Characteristics of sodium tail currents in Myxicola axons. Comparison with membrane asymmetry currents.

Sodium currents after repolarization to more negative potentials after initial activation were digitally recorded in voltage-clamped Myxicola axons compensated for series resistance. The results are inconsistent with a Hodgkin-Huxley-type kinetic scheme. At potentials more negative than -50 mV, the Na+ tails show two distinct time constants, while at more positive potentials only a single exponential process can be resolved. The time-course of the tail currents was totally unaffected when tetrodotoxin (TTX) was added to reduce gNa to low values, demonstrating the absence of any artifact dependent on membrane current. Tail currents were altered by [Ca++] in a manner consistent with a simple alteration in surface potential. Asymmetry current "off" responses are well described by a single exponential. The time constant for this response averaged 2.3 times larger than that for the rapid component of the Na+ repolarization current and was not sensitive to pulse amplitude or duration, although it did vary with holding potential. Other asymmetry current observations confirm previous reports on Myxicola.