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 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 Å2and KFe= 4.5 M−1for 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+.