Interaction of Local Anesthetics with the K%2B Channel Pore Domain: KcsA as a Model for Drug-Dependent Tetramer Stability .

Voltage-gated Na channels (Nav channels) are recognized as the primary target of a large number of diverse local anesthetic (LA) drugs including tertiary amines such as lidocaine and tetracaine that suppress firing of action potentials. Likewise, LA drugs are important anti-arrhythmic, anti-epileptic and anti-myotonic agents used to stabilize abnormal conditions of hyperexcitability by blocking particular protein products(s) of the nine gene isoforms of human Nav channels. However, many tetrameric K channels are also blocked by hydrophobic organic cations such as alkyl derivatives of tetraethylammonium (TEA) and amine derivatives with LA activity. Block of certain drug-sensitive K channels may induce or suppress abnormal symptoms of electrical excitability. For example, a form of potentially fatal ventricular fibrillation known as acquired long-QT syndrome is triggered by promiscuous drug block of the human hERG (Kv11.1) cardiac channel that mediates repolarization of the ventricular action potential. In contrast, drugs that block the human Kv1.5 channel may be useful in suppressing atrial fibrillation. Local anesthetics and related drugs block ionic currents of Na+, K+ and Ca2+ conducted across the cell membrane by voltage-dependent ion channels. Many of these drugs bind in the permeation pathway, occlude the pore and stop ion movement. However channel-blocking drugs have also been associated with decreased membrane stability of certain tetrameric K+ channels, similar to the destabilization of channel function observed at low extracellular K+ concentration. Such drugand K+-dependent stability may result from electrostatic repulsion of K+ from the selectivity filter by a cationic drug molecule bound in the central cavity of the channel. In this study we used the pore domain of the KcsA K+ channel protein to test this hypothesis experimentally with a biochemical assay of tetramer stability and theoretically by computational simulation of local anesthetic docking to the central cavity. We find that two common local anesthetics, lidocaine and tetracaine, promote thermal dissociation of the KcsA tetramer in a K+-dependent fashion. Docking simulations of these drugs with open, open-inactivated and closed crystal structures of KcsA yield many energetically favorable drug-channel complexes characterized by nonbonded interactions with pore-lining residues and electrostatic repulsion of K+. The results suggest that binding of cationic drugs to the inner cavity can reduce tetramer stability of K+ channels. Interaction of local anesthetics with the K+ channel pore domain KcsA as a model for drug-dependent tetramer stability