Development of electrophysiological and biochemical membrane properties during differentiation of embryonic skeletal muscle in culture.

Newly fused chick myotubes undergo simultaneous and rapid changes in cell membrane properties during synchronous differentiation in culture. These changes are coordinately regulated and include increases in acetylcholine receptor, acetylcholinesterase, and resting potential, as well as the appearance of action potentials in discrete membrane areas upon stimulation. Subsequently, the acetylcholine receptor reaches maximal levels, whereas the development of electrical properties is marked by a further increase in resting potential, changes in the characteristics of the elicited action potential, and the recruitment of additional membrane areas for action potential generation. Maturation of electrical excitability, marked by the acquisition of the ability to fire repetitively and to conduct action potentials along the membrane, occurs well after resting potential has reached a maximum. During post-maturational development, myotubes exhibit spontaneous electrical and contractile activity, and levels of acetylcholine receptor accessible to externally applied 125I-labeled alpha-bungarotoxin decrease markedly. It is suggested that electrophysiological membrane maturation is autonomously regulated with no requirement for neuronal intervention and involves the coordinated biosynthesis of discrete membrane components and their subsequent organization in the myotube membrane.