Toxins as tools in the study of sodium channel distribution in the muscle fibre membrane.

The number of tetrodotoxin molecules bound to the membrane of the fibres of muscles in normal conditions and after detubulation produced by glycerol-induced osmotic shock pointed to a higher sodium channel density at the surface membrane than at the membrane in the transverse tubules. Study of the maximum rate of rise of the action potential at the junctional and nonjunctional regions of the muscle fibre membrane suggested that the Na+ channel density is also not the same along the muscle fibre membrane, being higher at the junctional region. Further studies on the distribution of the Na+ channel along the muscle fibre membrane were carried out with the use of (1) the loose patch voltage-clamp technique, (2) labelling the Na+ channels with fluorescently labelled scorpion toxins, (3) autoradiography of localized Na+ channels with 125I-labelled scorpion toxins, and (4) toxins that induce persistent activation of the Na+ channel. The studies referred to in (1), (2) and (3) demonstrate that the density of the Na+ channel is much higher at the junctional region than elsewhere in the membrane of the muscle fibre. On the other hand, in experiments carried out on curarized rat diaphragms several sodium channel activating toxins (crotamine, Phoneutria nigriventer venom, its toxin PhTx2, veratrine) were found to produce a much greater depolarization of the membrane at the junctional region than at nonjunctional regions. However, it was also found that some toxins (veratridine, batrachotoxin) depolarized equally well the junctional and nonjunctional regions. Two alternative hypotheses to explain the uniform depolarization of the muscle fibre membrane induced by these toxins are suggested.