Evaluation of a Two-Site, Three-Barrier Model for Permeation in CaV3.1 (α1G) T-Type Calcium Channels: Ca²⁺, Ba²⁺, Mg²⁺, and Na

• Published in: The Journal of membrane biology 2010, Vol.235 (2), p.131-143
• Main Authors: Lopin, Kyle V, Obejero-Paz, Carlos A, Jones, Stephen W
• Format: Article
• Language: English
• Subjects: Biochemistry; Biomedical and Life Sciences; Channel block; Chord conductance; Eyring model; Human Physiology; Ion selectivity; Life Sciences; Patch clamp; Rate theory; Reversal potential
• ISSN: 0022-2631; 1432-1424
• DOI: 10.1007/s00232-010-9264-3

We explored the ability of a two-site, three-barrier (2S3B) Eyring model to describe recently reported data on current flow through open CaV3.1 T-type calcium channels, varying Ca²⁺ and Ba²⁺ over a wide range (100 nm-110 mm) while recording whole-cell currents over a wide voltage range (−150 mV to +100 mV) from channels stably expressed in HEK 293 cells. Effects on permeation were isolated using instantaneous current-voltage relationships (IIV) after strong, brief depolarizations to activate channels with minimal inactivation. Most experimental results were reproduced by a 2S3B model. The model described the IIV relationships, apparent affinities for permeation and block for Ca²⁺ and Ba²⁺, and shifts in reversal potential between Ca²⁺ and Ba²⁺. The fit to block by 1 mm [graphic removed] was reasonable, but block by [graphic removed] was described less well. Surprisingly, fits were comparable with strong ion-ion repulsion, with no repulsion, or with intermediate values. With weak repulsion, there was a single high-affinity site, with a low-affinity site near the cytoplasmic side of the pore. With strong repulsion, the net charge of ions in the pore was near +2 over a relatively wide range of concentration and voltage, suggesting a knockoff mechanism. With strong repulsion, Ba²⁺ preferred the inner site, while Ca²⁺ preferred the outer site, potentially explaining faster entry of Ni²⁺ and other pore blockers when Ba²⁺ is the charge carrier.