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 Fe 2+ 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 Fe 2+ 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 Fe 2+ rapidly blocked currents with 2 mM extracellular Ca 2+ 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 Fe 2+ also shifted activation to more-depolarized voltages (by ∼10 mV with 1.8 mM extracellular Fe 2+ ) somewhat more strongly than did extracellular Ca 2+ or Mg 2+ , which is consistent with a Gouy-Chapman-Stern model with surface charge density σ = 1 e − /98 A 2 and K Fe = 4.5 M −1 for extracellular Fe 2+ . In the absence of extracellular Ca 2+ (and with extracellular Na + replaced by TEA), Fe 2+ carried detectable, whole-cell, inward currents at millimolar concentrations (73 ± 7 pA at −60 mV with 10 mM extracellular Fe 2+ ). With a two-site/three-barrier Eyring model for permeation of Ca V 3.1 channels, we estimated a transport rate for Fe 2+ of ∼20 ions/s for each open channel at −60 mV and pH 7.2, with 1 μM extracellular Fe 2+ (with 2 mM extracellular Ca 2+ ). 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 Fe 2+ entry into cells with clinically relevant concentrations of extracellular Fe 2+ .