alpha-bungarotoxin binding sites (acetylcholine receptors) in denervated mammalian sarcolemma

The nonsynaptic sarcolemma of denervated skeletal muscle of rat shows an abundance of approximately 15 nm intramembranous particles on the P face. These particles are either singly distributed or are in clusters, and they are essentialy lacking from the comparable freeze-fractures of the innervated sarcolemma. Autoradiographic studies using 125I-alpha-bungarotoxin (BGT) on 1 mu-thick sections, and freeze-etch studies using ferritin-alpha-BGT conjugates on membrane fractions, show that the distribution of the label corresponds to the distribution of the 15-nm particles in the nonsynaptic sarcolemma. On the basis of these results and existing physiologic and biochemical data, it is suggested that the 15-nm intramembranous particles are components of the alpha-BGT binding sites, ie, acetylcholine (Ach) receptors, in the nonsynaptic sarcolemma of denervated muscle and that the two types of distributions represent two spatial manifestations of Ach receptor molecules. The significance of these findings in relation to synapse formation in denervated muscle is discussed.