Early cross-striation formation in twitching Xenopus myocytes in culture.

Spontaneous release of neurotransmitter has been demonstrated in various types of synapses. Its physiological significance, however, is still unknown. In nerve-muscle cultures of embryonic Xenopus laevis, we observed that acetylcholine, which is released spontaneously at the synaptic terminal, caused frequent twitches of muscle cells. These muscle cells developed cross-striations earlier than neighboring non-twitching cells. This effect of innervation was unaffected by tetrodotoxin but was blocked by alpha-bungarotoxin. Repeated iontophoretic application of acetylcholine or KCl to muscle cells caused twitches and also accelerated the formation of cross-striations. Thus twitching apparently promotes lateral alignment of myofibrils. It is also known that myosin synthesis is higher in twitching muscle cells. Therefore, successfully innervated twitching muscle cells may have an advantage for faster differentiation over neighboring non-twitching muscle cells. We suggest that spontaneously released transmitter may serve as a mediator for trophic interaction at forming synapses.