Monovalent Permeability, Rectification, and Ionic Block of Store-operated Calcium Channels in Jurkat T Lymphocytes

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 Ca(2+), 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(+), P (X)/P (Na), in order to estimate the diameter of the conducting pore. Ammonium (NH(4) (+)) exhibited the highest relative permeability (P (NH4)/P (Na )= 1.37). The largest permeant ion, tetramethylammonium with a diameter of 0.55 nm, had P (TMA)/P (Na) of 0.09. N-methyl-d-glucamine (0.50 × 0.64 × 1.20 nm) was not measurably permeant. In addition to carrying monovalent current, NH(4) (+) reduced the slow decline of monovalent current (“inactivation”) upon lowering [Ca(2+)](o). This kinetic effect of extracellular NH(4) (+) can be accounted for by an increase in intracellular pH (pH(i)), since raising intracellular pH above 8 reduced the extent of inactivation. In addition, decreasing pH(i) reduced monovalent and divalent current amplitudes through CRAC channels with a pK(a) of 6.8. In several channel types, Mg(2+) has been shown to produce rectification by a voltage-dependent block mechanism. Mg(2+) 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 Mg(2+) contributes to inward rectification by blocking in a voltage-dependent manner, with a zδ product of 1.88. Ca(2+) block from the outside was also found to be voltage dependent with zδ of 1.62. These experiments indicate that the CRAC channel, like voltage-gated Ca(2+) channels, achieves selectivity for Ca(2+) by selective binding in a large pore with current–voltage characteristics shaped by internal Mg(2+).