Block of voltage-operated sodium channels by 2,6-dimethylphenol, a structural analogue of lidocaine's aromatic tail.

1. The structural features that determine the state-dependent interaction of local anaesthetics with voltage-operated sodium channels are still a matter of debate. We have studied the blockade of sodium channels by 2,6-dimethylphenol, a phenol derivative which resembles the aromatic tail of lidocaine, etidocaine, and bupivacaine. 2. The effects of 2,6-dimethylphenol were studied on heterologously (HEK 293) expressed rat neuronal (rat brain IIA) and human skeletal muscle (hSkM1) sodium channels using whole-cell voltage-clamp experiments. 3. 2,6-Dimethylphenol was effective in blocking whole-cell sodium inward currents. Its potency was comparable to the potency of lidocaine previously obtained with similar protocols by others. The IC(50) at -70 mV holding potential was 150 and 187 microM for the skeletal muscle and the neuronal isoform, respectively. In both isoforms, the blocking potency increased with the fraction of inactivated channels at depolarized holding potentials. However, the block achieved at -70 mV with respect to -150 mV holding potential was significantly higher only in the skeletal muscle isoform. The estimated dissociation constant K(d) from the inactivated state was 25 microM and 28 microM in the skeletal muscle and the neuronal isoform, respectively. The kinetics of drug equilibration between resting and inactivated channel states were about 10 fold faster compared with lidocaine. 4. Our results show that the blockade induced by 2,6-dimethylphenol retains voltage-dependency, a typical feature of lidocaine-like local anaesthetics. This is consistent with the hypothesis that the 'aromatic tail' determines the state-dependent interaction of local anaesthetics with the sodium channel.