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Abstract

IN most vertebrate skeletal muscles only the endplate region of muscle membrane is sensitive to acetylcholine (ACh), but after denervation the entire extrajunctional membrane becomes sensitive1,2. In rat diaphragm fibres the degree of extrajunctional ACh sensitivity increases steadily during the 7–9 d following denervation and then remains relatively constant at a high level for at least several weeks. It has been shown that this increase in ACh sensitivity results from the appearance of new ACh receptors in the extrasynaptic membrane3,4, and an intriguing question is whether in already fully supersensitive fibres new ACh receptors continue to appear in the surface membrane. α-Bungarotoxin (α-BuTX) blocks the depolarising action of ACh on supersensitive muscle membrane4, and is thought to bind specifically and almost irreversibly to ACh receptors. Thus one can block all or most receptors of super-sensitive muscle fibres at a certain time. I report here experiments in which the reappearance of extrajunctional ACh sensitivity was followed after blocking sensitivity with α-BuTX, using denervated rat diaphragm muscle, maintained in organ culture5.