NONINDEPENDENT K+ MOVEMENT THROUGH THE PORE IN IRK1 POTASSIUM CHANNELS

We measured unidirectional K+ in- and efflux through an inward rectifi er K channel (IRK1) expressed in Xenopus oocytes. The ratio of these u nidirectional fluxes differed significantly from expectations based on independent ion movement. In an extracellular solution with a K+ conc entration of 25 mM, the data were described by a Ussing flux-ratio exp onent, n', of similar to 2.2 and was constant over a voltage range fro m -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' val ue of 2.2 is significantly smaller than the value of 3.5 obtained for Shaker K channels under identical conditions. To determine if other pe rmeation properties that reflect multi-ion behavior differed between t hese two channel types, we measured the conductance (at 0 mV) of singl e IRK1 channels as a function of symmetricals K+ concentration. The co nductance could be fit by a saturating hyperbola with a half-saturatio n K+ activity of 40 mM, substantially less than the reported value of 300 mM for Shaker Ii channels. We investigated the ability of simple p ermeation models based on absolute reaction rate theory to simulate IR K1 current-voltage, conductance, and flux-ratio data. Certain classes of four-barrier, three-site permeation models are inconsistent with th e data, but models with high lateral barriers and a deep central well were able to account for the nux-ratio and single channel data. We con clude that while the pore in IRK1 and Shaker channels share important similarities, including K+ selectivity and multi-ion occupancy, they d iffer in other properties, including the sensitivity of pore conductan ce to K+ concentration, and may differ in the number of K+ ions that c an simultaneously occupy the pore: IRK1 channels may contain three ion s, but the pore in Shaker channels can accommodate four or more ions.