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Biophys J
2008 Feb 15;944:1241-51. doi: 10.1529/biophysj.107.117796.
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Mechanism of the modulation of Kv4:KChIP-1 channels by external K+.
Kaulin YA
,
De Santiago-Castillo JA
,
Rocha CA
,
Covarrubias M
.
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In response to a prolonged membrane depolarization, inactivation autoregulates the activity of voltage-gated ion channels. Slow inactivation involving a localized constriction of the selectivity filter (P/C-type mechanism) is prevalent in many voltage-gated K(+) channels of the Kv1 subfamily. However, the generalization of this mechanism to other Kv channel subfamilies has remained uncertain and controversial. In agreement with a "foot-in-the-door" mechanism and the presence of ion-ion interactions in the pore, elevated external K(+) slows the development of P/C-type inactivation and accelerates its recovery. In sharp contrast and resembling the regulation of the hippocampal A-type K(+) current, we found that Kv4.x channels associated with KChIP-1 (an auxiliary subunit) exhibit accelerated inactivation and unaffected recovery from inactivation when exposed to elevated external K(+). This regulation depends on the ability of a permeant ion to enter the selectivity filter (K(+) = Rb(+) = NH4(+) > Cs(+) > Na(+)); and the apparent equilibrium dissociation constant of a single regulatory site is 8 mM for K(+). By applying a robust quantitative global kinetic modeling approach to all macroscopic properties over a 210-mV range of membrane potentials, we determined that elevated external K(+) inhibits unstable closed states outside the main activation pathway and thereby promotes preferential closed-state inactivation. These results suggest the presence of a vestigial and unstable P/C-type mechanism of inactivation in Kv4 channels and strengthen the concept of novel mechanisms of closed-state inactivation. Regulation of Kv4 channel inactivation by hyperkalemia may help to explain the pathophysiology of electrolyte imbalances in excitable tissues.
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