title: Low affinity block of native and cloned hyperpolarization-activated Ih channels by Ba2+ ions.
abstract: Ba2+ is commonly used to discriminate two classes of ion currents. The classical inward-rectifying K+ current, I(Kir), is blocked by low millimolar concentrations of Ba2+, whereas the hyperpolarization-activated cation current, I(h), is assumed not to be sensitive to Ba2+. Here we investigated the effects of Ba2+ on I(h) currents recorded from rat hippocampal CA1 pyramidal neurons, and on cloned I(h) channels composed of either HCN1 or HCN2 subunits transiently expressed in Human Embryonic Kidney (HEK) 293 cells. The results show that low millimolar concentrations of Ba2+ reduce the maximal I(h) conductance (IC50 approximately 3-5 mM) in both CA1 pyramidal neurons and in HEK 293 cells without specificity for HCN1 or HCN2 subunits. In addition, Ba2+ decreases the rate of activation and increases the rate of deactivation of I(h) currents. Neither the half-maximal voltage of activation, V(h), nor the reversal potential of the I(h) channels were affected by Ba2+. The combined results suggest that B2+, at concentrations commonly used to block I(Kir) currents, also reduces the conductance of I(h) channels without subunit specificity, and affects the kinetics of I(h) channel gating.
authors.affiliation: Swammerdam Institute for Life Sciences, Section Neurobiology, University of Amsterdam, P.O. Box 94084, 1090 GB Amsterdam, The Netherlands.
authors.forename: Ingrid
authors.initials: I
authors.lastname: van Welie
authors.forename: Wytse J
authors.initials: WJ
authors.lastname: Wadman
authors.forename: Johannes A
authors.initials: JA
authors.lastname: van Hooft
mesh_terms: D000818:Animals; D001464:Barium / Q000494:pharmacology*; D002460:Cell Line; D003001:Cloning, Molecular / Q000379:methods*; D054815:Cyclic Nucleotide-Gated Cation Channels; D006801:Humans; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D007473:Ion Channels / Q000037:antagonists & inhibitors* / Q000502:physiology; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D015221:Potassium Channels; D051381:Rats; D017208:Rats, Wistar
chemical_list: D054815:Cyclic Nucleotide-Gated Cation Channels; C578665:HCN1 protein, human; C578710:HCN2 protein, human; C578667:Hcn1 protein, rat; C578712:Hcn2 protein, rat; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D007473:Ion Channels; D015221:Potassium Channels; D001464:Barium