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 Ca2+, but can be induced to carry a slowly declining Na curs rent when external divalent ions are reduced to micromolar levels. Using a series of organic cations as probes of var)ring 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 (NH4+) 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 x 0.64 x 1.20 nm) was not measurably permeant. In addition to carrying monovalent current, NH,+ reduced the slow decline of monovalent current ("inactivation") upon lowering [Ca2+](o). This kinetic effect of extracellular NH,+ 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 0.8. 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+.