Constitutive expression of a Mg2+ - inhibited K+ current and a TRPM7-like current in human erythroleukemia cells

Mason MJ, Schaffner C, Floto RA, Teo QA. Constitutive expression of a Mg2+ -inhibited K+ current and a TRPM7-like current in human erythroleukemia cells. Am J Physiol Cell Physiol 302: C853-C867, 2012. First published November 30, 2011; doi: 10.1152/ajpcell.00071.2011.Whole cell patch-clamp experiments were undertaken to define the basal K+ conductance(s) in human erythroleukemia cells and its contribution to the setting of resting membrane potential. Experiments revealed a non-voltage-activated, noninactivating K+ current. The magnitude of the current recorded under whole cell conditions was inhibited by an increase in free intracellular Mg2+ concentration. Activation or inactivation of the Mg2+ -inhibited K+ current (MIP) was paralleled by activation or inactivation of a Mg2+ -inhibited TRPM7-like current displaying characteristics indistinguishable from those reported for molecularly identified TRPM7 current. The MIP and TRPM7 currents were inhibited by 5-lipoxygenase inhibitors. However, inhibition of the MIP current was temporally distinct from inhibition of TRPM7 current, allowing for isolation of the MIP current. Isolation of the MIP conductance revealed a current reversing near the K+ equilibrium potential, indicative of a highly K+ -selective conductance. Consistent with this finding, coactivation of the nonselective cation current TRPM7 and the MIP current following dialysis with nominally Mg2+ -free pipette solution resulted in hyperpolarized whole cell reversal potentials, consistent with an important role for the MIP current in the setting of a negative resting membrane potential. The MIP and TRPM7-like conductances were constitutively expressed under in vivo conditions of intracellular Mg2+, as judged by their initial detection and subsequent inactivation following dialysis with a pipette solution containing 5 mM free Mg2+. The MIP current was blocked in a voltage-dependent fashion by extracellular Cs+ and, to a lesser degree, by Ba2+ and was blocked by extracellular La3+ and 2-aminoethoxydiphenyl borate. MIP currents were unaffected by blockers of ATP-sensitive K+ channels, human ether-a-go-go-related gene current, and intermediate-conductance Ca2+ -activated K+ channels. In addition, the MIP current displayed characteristics distinct from conventional inwardly rectifying K+ channels. A similar current was detected in the leukemic cell line CHRF-288-11, consistent with this current being more generally expressed in cells of leukemic origin.