title: Monovalent permeability, rectification, and ionic block of store-operated calcium channels in Jurkat T lymphocytes.
abstract: We used whole-cell recording to characterize ion permeation, rectification, and block of monovalent current through calcium release-activated calcium (CRAC) channels in Jurkat T lymphocytes. Under physiological conditions, CRAC channels exhibit a high degree of selectivity for Ca2+, but can be induced to carry a slowly declining Na+ current when external divalent ions are reduced to micromolar levels. Using a series of organic cations as probes of varying size, we measured reversal potentials and calculated permeability ratios relative to Na+, PX/PNa, in order to estimate the diameter of the conducting pore. Ammonium (NH4+) exhibited the highest relative permeability (PNH4/PNa = 1.37). The largest permeant ion, tetramethylammonium with a diameter of 0.55 nm, had PTMA/PNa of 0.09. N-methyl-D-glucamine (0.50 x 0.64 x 1.20 nm) was not measurably permeant. In addition to carrying monovalent current, NH4+ reduced the slow decline of monovalent current ("inactivation") upon lowering [Ca2+]o. This kinetic effect of extracellular NH4+ can be accounted for by an increase in intracellular pH (pHi), since raising intracellular pH above 8 reduced the extent of inactivation. In addition, decreasing pHi reduced monovalent and divalent current amplitudes through CRAC channels with a pKa of 6.8. In several channel types, Mg2+ has been shown to produce rectification by a voltage-dependent block mechanism. Mg2+ removal from the pipette solution permitted large outward monovalent currents to flow through CRAC channels while also increasing the channel's relative Cs+ conductance and eliminating the inactivation of monovalent current. Boltzmann fits indicate that intracellular Mg2+ contributes to inward rectification by blocking in a voltage-dependent manner, with a z delta product of 1.88. Ca2+ block from the outside was also found to be voltage dependent with z delta of 1.62. These experiments indicate that the CRAC channel, like voltage-gated Ca2+ channels, achieves selectivity for Ca2+ by selective binding in a large pore with current-voltage characteristics shaped by internal Mg2+.
authors.affiliation: Department of Physiology and Biophysics, University of California, Irvine 92697, USA.
authors.forename: H H
authors.initials: HH
authors.lastname: Kerschbaum
authors.forename: M D
authors.initials: MD
authors.lastname: Cahalan
mesh_terms: D002118:Calcium / Q000378:metabolism; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000378:metabolism*; D002414:Cations, Monovalent / Q000378:metabolism; D004553:Electric Conductivity; D004594:Electrophysiology; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007477:Ions; D019169:Jurkat Cells / Q000378:metabolism*; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D010539:Permeability
chemical_list: D002121:Calcium Channel Blockers; D015220:Calcium Channels; D002414:Cations, Monovalent; D007477:Ions; D008274:Magnesium; D002118:Calcium