C‐type inactivation controls recovery in a fast inactivating cardiac K+ channel (Kv1.4) expressed in Xenopus oocytes.

1. A fast inactivating transient K+ current (FK1) cloned from ferret ventricle and expressed in Xenopus oocytes was studied using the two‐electrode voltage clamp technique. Removal of the NH2‐terminal domain of FK1 (FK1 delta 2‐146) removed fast inactivation consistent with previous findings in Kv1.4 channels. The NH2‐terminal deletion mutation revealed a slow inactivation process, which matches the criteria for C‐type inactivation described for Shaker B channels. 2. Inactivation of FK1 delta 2‐146 at depolarized potentials was well described by a single exponential process with a voltage‐insensitive time constant. in the range ‐90 to +20 mV, steady‐state C‐type inactivation was well described by a Boltzmann relationship that compares closely with inactivation measured in the presence of the NH2‐terminus. These results suggest that C‐type inactivation is coupled to activation. 3. The coupling of C‐type inactivation to activation was assessed by mutation of the fourth positively charged residue (arginine 454) in the S4 voltage sensor to glutamine (R454Q). This mutation produced a hyperpolarizing shift in the inactivation relationship of both FK1 and FK1 delta 2‐146 without altering the rate of inactivation of either clone. 4. The rates of recovery from inactivation are nearly identical in FK1 and FK1 delta 2‐146. 5. To assess the mechanisms underlying recovery from inactivation the effects of elevated [K+]o and selective mutations in the extracellular pore and the S4 voltage sensor were compared in FK1 and FK1 delta 2‐146. The similarity in recovery rates in response to these perturbations suggests that recovery from C‐type inactivation governs the overall rate of recovery of inactivated channels for both FK1 and FK1 delta 2‐146. 6. Analysis of the rate of recovery of FK1 channels for inactivating pulses of different durations (70‐2000 ms) indicates that recovery rate is insensitive to the duration of the inactivating pulse.