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

Abstract

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 (FK1A2-146) 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 FK1A2-146 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 FK1A2-146 without altering the rate of inactivation of either clone. 4. The rates of recovery from inactivation are nearly identical in FK1 and FK1A2-146. 5. To assess the mechanisms underlying recovery from inactivation the effects of elevated [K+]. and selective mutations in the extracellular pore and the S4 voltage sensor were compared in FK1 and FK1A2-146. 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 . 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.

A quantitative analysis of the activation and inactivation kinetics of HERG expressed in Xenopus oocytes

“…This 'bell-shaped' voltage dependence is distinct from both N-type and C-type inactivation mechanisms described for voltage-gated K¤ channels. In both Shaker and Kv1.4 channels C-type inactivation is a voltage-insensitive process at positive potentials (Hoshi et al 1990;Rasmusson et al 1995). Any apparent voltage dependence arises from direct coupling to activation.…”

“…(2) The rate of development of C_type inactivation is voltage insensitive at potentials positive to the threshold for activation (Hoshi et al 1990;Rasmusson et al 1995); in contrast HERG inactivation is voltage sensitive at such potentials. (3) Shaker and Kv1.4 mutant channels that have a fast rate of development of C_type inactivation usually have a slow rate of recovery (Hoshi et al 1990; LopezBarneo, Hoshi, Heinemann & Aldrich, 1993;; HERG has both a rapid development of C-type inactivation and a rapid recovery from inactivation.…”

“…Our results suggest that inactivation of HERG has a unique intrinsic voltage sensor and a [K¤]ï dependence. These properties differentiate inactivation of HERG from classic C-type inactivation, which is generally voltage insensitive and in which recovery is accelerated by increased [K¤]ï (Hoshi, Zagotta & Aldrich, 1990;Rasmusson, Morales, Castellino, Zhang, Campbell & Strauss, 1995). This suggests that the conformational changes which result in C-type inactivation involve more than superficial external sites which lie outside the membrane field and may involve significant movement of the membrane-spanning domains during C-type inactivation.…”

Effects of diltiazem and propafenone on the inactivation and recovery kinetics of fKv1.4 channel currents expressed in Xenopus oocytes

“…N-type inactivation results from the occlusion of the intracellular side of the pore by a "ball and chain" mechanism formed by the NH 2 terminus of the channel molecule [4][5][6][7][8][9] , while C-type inactivation involves conformational changes on the extracellular side of the pore [10] . These two mechanism are coupled [5] ; C-type inactivation is more rapid in the presence of N-type inactivation [11] and can be affected by open channel blockers. Recovery from inactivation is controlled by the slower C-type mechanism [11] , which makes it physiologically important.…”

“…These two mechanism are coupled [5] ; C-type inactivation is more rapid in the presence of N-type inactivation [11] and can be affected by open channel blockers. Recovery from inactivation is controlled by the slower C-type mechanism [11] , which makes it physiologically important.…”