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

(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 $\text{1}\text{f}$ component and a Lorentzian function. The time constant $\text{τ}{}_{\mathrm{<mtext>c</mtext>}}\text{= 1/(2π?}{}_{\mathrm{<mtext>c</mtext>}}\text{)}$ obtained from the corner frequency ?_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.