title: Multiple ion binding sites in Ih channels of rod photoreceptors from tiger salamanders.
abstract: The mechanism of ion permeation in K+/Na(+)-permeable Ih channels of tiger salamander rod photoreceptors was investigated using the whole-cell voltage-clamp technique. Ih channels showed features indicative of pores with multiple ion binding sites: in mixtures of K+ and thallium (T1+), the amplitude of the time-dependent current showed an anomalous mole fraction dependence, and K+ permeation was blocked by other permeant ions (with K0.5 values: T1+, 44 microM; Rb+, 220 microM and NH4+, 1100 microM) as well as by essentially impermeant ions (Cs+, 22 microM Ba2+, 9200 microM) which apparently block Ih by binding in the pore. In contrast, Na+ had little blocking action on K+ permeation. The block by all of these ions was sensitive to external K+ with the block by Cs+ being the least sensitive. Na+ was more effective than K+ in reducing the block by T1+, Rb+ and NH4+, but was less effective for the block by Cs+ and Ba2+. The blocking action of Cs+ and Ba2+ was non-competitive, suggesting that they block Ih channels at independent sites. Based on the efficacy of block by the different ions, the degree to which K+ and Na+ antagonize this block and the noncompetitive blocking action of Cs+ and Ba2+, the permeation pathway of Ih channels appears to contain at least three ion binding sites with at least two sites having a higher affinity for K+ over Na+ and another site with a higher affinity for Na+ over K+.
authors.affiliation: Department of Physiology and Biophysics, University of Washington School of Medicine, Seattle 98195, USA.
authors.forename: L P
authors.initials: LP
authors.lastname: Wollmuth
mesh_terms: D000557:Ambystoma; D000818:Animals; D001464:Barium / Q000378:metabolism; D001665:Binding Sites; D001667:Binding, Competitive; D002586:Cesium / Q000378:metabolism; D066298:In Vitro Techniques; D015640:Ion Channel Gating; D007473:Ion Channels / Q000378:metabolism*; D017136:Ion Transport; D008564:Membrane Potentials; D018408:Patch-Clamp Techniques; D010539:Permeability; D011188:Potassium / Q000378:metabolism; D000644:Quaternary Ammonium Compounds / Q000378:metabolism; D017948:Retinal Rod Photoreceptor Cells / Q000378:metabolism* / Q000502:physiology; D012413:Rubidium / Q000378:metabolism; D012964:Sodium / Q000378:metabolism; D013793:Thallium / Q000378:metabolism
chemical_list: D007473:Ion Channels; D000644:Quaternary Ammonium Compounds; D002586:Cesium; D001464:Barium; D012964:Sodium; D013793:Thallium; D012413:Rubidium; D011188:Potassium