The conductance and density of sodium channels in the cut-open squid giant axon

Non-stationary Na current fluctuations in small voltage-clamped patches of cut-open squid giant axon were analysed by an ensemble-average technique to yield the single Na channel conductance gamma Na and the Na channel density in the patch. gamma Na appeared to be voltage independent over the range -30 to +40 mV and had a mean value of 4.4 +/- 1.1 pS in 514 mM-Na/20 mM-Na at 5 kHz band width and temperature between 3.5 and 5.0 degrees C. gamma Na did not change significantly at band widths to 20 kHz. gamma Na in reduced Na solutions, 103 mM-Na/4 mM-Na, at 3.5-5.0 degrees C had a mean value of 1.2 +/- 0.3 pS. Internal solutions containing 50 mM-tetraethylammonium (TEA) depressed both gamma Na and the mean Na currents by roughly the same factor, compared with solutions without TEA. The reduced gamma Na had a mean value of 2.2 +/- 0.7 pS. The mean Na channel density in the standard 514 mM-Na/20 mM-Na solution was estimated to be 180 +/- 100 microM-2. The densities in the other solutions mentioned above were not significantly different from this value.