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

Abstract

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-à-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.