Sodium currents and sodium-current fluctuations in rat myelinated nerve fibres.

1. Sodium currents and fluctuations of sodium currents were measured in myelinated fibres of rat sciatic nerve under voltage clamp at 20 degrees C.2. Relaxations of sodium currents during various test potentials were recorded in the presence of 6 nM-TTX in the extracellular solution. The activation of sodium currents at low depolarizations could be described with the m(2) formulation. At increasing potentials higher powers of m up to 4 were required. The mid-point of the P(Na) (E) curve was located near E = -32 mV. Sodium inactivation during various depolarizations developed in two phases.3. The resistance in series with the nodal membrane was calculated from peak sodium currents without and with 6 nM-TTX in the extracellular solution. The resistance varied between different fibres and ranged between 190 and 620 kOmega.4. From peak sodium currents at the same mambrane potential without and in the presence of TTX an apparent equilibrium dissociation constant of 1.6 nM was calculated for TTX binding to sodium channels.5. The conductance gamma and the number N(0) (corrected for series-resistance effects) of sodium channels were evaluated from ensemble average values of the mean sodium current and the variance of sodium-current fluctuations at the beginning of a test pulse. The mean values were gamma = 14.5 pS, N(0) = 21,000 per node.6. The spectral density of stationary sodium-current fluctuations exhibited two relaxation components whose time constants were comparable to those of sodium activation and inactivation. At low depolarizations the variance produced by inactivation fluctuations was larger than predicted by the m(3). h formulation.7. It is concluded that individual sodium channels of rat and frog nerve have similar gating properties. In mammalian nodes the number of sodium channels is lower and the single-channel conductance higher than in amphibian nodes.