A pharmacological approach to the structure of sodium channels in myelinated axons

Figure 6 summarizes the present state of our knowledge on the sodium channel in myelinated nerve fibers. Two sites have been discussed in detail: a metal cation binding site accessible by tetrodotoxin and saxitoxin from the outside surface only; and a second site accessible from the inside surface with which local anesthetics combine. Hydrogen ions gain access to this region of the sodium channel (and hence determine the relative local concentration of protonated drug) more readily from the extracellular fluid than from the axoplasm (Schwarz et al 1977). In addition, a variety of other sites have been mentioned, binding of drugs to which alters selectively the kinetics of opening and closing of the h and m gates. In myelinated nerve fibers these channels are packed tightly on the nodal membrane. The highest estimate for the sodium channel density in the mammalian node is 10,000 micron2. A re-evaluation of the effective nodal area, however, might reduce this value to 3000-5000/micron 2. This would still leave the nodal membrane rather crowded with sodium channels. Furthermore, the channel density would still be greater than the density of particles, sometimes believed to be sodium channels seen in freeze fracture studies (Rosenbluth 1976). One possibility for resolving this problem is that the units detected by X-ray inactivation (Levinson & Ellory 1973), and those seen in freeze-fracture studies (Rosenbluth 1976) represent not single sodium channels but groups of three. Catterall & Morrow (1978) in a comparison of the binding of saxitoxin and Leiurus sculpturatus scorpion toxin venom have concluded that there are three saxitoxin binding sites for each scorpion toxin binding site. On this basis, three saxitoxin molecules might act to block independently each of the three openings of the channels; while the the conformational change produced by the scorpion venom molecule would affect the inactivation process of all three channels.