Determinants of Antiarrhythmic Drug Action

Abstract

Abstract The molecular basis of antiarrhythmic drug action is still poorly understood. We recently reported that block of the human cardiac hKv1.5 channel by quinidine displayed similarity with internal quaternary ammonium block of squid and Shaker potassium channels. To gain further insight into the molecular determinants of the affinity and the stereoselectivity of antiarrhythmic drug action, we studied the effects of quinine (a diastereomer of quinidine), clofilium (a quaternary ammonium class III agent), and tetrapentylammonium (TPeA, a biophysical reference probe for the internal quaternary ammonium binding site). For all compounds, block was voltage dependent, with a steep increase over the voltage range of channel opening and a superimposed weaker voltage dependence at more positive potentials. The latter electrostatic component was similar for all drugs, consistent with a binding reaction sensing ≈20% of the transmembrane electrical field. Clofilium and TPeA displayed a higher apparent affinity (0.15 and 0.28 μmol/L, respectively), and quinine displayed a lower one (21 μmol/L) compared with quinidine (6.2 μmol/L). Block development upon depolarization was time dependent for clofilium and TPeA but slow compared with quinidine. A time-dependent component was difficult to resolve for quinine, but the time course of deactivating tail currents was slower than in the control condition. The resulting crossover phenomenon was also observed for the quaternary drugs. Compared with TPeA alone, the combined application of quinine and TPeA resulted in a reduced current that decayed slower, consistent with competition. When a bimolecular model for open-channel block was used, the apparent association rate constants for these drugs were found to be similar [range, 4.5 to 7.7 (μmol/L) −1 · s −1 ]. The apparent dissociation rate constants for clofilium (1.9 s −1 ) and TPeA (3.6 s −1 ) were smaller compared with quinidine (34 s −1 ), whereas that for quinine was faster (≥125 s −1 ). This large range in dissociation rate constants could explain the differences between these drugs both in kinetics and affinity. The results are consistent with a general model in which these agents act as cationic open-channel blockers but with an affinity largely determined by the intrinsic stability of the drug-receptor complex. Hydrophobic interactions are most likely involved in this stabilization of binding.