Identification of a Specific Radioligand for the Cardiac Rapidly Activating Delayed

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 [3H]dofetilide could be used as a specific radioligand for the rapidly activating delayed rectifier K ' channel of the heart. We find that [3H]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 Kd of 70±6 nM and a maximal binding capacity of 0.30±0.02 pmol/mg protein. [3H]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 coefflicient, 0.951; slope, 0.99±0.19; p=0.014). High-affinity [3H]dofetilide binding is not detected in rat myocytes, which are devoid of delayed rectifier K ' current. We conclude that [3H]dofetilide specifically binds to sites associated with the rapidly activating delayed rectifier K ' channel of guinea pig myocardium. (Circulation Research 1993;72:707-714) KEY WoRDs * dofetilide * radioligands * delayed rectifier K ' channels * antiarrhythmic agents A ntiarrhythmic therapy has focused on modulatio