Nerve‐induced and spontaneous redistribution of acetylcholine receptors on cultured muscle cells.

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Abstract
Three day old cultures of myotomal muscle, obtained from embryos of Xenopus laevis, were stained with fluorescent conjugates of .alpha.-bungarotoxin and maintained in native toxin in order to ensure that ACh [acetylcholine] receptors subsequently inserted into the sarcolemma were not stained. Neural tube cells were then added to the cultures. When cultures were examined 1-3 days later, fluorescent stain was associated with sites of nerve-muscle contact. In some cases the stain along the path of contact extended for greater distances than the patches of stain seen on non-contacted muscle cells. The development of new areas of fluorescent stain at sites of nerve-muscle contact was confirmed by making successive observations on the same muscle cell over a period of a day. Similar experiments on muscle cells not contacted by nerve revealed the formation of new receptor patches, usually in areas of cell growth. The majority of fluorescent patches on non-contacted muscle cells did not undergo changes in size or shape over the course of 1-2 days. Some examples of enlargement, shrinkage and disappearance were observed. ACh receptors aggregated within the sarcolemma, spontaneously and in response to innervation. In the latter case extrajunctional receptors accumulated at the site of nerve contact thereby contributing to the development of high receptor density in the subneural muscle membrane. This process of receptor redistribution occurred in the absence of synaptic or contractile activity. Possible mechanisms involved in the redistribution of ACh receptors are discussed in relation to those which appear to modulate ligand-induced changes in the distribution of lectin and immunoglobulin receptors.