Development of sodium channels during differentiation of chick skeletal muscle in culture. I. Binding studies

[125I]Scorpion toxin and [3H]saxitoxin, two neurotoxins that bind specifically to different sites on sodium channels, were used to monitor the development of sodium channels during synchronous differentiation of chick skeletal muscle in culture. [3H]Saxitoxin-binding sites are not detectable in myoblasts. They appear only after myoblasts fuse to form myotubes and increase gradually to reach a maximal value of 58 fmol/mg of protein by 8 days in culture. In contrast, [125I]scorpion toxin-binding sites are first detected during the period of cell fusion and increase after fusion to reach a maximum by 4 days in culture, well before either the maximal value of the [3H]saxitoxin binding or the appearance of fully developed sodium action potentials. Neither [3H]saxitoxin binding nor [125I] scorpion toxin binding affinity changes during development; they have values of 1.4 and 62 nM, respectively. These data, and data described in the companion paper (Baumgold, J., J. B. Parent, and I. Spector (1983) J. Neurosci. 3: 1004-1013), suggest that sodium channel development involves two sequential steps: incorporation into the cell membrane of an immature form of the sodium channel capable of binding scorpion toxin alone, followed by the acquisition of the ability to bind saxitoxin and thus to become a mature and functional channel.