Fe2+ Block and Permeation of CaV3.1 (α1G) T-Type Calcium Channels: Candidate Mechanism for Non-Transferrin-Mediated Fe2+ Influx

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 delta = 0.17. Extracellular Fe2+ also shifted activation to more-depolarized voltages (by similar to 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 sigma = 1 e(-)/98 angstrom(2) 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 similar to 20 ions/s for each open channel at -60 mV and pH 7.2, with 1 mu M extracellular Fe2+ (with 2 mM extracellular Ca2+). Because Ca(V)3.1 channels exhibit a significant window current at that voltage (open probability, similar to 1%), Ca(V)3.1 channels represent a likely pathway for Fe2+ entry into cells with clinically relevant concentrations of extracellular Fe2+.