Block of CaV1.2 channels by Gd3+ reveals preopening transitions in the selectivity filter.
Using the lanthanide gadolinium (Gd(3+)) as a Ca(2+) replacing probe, we investigated the voltage dependence of pore blockage of Ca(V)1.2 channels. Gd(+3) reduces peak currents (tonic block) and accelerates decay of ionic current during depolarization (use-dependent block). Because diffusion of Gd(3+) at concentrations used (<1 microM) is much slower than activation of the channel, the tonic effect is likely to be due to the blockage that occurred in closed channels before depolarization. We found that the dose-response curves for the two blocking effects of Gd(3+) shifted in parallel for Ba(2+), Sr(2+), and Ca(2+) currents through the wild-type channel, and for Ca(2+) currents through the selectivity filter mutation EEQE that lowers the blocking potency of Gd(3+). The correlation indicates that Gd(3+) binding to the same site causes both tonic and use-dependent blocking effects. The apparent on-rate for the tonic block increases with the prepulse voltage in the range -60 to -45 mV, where significant gating current but no ionic current occurs. When plotted together against voltage, the on-rates of tonic block (-100 to -45 mV) and of use-dependent block (-40 to 40 mV) fall on a single sigmoid that parallels the voltage dependence of the gating charge. The on-rate of tonic block by Gd(3+) decreases with concentration of Ba(2+), indicating that the apparent affinity of the site to permeant ions is about 1 mM in closed channels. Therefore, we propose that at submicromolar concentrations, Gd(3+) binds at the entry to the selectivity locus and that the affinity of the site for permeant ions decreases during preopening transitions of the channel.
Babich
Olga
O
Department of Pharmacology and Physiology, University of Medicine and Dentistry of New Jersey, New Jersey Medical School, Newark, NJ 07103, USA.
Reeves
John
J
Shirokov
Roman
R
D000818:Animals; D001464:Barium / Q000378:metabolism; D001667:Binding, Competitive; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000378:metabolism*; D020746:Calcium Channels, L-Type / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D002460:Cell Line; D002463:Cell Membrane Permeability; D005682:Gadolinium / Q000378:metabolism*; D006801:Humans; D015640:Ion Channel Gating; D007700:Kinetics; D008564:Membrane Potentials; D008954:Models, Biological; D009154:Mutation; D018408:Patch-Clamp Techniques; D011487:Protein Conformation; D021122:Protein Subunits / Q000378:metabolism; D013324:Strontium / Q000378:metabolism; D014162:Transfection
D002121:Calcium Channel Blockers; D020746:Calcium Channels, L-Type; C419385:L-type calcium channel alpha(1C); D021122:Protein Subunits; D001464:Barium; D005682:Gadolinium; D002118:Calcium; D013324:Strontium