Molecular and Structural Basis of Resting and Use-dependent Block of Sodium Current Defined Using Disopyramide Analogues

The effects of disopyramide (Norpace) and 14 closely related structural analogues on the Na current of voltage clamped squid axons were examined to determine which physico-chemical properties and which changes in the structure of the Norpace molecule can alter the nature of its sodium channel blocking actions. Conventional voltage clamp technique for internally perfused giant axons was used. Axons were exposed to 100 microM concentrations via the internal perfusion solution, and the actions of the 15 analogues to produce resting and use-dependent block of Na current were assessed. The roles of Na ions and the activation and inactivation processes in the development of and recovery from use-dependent block of Na current induced by the Norpace analogues were also examined. The results indicate that for both mono-tertiary and bis-tertiary amines the potency to produce use-dependent block was proportional to molecular weight, whereas the correlation between potency to produce resting block and molecular weight was significant only for bis-tertiary amines. The mono- were more potent than the bis-compounds. However, comparisons between compounds having similar molecular weights and/or pKa values indicate that other factors also can influence blocking potency. For compounds within each homologous mono- or bis-tertiary amine series, hydrophobicity as estimated from log P values (P = octanol/water partition coefficient) was found to influence the potency to produce use dependent block of Na current. Use-dependent block was extant in axons internally exposed to pronase to remove the inactivation process, which indicates that inactivation is not an obligate condition for development of use-dependent block of Na current. An important role for the activation process in the development of use-dependent block of Na current is suggested by the finding that, in general, the voltage dependence of Na current activation paralleled that of use-dependent block. However, the potential dependence of use-dependent block produced by less hydrophobic but not by more hydrophobic compounds was shifted in the hyperpolarizing direction by removing Na+ from the external solution. Compounds with intermediate hydrophobicities altered the time course of Na current during its activating and inactivating phases. This finding can be explained by the kinetics of association and dissociation of drug molecules with channel receptor sites during the development and relaxation of use-dependent block rather than by postulating any major effect of drug to alter channel gating kinetics. In summary, a comprehensive study of the structure-activity relationship of the Norpace molecule was achieved and the implications of the findings with respect to several factors believed to influence drug potency for resting and use-dependent block of the Na current in squid axon are examined and discussed.