Evidence that action potentials activate an internodal potassium conductance in lizard myelinated axons.

Abstract

1. We have studied action potentials and after‐potentials evoked in the internodal region of visualized lizard intramuscular nerve fibres by stimulation of the proximal nerve trunk. Voltage recordings were obtained using microelectrodes inserted into the axon (intra‐axonal) or into the layers of myelin (peri‐internodal), with the goal of studying conditions required to activate internodal K+ currents. 2. Peri‐internodal recordings made using K2SO4‐, KCl‐ or NaCl‐filled electrodes exhibited a negligible resting potential (less than 2 mV), but showed action potentials with peak amplitudes of up to 78 mV and a duration less than or equal to that of the intra‐axonally recorded action potential. 3. Following ionophoretic application of potassium from a peri‐internodal microelectrode, the peri‐internodal action potential was followed by a prolonged (hundreds of milliseconds) negative plateau. This plateau was not seen following peri‐internodal ionophoresis of sodium. The prolonged negative potential (PNP) was confined to the K(+)‐injected internode: it could be recorded by a second peri‐internodal microelectrode inserted into the same internode, but not into an adjacent internode. 4. The peri‐internodally recorded PNP was accompanied by an equally prolonged intra‐axonal depolarizing after‐potential, and by an increase in the conductance of the internodal axolemma. However, the K+ ionophoresis that produced the PNP had little or no detectable effect on the intra‐axonally or peri‐internodally recorded resting potential or action potential. These findings suggest that the PNP is generated by an inward current across the axolemma of the K(+)‐injected internode, through channels opened following the action potential. 5. Following peri‐internodal K+ ionophoresis a PNP could also be evoked by passage of depolarizing current pulses through an intra‐axonal electrode or by passage of negative current pulses through an electrode in the K(+)‐filled peri‐internodal region. The threshold for evoking a PNP was less than the threshold for evoking an action potential, and the PNP persisted in 10 microM‐tetrodotoxin. Thus the PNP is evoked by depolarization of the axolemma rather than by Na+ influx. 6. The PNP was reversibly blocked by tetraethylammonium (TEA, 2‐10 mM), but was not blocked by 100 microM‐3,4‐diaminopyridine or 5 mM‐4‐aminopyridine.(ABSTRACT TRUNCATED AT 400 WORDS)