Development of sodium channels during differentiation of chick skeletal muscle in culture. II. 22Na+ uptake and electrophysiological studies.

The development of the functional properties of the sodium channel of chick skeletal muscle grown in culture was studied using 22Na uptake and electrophysiological techniques. In accord with the biochemical data in the preceding paper (Baumgold, J., J. B. Parent, and I. Spector (1983) J. Neurosci. 3: 995-1003), the functional manifestations of sodium channel expression are initiated shortly after cell fusion. At this stage, the myotubes have barely detectable sodium-dependent action potentials (Vmax = 1 to 9 V/sec) and exhibit a very small amount of batrachotoxin (BTX)-stimulated 22Na+ uptake. However, when these cultures are treated with scorpion toxin (ScTX), the amplitude and rate of rise of the sodium action potential increase dramatically (Vmax = 35 to 50 V/sec) and the (BTX)-stimulated 22Na+ uptake is much larger, suggesting that ScTX unmasks channels that are already present but nonfunctional in these immature myotubes. The two different rates of development of the biochemical properties of the sodium channel described in the preceding paper are also reflected in the two separate rates of development of its functional properties. In the absence of ScTX, the amplitude and Vmax of the action potential develop at a slow rate similar to that of the [3H]saxitoxin binding, eventually reaching a Vmax of 158 V/sec by day 10; the BTX-stimulated 22Na+ uptake also rises gradually, reaching 12 nmol of 22Na+/culture/min by day 7.5. In contrast, in the presence of ScTX, the Vmax of the Na+ action potential increases more rapidly, reaching 158 V/sec by day 5 and 220 V/sec by day 10. The BTX-stimulated 22Na+ uptake also increases more rapidly in the presence of ScTX. This rapid rate of development is very similar to that for [125I]ScTX binding. These findings and those in the preceding paper suggest the existence of two types of Na+ channels: an immature, nonfunctional channel capable of binding [125I]ScTX alone, and a mature, functional channel capable of binding both [125I]ScTX and [3H]saxitoxin. They further suggest that the insertion of the immature form of the channel protein into the cell membrane shortly after cell fusion is the first event in the expression of the sodium channel. During development, the sodium channel undergoes a structural change which renders it functional. The possibility that both the appearance of functional sodium channels during development and the rapid induction of functional channels by ScTX in immature myotubes reflect a post-translational modification or aggregation of immature nonfunctional channels is discussed.