PFS-1144

[3H]EO-122, a radiolabeled class I antiarrhythmic drug, has been used to characterize a new specific binding system to rat heart membranes. The binding is saturable and competitive with unlabeled EO-122 and other antiarrhythmic drugs. In this system, [3H]EO-122 binds to two sites. Site A with an apparent Kd of 33.5 +/- 1.5 nM, Bmax of 1.05 +/- 0.15 pmol/mg protein and Hill coefficient nH = 4. Site B with an apparent Kd of 233 +/- 25 nM, Bmax equals 5.7 +/- 0.61 pmol/mg proteins and nH = 6. The binding to site B indicates that this site is pharmacologically relevant to known class IA antiarrhythmic drugs such as quinidine and procainamide. Lidocaine (class IB) does not interact with this site. Interpretation of the high Hill coefficient suggests that the binding of an antiarrhythmic drug to its pharmacologically relevant binding site exposes additional binding sites and/or modulates the affinity of adjacent binding sites.

The molecular properties of quinidine, EO 122, and lidocaine were investigated using theoretical methods (Molecular Modeling). The binding pattern of the molecules were investigated by calculating interaction energies with a negative charged fragment (receptor model). Based on these calculations a model for the differentiation of class Ia and class Ib antiarrhythmic drugs could be deduced. The results explain data which were formulated in the modulated receptor hypothesis. In this way the molecular basis for the preferred affinity of quinidine to the open state of the sodium channel as well as the equal affinity of lidocaine to the open and inactivated state of the channel were defined.