Fluctuation analysis of Na + channels modified by batrachotoxin in myelinated nerve

Abstract

(1) Single myelinated nerve fibres of Rana esculenta were treated with the steroidal alkaloid batrachotoxin, and Na + currents and Na +-current fluctuations were measured near the resting potential under voltage-clamp conditions. Between test pulses fibres were held at hyperpolarizing membrane potentials. (2) The spectral density of Na +-current fluctuations was fitted by the sum of a 1 f component and a Lorentzian function. The time constant τ c = 1/(2πƒ c ) obtained from the corner frequency ƒ c of the Lorentzian function approximately agreed with the activation time constant τ m of the macroscopic currents. (3) The conductance γ of a single Na + channel modified by batrachotoxin was calculated from the integral of the Lorentzian function and the steady-state Na + current. At the resting potential V = O we obtained γ = 1.6 pS, higher γ-values of 3.2 and 3.45 pS were found at V = −8 and −16 mV, respectively. (4) The conductance of a modified Na + channel is significantly lower than the values 6.4 to 8.85 pS reported in the literature for normal Na + channels. Hence, our experiments are in agreement with the view that batrachotoxin acts in an ‘all-or-none’ manner on Na + channels and creates a distinct population of modified channels.