| Abstract: | We have studied the development of the action potential Na+ channels in PC12 cells, an established line that has been useful as a model for neuronal differentiation. In continuous culture PC12 cells, although electrically inexcitable, nevertheless have a low level of Na+ channels as judged by the increase in 22Na+ uptake in the presence of veratridine and scorpion toxin. These two neurotoxins have been shown to promote activation of Na+ channels in a variety of electrically excitable cells. Following treatment with nerve growth factor (NGF), conditions which induce differentiation to an electrically excitably neuronal-cell type, the neurotoxin-activated 22Na+ uptake increases approximately 12-fold, on a per cell basis, reaching a maximum in 12-16 days. The dose-response curves for veratridine and scorpion toxin are unchanged by NGF treatment (K0.5 for veratridine, 18-14 microM; K0.5 for scorpion toxin, 120-96 nM). Na+ channels in both undifferentiated and differentiated cells are tetrodotoxin sensitive and NGF treatment has no effect on the inhibition constant (Ki, 10-12 nM). Na+ channel sites were measured directly by the specific binding of 3H]saxitoxin. In NGF-treated cells, the saxitoxin receptor density reaches 154 fmol/mg protein (Kd, 1.3 nM), a level comparable to other excitable cells. Levels in control cells were too low to measure accurately. These findings show that NGF treatment of PC12 cells leads to a substantial increase in the expression of neurotoxin-sensitive Na+ channels. Furthermore, these channels are pharmacologically similar, if not identical, to those which exist in undifferentiated cells and therefore do not appear to result from the conversion of preexisting channels. |
