Brain extract induces the tetrodotoxin-sensitive action potentials in a rat skeletal muscle cell line (L6).
Tetrodotoxin (TTX) usually blocks the action potential in adult vertebrate innervated skeletal muscle, i.e. the action potential is operated by a TTX-sensitive sodium channel. The L6 cultured myotube, however, has TTX-resistant action potential that is usually found in skeletal muscle following denervation or during fetal and neonatal stages in rat. The TTX-resistivity of the L6 myotubes might be due to a lack of a neurotrophic substance. The present study revealed that brain extract had a trophic substance that is capable of inducing the TTX-sensitive sodium channels in uninnervated cultured L6 myotubes. Seventeen to 18 days old cultures were used and the maximum rate of rise of action potential was measured as an index of the sodium channel density. In control cultures, the maximum rate of rise was about 120 V/s and did not change by TTX application. However, in cultures with the brain extract the value was 142 V/s and reduced to 86 V/s by TTX. The reduced maximum rate of rise by TTX in the latter cultures means that exposure of L6 myotubes to brain extract resulted in an increase in TTX-sensitive sodium channels and a decrease in TTX-resistant sodium channels. Therefore, exposure to a factor in brain extract can elicit development of TTX-sensitive sodium channels in isolated L6 muscle cells that have been grown as a cell line in the absence of neurons for over 10 years.