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1. We have examined the binding of tritium-labelled and unlabelled tetrodotoxin to frog twitch muscle. Bio-assay as well as radioisotope experiments show a saturable component of tetrodotoxin binding with a binding capacity of about 22 p-mole/g wet wt., and a dissociation constant of about 5 nM. 2. If the observed uptake of tetrodotoxin by muscles represents one-to-one binding of the drug to sodium channels, the channel density is about 380 channels/mum2 of a muscle fibre's surface membrane. On the basis of this result and electrical measurements of sodium conductance in frog muscle, we calculate that the conductance of a single sodium channel is of the order of 10(-12) reciprocal ohms. This is one to two orders of magnitude less than previous estimates. 3. We have looked for an effect of membrane depolarization on saturable tetrodotoxin binding, and have found none. This suggests that there is little molecular interaction between the "gating" portion of the sodium channel molecule, and that which binds tetrodotoxin.

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