Regulation of Kv4.3 voltage-dependent gating kinetics by KChIP2 isoforms

We conducted a kinetic analysis of the voltage dependence of macroscopic inactivation (τ_fast, τ_slow), closed-state inactivation (τ_closed,inact), recovery (τ _rec), activation (τ_act), and deactivation (τ _deact) of Kv4.3 channels expressed alone in Xenopus oocytes and in the presence of the calcium-binding ancillary subunits KChIP2b and KChIP2d. We demonstrate that for all expression conditions, τ _rec, τ_closed,inact and τ _fast are components of closed-state inactivation transitions. The values of τ_closed,inact and τ _fas't monotonically merge from -30 to -20 mV while the values of τ_closed,inact and τ_rec approach each other from -60 to -50 mV. These data generate classic bell-shaped time-constant-potential curves. With the KChIPs, these curves are distinct from that of Kv4.3 expressed alone due to acceleration of τ _rec and slowing of τ_closed,inact and τ _fast. Only at depolarized potentials where channels open is τ_slow detectable suggesting that it represents an open-state inactivation mechanism. With increasing depolarization, KChIPs favour this open-state inactivation mechanism, supported by the observation of larger transient reopening currents upon membrane hyperpolarization compared to Kv4.3 expressed alone. We propose a Kv4.3 gating model wherein KChIP2 isoforms accelerate recovery, slow closed-state inactivation, and promote open-state inactivation. This model supports the observations that with KChIPs, closed-state inactivation transitions are [Ca²⁺]_i-independent, while open-state inactivation is [Ca²⁺]_i-dependent. The selective KChIP- and Ca²⁺-dependent modulation of Kv4.3 inactivation mechanisms predicted by this model provides a basis for dynamic modulation of the native cardiac transient outward current by intracellular Ca²⁺ fluxes during the action potential.