Sodium and potassium currents in acutely demyelinated internodes of rabbit sciatic nerves.

1. Voltage-clamp experiments were performed on single internodes isolated from rabbit sciatic nerve fibres acutely demyelinated with the detergent lysolecithin or a synthetic analogue, lysophosphatidyl choline palmitoyl. 2. The extent of demyelination was monitored by a gradual increase in the internodal leak conductance and capacitance. Voltage- and time-dependent inward and outward currents, absent during the early phase (30-40 min) of detergent treatment, appeared during the final phase (40-60 min) of treatment. 3. The internodal ionic currents elicited by depolarizations consisted of three components pharmacologically identified as (a) a transient sodium current which was inhibited by tetrodotoxin, (b) a delayed rectifying potassium current which was inhibited by internal caesium and (c) a time-dependent current that was abolished by replacement of external chloride with ascorbate. 4. The current-voltage relations and h infinity curves for the internodal sodium current were similar in shapes to those of the nodal sodium current. 5. The amplitudes of the three internodal currents increased with the increase in the measured internodal capacity during the final phase of demyelination. 6. At high degrees of demyelination a peak sodium current of about 90 nA could be observed in an internodal segment of 100 micron length. 7. Interestingly, the membrane capacity measured at the time of such a large sodium current was about 10 times larger than could be accounted for by the axonal membrane in the recording pool alone. A suggestion is made that this represents lysolecithin-induced membrane fusion between the Schwann cell and the internodal axon.