Block of voltage-gated potassium channels by acidic pH and Class III antiarrhythmic agents

Abstract

The goal of experiments performed in this dissertation was to determine molecular mechanisms underlying cardiac action potential prolongation, induced by acidosis and by the administration of the Class III antiarrhythmic agents clofilium and amiodarone. Kv1.2 and Kv1.5 are two voltage-gated potassium channels expressed in heart that contribute to cardiac action potential repolarization. I found that Kv1.5 channels are blocked at acidic pH while Kv1.2 channels are not. Kv1.5 channels show enhanced C-type inactivation at acidic pH, and accumulation of channels in this inactivated state likely explains pH-dependent block of the channel subtype. A histidine residue in the third extracellular loop of Kv1.5 (H452) accounts for the difference in pH-sensitivity between the Kv1.5 and Kv1.2 channels. Mutation of H452 to a neutral glutamine prevents enhanced C-type inactivation of Kv1.5 channels at acidic pH. These data provide insight into the molecular mechanisms subserving increases in cardiac action potential duration induced by acidosis. Class III antiarrhythmic agents such as clofilium and amiodarone exert their therapeutic effects by prolonging cardiac action potential duration. I found that Kv1.5 potassium channels are blocked by clofilium in a state-dependent manner, and that structural modifications of clofilium alter properties of Kv1.5 channel block. Clofilium and other quaternary amine analogs seem to bind and unbind primarily from the open channel conformation, and rapid closing at negative potentials "traps" the charged blocking agents. Conversely, a tertiary amine analog of clofilium appears to access and exit the receptor site in closed as well as open channel states, presumably because the neutral compound can escape from the closed via an alternative hydrophobic pathway inaccessible to the charged compounds. Amiodarone blocks Kv1.2 channels by shifting the voltage-dependence of channel activation to more depolarized voltages. This effect occurs at acidic but not neutral pH, and could be due to acid-induced changes in either receptor or drug conformation. In summary, results presented in this dissertation provide a novel characterization of mechanisms underlying prolongation of the cardiac action potential induced by acidosis and the administration of the Class III antiarrhythmic compounds, clofilium and amiodarone.