Kinetic parameters of the ionic currents in myelinated axons: characterization of temperature effects in a hibernator and a nonhibernator.

Na+ and K+ currents were measured by the patch-clamp method in the paranodal region of single sciatic nerve fibres of rats and of warm-adapted and cold-adapted golden hamsters. Kinetic parameters and temperature dependence of the Na+ currents were determined. The time constant for activation (about 0.2 ms for rats and hamsters) as well as the time constant for inactivation (about 1.6 ms for rats and hamsters) at 15 degrees C and at -35 mV compared well with single fibre voltage-clamp data from the rat. Differences amongst the three groups of animals were not significant. The temperature coefficient, Q10, for the activation and the inactivation time constant as well as for the time-to-peak of the Na+ current ranged between 2.3 and 3.1. No data have previously been published on the temperature dependence of the delayed-rectifier K channels of mammalian nerve fibres. Most of the K+ current was carried by intermediate (KI) and fast (KF) K channels. Dendrotoxin block indicated that "approximate"55% of the K+ current was due to KI channels, with no significant difference amongst the three groups of animals tested. The Arrhenius plot of the time constant of K+ current activation, "tau"n, yielded a mean Q10 of 3.3 at -40 mV (4. 0 at + 60 mV). No significant differences of the channel kinetics between rats, warm-adapted hamsters and cold-adapted hamsters were detected. We observed, however, a significant decrease of the Na channel density in the paranodal region of cold-adapted hamsters.