Evidence for multiple antiarrhythmic binding sites on the cardiac rapidly activating delayed rectifier K+ channel

We have previously shown that [3H]dofetilide binds with high affinity to sites associated with the guinea pig cardiac rapidly activating delayed rectifier K+ (IKr) channel and that class III antiarrhythmic agents, including dofetilide, clofilium, quinidine, sotalol, and sematilide, competitively displace [3H]dofetilide with IC50 values that correlate with those for blockade of the IKr channel. In this report, we show that other class III antiarrhythmic agents, namely, E-4031 (1-[2-(6-methyl-2-pyridyl)ethyl]-4-(4-methylsulfonylamidobenzoyl)piperidine) and L-691, 121 (3,4-dihydro-1′-[2-(benzofurazan-5-yl)ethyl]-6-methanesfulonamidospiro[(2H)-1-benzopyran-2,4′-piperidin]-4-one), potently block guinea pig IKr channels with respective IC50 values of 29 and 8 nM, yet have a low potency for displacement of [3H]dofetilide. Moreover, WIN 61773-2 [(R)(+)-4,5-dihydro-4-methyl-1-phenyl-3(2-phenylethyl)-(1H)-2,4-benzodiazepine monohydrochloride] biphasically displaces [3H]dofetilide according to a two site competitive binding model (site 1 = 21% displacement, IC50 = 116 nM; site 2 = 79% displacement, IC50 = 50 mM) with correlation to IKr block in the first phase (IC50 = 92 nM). These findings suggest that E--4031, L-691, 121, and WIN 61773-2 inhibit IKr channels by interacting at sites distinct from the high affinity [3H]dofetilide binding site. The partial displacement of [3H]dofetilide by low concentrations of WIN 61773-2, correlated with complete block of IKr, suggests allosteric modulation of the dofetilide binding site by this agent. ©1995 Wiley-Liss, Inc.