COMPARISON OF BINDING TO RAPIDLY ACTIVATING DELAYED RECTIFIER K+ CHANNEL, I-KR, AND EFFECTS ON MYOCARDIAL REFRACTORINESS FOR CLASS-III ANTIARRHYTHMIC AGENTS

Saturation binding studies in guinea pig ventricular myocytes with H-3-dofetilide, a radioligand for the cardiac rapidly activating delayed rectifier K+ I-Kr channel, indicated specific high-affinity binding with a K-d of 83 nM and a B-max of 0.18 pmol/mg cellular protein (1.36 x 10(6) sites/cell). Using displacement of high-affinity H-3-dofetilide binding as a measure of interaction with the I-Kr channel, potencies (K-i values) for binding to the I-Kr channel in guinea pig myocytes for six class III antiarrhythmic agents were characterized and compared to indices of functional electrophysiologic activity in isolated guinea pig papillary muscles [EC(25) values, concentration required to increase effective refractory period (ERP) 25% above baseline]. Dofetilide, E-4031, sematilide, and d-sotalol, which have been characterized previously as selective I-Kr blockers, displayed good agreement between K-i values for displacement of H-3-dofetilide binding (47 +/- 7 nM, 38 +/- 8 nM, 12 +/- 5 mu M, and similar to 100 mu M, respectively) and EC(25) values for increasing ERP in papillary muscles (45.0 nM, 76.9 nM, 20.2 mu M and 63.5 mu M, respectively). Ibutilide and RP58866, which have been reported to act via mechanisms other than I-Kr block, had K-i values for displacement of H-3-dofetilide binding (16 +/- 7 nM and 17 +/- 2 nM, respectively) that were similar to 10-fold lower than EC(25) values for increasing ERP in papillary muscles (185.8 nM and 223.5 nM, respectively). The potent displacement of high-affinity H-3-dofetilide binding by ibutilide and RP58866 strongly suggest a role for interaction with I-Kr in their actions. The discrepant functional activities of these agents, however, suggest a combination of effects beyond those on I-Kr and implicate modulation of Na+ or other K+ current subtypes.