title: Polyvalent cations as permeant probes of MIC and TRPM7 pores.
abstract: Recent studies in Jurkat T cells and in rat basophilic leukemia cells revealed an Mg(2+)-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 Mg(2+) to block monovalent MIC current from the outside. Putrescine, spermidine, spermine, PhTX-343 (a derivative of the naturally occurring polyamine toxin philanthotoxin), and Mg(2+) 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 Mg(2+) are permeant blockers, as judged by relief of block at strongly negative membrane potentials. Intracellular dialysis with spermine (300 microM) had no effect, indicating an asymmetrical pore. At the single-channel level, spermine and Mg(2+) 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.
authors.affiliation: Department of Physiology and Biophysics, University of California, Irvine, California 92697-4561 USA.
authors.forename: Hubert H
authors.initials: HH
authors.lastname: Kerschbaum
authors.forename: J Ashot
authors.initials: JA
authors.lastname: Kozak
authors.forename: Michael D
authors.initials: MD
authors.lastname: Cahalan
mesh_terms: D000818:Animals; D016466:CHO Cells / Q000187:drug effects / Q000502:physiology; D015220:Calcium Channels / Q000145:classification / Q000187:drug effects* / Q000502:physiology*; D027682:Cation Transport Proteins / Q000037:antagonists & inhibitors* / Q000145:classification / Q000502:physiology*; D002412:Cations; D002460:Cell Line; D045744:Cell Line, Tumor; D002463:Cell Membrane Permeability / Q000187:drug effects; D006224:Cricetinae; D004305:Dose-Response Relationship, Drug; D004553:Electric Conductivity; D006801:Humans; D015640:Ion Channel Gating / Q000187:drug effects*; D007473:Ion Channels; D019169:Jurkat Cells / Q000187:drug effects / Q000502:physiology; D015471:Leukemia, Basophilic, Acute / Q000378:metabolism / Q000503:physiopathology; D008274:Magnesium / Q000494:pharmacology*; D008564:Membrane Potentials / Q000187:drug effects; D008565:Membrane Proteins; D010636:Phenols / Q000494:pharmacology; D011073:Polyamines / Q000494:pharmacology*; D011700:Putrescine / Q000494:pharmacology; D051381:Rats; D013095:Spermidine / Q000494:pharmacology; D013096:Spermine / Q000494:pharmacology; D050053:TRPM Cation Channels
chemical_list: D015220:Calcium Channels; D027682:Cation Transport Proteins; D002412:Cations; D007473:Ion Channels; D008565:Membrane Proteins; D010636:Phenols; D011073:Polyamines; D050053:TRPM Cation Channels; C121934:TRPM2 protein, human; C074003:philanthotoxin 343; D013096:Spermine; D008274:Magnesium; D013095:Spermidine; D011700:Putrescine