Effect of lidocaine on the slow Na+ channels of Xenopus oocytes

The membrane of immature Xenopus oocytes is known to possess a peculiar type of sodium channels, which are not activatable unless the membrane has been depolarized for some time. Once induced by a long-lasting depolarization, the channels behave like voltage-dependent channels, but they slowly activate and apparently do not inactivate. In addition, these channels were shown to be insensitive to the toxins classically used to inhibit the voltage-dependent Na+ channels. The effects of lidocaine on these slow Na+ channels were investigated using current-and voltage-clamped oocytes. Lidocaine reversibly blocked the channels when they were in their open configuration, but not when the channels were in their closed state. The concentration of lidocaine required for half-inhibition of the slow inward current was 270 +/- 67 micromol/l. The current/voltage relationships indicated that lidocaine blocked the sodium current (inward as well as outward) for all the potentials investigated. At a concentration of 0.3 mmol/l, lidocaine caused a shift of 5 +/- 1 mV of the activation curve. This suggests that the gating properties of the channels were alterated. The effect of lidocaine was found to be non-selective since at least two other channels were affected by the drug, namely the voltage-dependent calcium channels and the monovalent non-selective channels.