Alteration of the conductance of Na + channels in the nodal membrane of frog nerve by holding potential and tetrodotoxin

Abstract

(1) Na + currents and Na +-current fluctuations were measured in myelinated frog nerve fibres at 15°C during 7.7 ms depolarizations to V = 40, 60 and 80 mV . (2) The conductance γ of a single Na + channel and the number N 0 of channels per node were calculated from ensemble average values of the mean Na + current and the variance of Na +-current fluctuations. (3) For a hyperpolarizing holding potential of V H = −28 mV the mean values of the channel conductance and number were γ = 9.8 pS and N 0 = 74 000 . (4) After changing the holding potential to the resting potential ( V H = 0 ) the conductance γ increased by a factor of 1.37 whereas the number N 0 decreased by a factor of 0.60. (5) Addition of 8 nM tetrodotoxin at a holding potential of V H = −28 mV increased γ by a factor of 1.55 and reduced N 0 by a factor of 0.25. (6) The increase of the channel conductance at reduced channel numbers suggests negative cooperativity between Na + channels in the nodal membrane.