Inactivation of voltage-gated cardiac K⁺ channels

Inactivation is the process by which an open channel enters a stable nonconducting conformation after a depolarizing change in membrane potential. Inactivation is a widespread property of many different types of voltage- gated ion channels. Recent advances in the molecular biology of K⁺ channels have elucidated two mechanistically distinct types of inactivation, N-type and C-type. N-type inactivation involves occlusion of the intracellular mouth of the pore through binding of a short segment of residues at the extreme N- terminal. In contrast to this 'tethered ball' mechanism of N-type inactivation, C-type inactivation involves movement of conserved core domain residues that result in closure of the external mouth of the pore. Although C-type inactivation can show rapid kinetics that approach those observed for N-type inactivation, it is often thought of as a slowly developing and slowly recovering process. Current models of C-type inactivation also suggest that this process involves a relatively localized change in conformation of residues near the external mouth of the permeation pathway. The rate of C- type inactivation and recovery can be strongly influenced by other factors, such as N-type inactivation, drug binding, and changes in [K⁺](o). These interactions make C-type inactivation an important biophysical process in determining such physiologically important properties as refractoriness and drug binding. C-type inactivation is currently viewed as arising from small- scale rearrangements at the external mouth of the pore. This review will examine the multiplicity of interactions of C-type inactivation with N- terminal-mediated inactivation and drug binding that suggest that our current view of C-type inactivation is incomplete. This review will suggest that C- type inactivation must involve larger-scale movements of transmembrane- spanning domains and that such movements contribute to the diversity of kinetic properties observed for C-type inactivation.