Kinetics of sodium current and gating current in the frog node of Ranvier.
The experiments were done on voltage-clamped nodes of Ranvier of the frog. The aim was to study the kinetics of sodium current INa and gating current Igat over a large potential range (-92 to -12 mV) and to compare the time constants for the turning-on of INa or Igat with those for the turning-off measured at the same potential. Sodium tail currents were recorded at different postpulse potentials. Inactivation was inhibited by a few min treatment with 0.5 mM chloramine-T (Wang 1984). The sodium permeability was activated by a 0.4 ms pulse from holding potential (-92 mV) to about 0 mV. At the peak of INa the membrane was repolarized to postpulse potentials between -92 and -12 mV. At E greater than -60 mV the tail currents decayed with two time constants, tau 1 and tau 2, reflecting presumably the turning-off and the inactivation of the sodium permeability. The relation between tau 1 and postpulse potential was bellshaped with a maximum at -32 mV. The tail currents could also be fitted by the Hodgkin-Huxley equation with the sodium activation variable m raised to the second or third power. At E less than -50 mV tau m off was equal to 2 tau 1 or 3 tau 1, respectively, whereas at E greater than -25 mV tau m off was equal to tau 1. In addition, the time constant of the turning-on of sodium activation m (tau m on) was determined, assuming INa approximately m2 (with a small initial delay) or INa approximately m3 (without an initial delay). At -22 mV and -12 mV the ratio tau m off/tau m on was close to 1.(ABSTRACT TRUNCATED AT 250 WORDS)
D000818:Animals; D002700:Chloramines / Q000494:pharmacology; D004558:Electric Stimulation; D004594:Electrophysiology; D066298:In Vitro Techniques; D007700:Kinetics; D008564:Membrane Potentials; D011893:Rana esculenta; D011901:Ranvier's Nodes / Q000187:drug effects / Q000502:physiology*; D012964:Sodium / Q000378:metabolism*; D013779:Tetrodotoxin / Q000494:pharmacology; D014105:Tosyl Compounds
D002700:Chloramines; D014105:Tosyl Compounds; C016300:chloramine-T; D013779:Tetrodotoxin; D012964:Sodium