by a Woodhull model with KD = 2.5 mM at 0 mV and an apparent electrical distance δ = 0.17. Fe2+o also shifted activation to more depolarized voltages (by ~10 mV at 1.8 mM Fe2+o), somewhat more strongly than Ca2+o or Mg2+o, consistent with a Gouy-Chapman-Stern model with a surface charge density σ = 1 e-/98 Å2 and KFe = 4.5 M-1 for Fe2+o. In the absence of Ca2+o (and with Na+o replaced by tetraethylammonium), Fe2+ carried detectable whole-cell inward currents at millimolar concentrations (73 ± 7 pA at -60 mV with 10 mM Fe2+o). From a 2-site 3-barrier Eyring model for permeation in CaV3.1, we estimate a transport rate for Fe2+ of ~20 ions per second per open channel at -60 mV and pH 7.2, in 1 μM Fe2+o (with 2 mM Ca2+o). Since CaV3.1 exhibits a significant 'window current' in that voltage range (PO ~1%), CaV3.1 channels are a likely pathway for Fe2+ entry into cells, at clinically relevant concentrations of Fe2+o. ER -