External pore collapse as an inactivation mechanism for Kv4.3 K+ channels.
Kv4 channels are thought to lack a C-type inactivation mechanism (collapse of the external pore) and to inactivate as a result of a concerted action of cytoplasmic regions of the channel. To investigate whether Kv4 channels have outer pore conformational changes during the inactivation process, the inactivation properties of Kv4.3 were characterized in 0 mM and in 2 mM external K+ in whole-cell voltage-clamp experiments. Removal of external K+ increased the inactivation rates and favored cumulative inactivation by repetitive stimulation. The reduction in current amplitude during repetitive stimulation and the faster inactivation rates in 0 mM external K+ were not due to changes in the voltage dependence of channel opening or to internal K+ depletion. The extent of the collapse of the K+ conductance upon removal of external K+ was more pronounced in NMG+-than in Na+-containing solutions. The reduction in the current amplitude during cumulative inactivation by repetitive stimulation is not associated with kinetic changes, suggesting that it is due to a diminished number of functional channels with unchanged gating properties. These observations meet the criteria for a typical C-type inactivation, as removal of external K+ destabilizes the conducting state, leading to the collapse of the pore. A tentative model is presented, in which K+ bound to high-affinity K+-binding sites in the selectivity filter destabilizes an outer neighboring K+ modulatory site that is saturated at approximately 2 mM external K+. We conclude that Kv4 channels have a C-type inactivation mechanism and that previously reported alterations in the inactivation rates after N- and C- termini mutagenesis may arise from secondary changes in the electrostatic interactions between K+-binding sites in the selectivity filter and the neighboring K+-modulatory site, that would result in changes in its K+ occupancy.
Eghbali
M
M
Department of Anesthesiology, David Geffen School of Medicine, University of California Los Angeles, Los Angeles, CA 90095-7155, USA.
Olcese
R
R
Zarei
M M
MM
Toro
L
L
Stefani
E
E
D002460:Cell Line; D051668:Delayed Rectifier Potassium Channels; D004553:Electric Conductivity; D006801:Humans; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology*; D007668:Kidney / Q000196:embryology; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D008954:Models, Biological; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000378:metabolism* / Q000494:pharmacology; D026902:Potassium Channel Blockers / Q000494:pharmacology; D015221:Potassium Channels / Q000187:drug effects / Q000502:physiology*; D024642:Potassium Channels, Voltage-Gated; D051672:Shal Potassium Channels
D051668:Delayed Rectifier Potassium Channels; C497021:KCND3 protein, human; D026902:Potassium Channel Blockers; D015221:Potassium Channels; D024642:Potassium Channels, Voltage-Gated; D051672:Shal Potassium Channels; D011188:Potassium