Class I antiarrhythmic drug receptor: biochemical evidence for state-dependent interaction with quinidine and...

The state-dependent binding of class I antiarrhythmic drugs to a receptor associated with the cardiac sodium channel was assessed using [3H]batrachotoxinin A 20-alpha-benzoate [( 3H]BTXB) binding. [3H]BTXB binds specifically to and stabilizes activated states of the sodium channel. Quinidine (IC50 = 40 microM) and lidocaine [IC50 = 61 microM) inhibited equilibrium [3H]BTXB binding to sodium channels present on freshly isolated rat cardiac myocytes. Scatchard analysis of [3H]BTXB binding in the presence of quinidine and lidocaine revealed two apparent patterns of inhibition. Quinidine (33 microM) increased the KD but had no significant effect on the Bmax, whereas lidocaine (91 microM) reduced the Bmax but had no significant effect on the KD. To address drug binding to activated and nonactivated states, we exploited the state-specific binding of [3H]BTXB. Drugs that increase the rate of dissociation (k-1) of [3H]BTXB must bind to sodium channels to which [3H]BTXB is already bound (i.e., activated channels). Therefore, drug-mediated increases in k-1 measure drug binding to activated states. Both quinidine and lidocaine increased the k-1 of [3H]BTXB, indicating drug binding to and destablization of activated sodium channels. However, the minimal affinities of quinidine and lidocaine for activated channels (KDact) were estimated to be 433 and 455 microM, respectively, concentrations much higher than the equilibrium IC50 values. Drugs that allosterically decrease the rate of association (k+1) of [3H]BTXB must bind to sodium channels to which [3H]BTXB is not already bound (i.e., nonactivated channels). Therefore, drug-mediated decreases in k+1 measures drug binding to nonactivated states. Quinidine and lidocaine decreased the k+1 of [3H]BTXB, indicating drug binding to and stablization of nonactivated sodium channels. The affinity of quinidine and lidocaine for nonactivated channels (KDnon) was estimated to be 10 and 35 microM, respectively, concentrations close to the equilibrium IC50 values. The markedly different KDact and KDnon values for both quinidine and lidocaine indicate state-dependent binding of quinidine and lidocaine to the class I receptor on the cardiac sodium channel. Both drugs destabilize activated channels and stabilize nonactivated channels. The Scatchard results suggest that quinidine and lidocaine may have different mechanisms of allosteric inhibition of [3H]BTXB binding.