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J. Gen. Physiol.,
Volume 112, Number 4, October 1, 1998 475-484
From the Department of Pharmacology and Physiology, University of Rochester Medical Center, Rochester, New York 14642
We measured unidirectional K+ in- and efflux through an inward rectifier K channel (IRK1) expressed in Xenopus oocytes. The ratio of these unidirectional fluxes differed significantly from expectations based
on independent ion movement. In an extracellular solution with a K+ concentration of 25 mM, the data were described by a Ussing flux-ratio exponent, n', of ~2.2 and was constant over a voltage range from 
50 to 25 mV.
This result indicates that the pore of IRK1 channels may be simultaneously occupied by at least three ions. The
IRK1 n' value of 2.2 is significantly smaller than the value of 3.5 obtained for Shaker K channels under identical
conditions. To determine if other permeation properties that reflect multi-ion behavior differed between these
two channel types, we measured the conductance (at 0 mV) of single IRK1 channels as a function of symmetrical
K+ concentration. The conductance could be fit by a saturating hyperbola with a half-saturation K+ activity of 40 mM, substantially less than the reported value of 300 mM for Shaker K channels. We investigated the ability of simple permeation models based on absolute reaction rate theory to simulate IRK1 current-voltage, conductance,
and flux-ratio data. Certain classes of four-barrier, three-site permeation models are inconsistent with the data, but
models with high lateral barriers and a deep central well were able to account for the flux-ratio and single channel
data. We conclude that while the pore in IRK1 and Shaker channels share important similarities, including K+ selectivity and multi-ion occupancy, they differ in other properties, including the sensitivity of pore conductance to
K+ concentration, and may differ in the number of K+ ions that can simultaneously occupy the pore: IRK1 channels may contain three ions, but the pore in Shaker channels can accommodate four or more ions.
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