Coupled K+-water flux through the HERG potassium channel measured by an osmotic pulse method.
The streaming potential (V(stream)) is a signature feature of ion channels in which permeating ions and water molecules move in a single file. V(stream) provides a quantitative measure of the ion and water flux (the water-ion coupling ratio), the knowledge of which is a prerequisite for elucidating the mechanisms of ion permeation. We have developed a method to measure V(stream) with the whole-cell patch-clamp configuration. A HEK293 cell stably expressing the HERG potassium channel was voltage clamped and exposed to hyperosmotic solutions for short periods of time (<1 s) by an ultrafast solution switching system (the osmotic pulse [quick jump-and-away] method). The reversal potentials were monitored by a series of voltage ramps before, during, and after the osmotic pulse. The shifts of the reversal potentials immediately after the osmotic jump gave V(stream). In symmetrical K+ solutions (10 mM), the V(stream)s measured at different osmolalities showed a linear relationship with a slope of -0.7 mV/DeltaOsm, from which the water-ion coupling ratio (n, the ratio of the flux of water to the flux of cations; Levitt, D.G., S.R. Elias, and J.M. Hautman. 1978. Biochim. Biophys. Acta. 512:436-451) was calculated to be 1.4. In symmetrical 100 mM K+ solutions, the coupling ratio was decreased significantly (n = 0.9), indicating that the permeation process through states with increased ion occupancy became significant. We presented a diagrammatic representation linking the water-ion coupling ratio to the mode of ion permeation and suggested that the coupling ratio of one may represent the least hydrated ion flux in the single-file pore.
Ando
Hiroyuki
H
Department of Molecular Physiology and Biophysics, University of Fukui Faculty of Medical Sciences, Fukui 910-1193, Japan.
Kuno
Miyuki
M
Shimizu
Hirofumi
H
Muramatsu
Ikunobu
I
Oiki
Shigetoshi
S
D002463:Cell Membrane Permeability / Q000187:drug effects / Q000502:physiology; D002478:Cells, Cultured / Q000502:physiology; D004553:Electric Conductivity; D051638:Ether-A-Go-Go Potassium Channels / Q000502:physiology*; D006801:Humans; D008954:Models, Biological; D009994:Osmolar Concentration; D009997:Osmotic Pressure / Q000187:drug effects; D018408:Patch-Clamp Techniques / Q000379:methods; D011188:Potassium / Q000494:pharmacology* / Q000502:physiology; D015221:Potassium Channels / Q000187:drug effects* / Q000502:physiology*; D014162:Transfection; D014867:Water / Q000494:pharmacology* / Q000502:physiology
D051638:Ether-A-Go-Go Potassium Channels; C092509:KCNH6 protein, human; D015221:Potassium Channels; D014867:Water; D011188:Potassium