Polyvalent Cations as Permeant Probes of MIC and TRPM7 Pores

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Recent studies in Jurkat T cells and in rat basophilic leukemia cells revealed an Mg2+-inhibited cation (MIC) channel that has electrophysiological properties similar to TRPM7 Eyring rate model expressed exogenously in mammalian cells. Here we compare the characteristics of several polyvalent cations and Mg2+ to block monovalent MIC current from the outside. Putrescine, spermidine, spermine, PhTX-343 (a derivative of the naturally occurring polyamine toxin philanthotoxin), and Mg2+ each blocked in a dose- and voltage-dependent manner, indicating a blocking site within the electric field of the ion channel. Spermine and the relatively bulky PhTX-343 exhibited voltage dependence steeper than that expected for the number of charges on the molecule. Polyamines and Mg2+ are permeant blockers, as judged by relief of block at strongly negative membrane potentials. Intracellular dialysis with spermine (300 μM) had no effect, indicating an asymmetrical pore. At the single-channel level, spermine and Mg2+ induced flickery block of 40-pS single channels. I/V characteristics and polyamine block are similar in expressed TRPM7 and in native MIC currents, consistent with the conclusion that native MIC channels are composed of TRPM7 subunits. An Eyring rate model is developed to account for I/V characteristics and block of MIC channels by polyvalent cations from the outside.