TTX binding to frog muscle and the conductance of a single sodium channel

Using both bioassay and radioisotope methods, the authors have examined the binding of tetrodotoxin (TTX) to frog sartorius muscle. This tissue has the advantage of allowing electrical as well as biochemical studies of membrane properties. The authors find a saturable component of TTX binding with a capacity of about 22 pMoles/g wet and half saturation at 5 nM. If this uptake represents a one to one binding of TTX to sodium channels, the channel density in a muscle of 0.3 ml/g extracellular space and 80 μm fiber diameter is about 380/μm² referred to unit area of surface membrane. In conjunction with voltage clamp measurements of muscle sodium conductance, the present result allows one to estimate the conductance of a single sodium channel as about 10^-12 mho. This is one to 2 orders of magnitude less than previous estimates obtained from TTX binding on tissues which do not allow measurement of the maximal sodium conductance. Their estimate is also lower that the single channel conductances of the 2 virtually non selective cation pores opened by acetylcholine and gramicidin-A. This makes good sense, since one might expect that a channel which does not discriminate between univalent cations, should permit higher transient rates than the relatively selective sodium channel, which first has to 'test' each approaching ion before allowing its passage. The authors also find that maintained depolarization does not measurably affect TTX binding. This suggests that there is little molecular interaction between the 'gating' position of the sodium channel molecule, and that which binds TTX.