IDENTIFICATION OF A SPECIFIC RADIOLIGAND FOR THE CARDIAC RAPIDLY ACTIVATING DELAYED RECTIFIER K+ CHANNEL

Class III antiarrhythmic drugs show promise as effective treatments for the suppression of potentially lethal cardiac arrhythmias. Dofetilide (UK-68,798), is a potent class III antiarrhythmic agent that is presently under clinical investigation. The objective of this study was to determine whether [H-3] dofetilide could be used as a specific radioligand for the rapidly activating delayed rectifier K+ channel of the heart. We find that [H-3]dofetilide binds to high-affinity sites on guinea pig cardiac myocytes. Competition studies using unlabeled dofetilide indicate that binding is characterized by an IC50 of 100+/-30 nM (mean+/-SD, n=13). Scatchard analyses of binding indicate a K(d) of 70+/-6 nM and a maximal binding capacity of 0.30+/-0.02 pmol/mg protein. [H-3]Dofetilide is displaced from guinea pig myocytes by dofetilide, clofilium, quinidine, sotalol, and sematilide with a rank order of potency that correlates with functional blockade of the rapidly activating delayed rectifier K+ current (correlation coefficient, 0.951; slope, 0.99+/-0.19; p=0.014). High-affinity [H-3]dofetilide binding is not detected in rat myocytes, which are devoid of delayed rectifier K+ current. We conclude that [H-3]dofetilide specifically binds to sites associated with the rapidly activating delayed rectifier K+ channel of guinea pig myocardium.