A correlation of the alpha-bungarotoxin binding sites (acetylcholine receptors) and intramembranous particles in denervated skeletal muscle of rat

Abstract

A quantitative distribution of alpha-bungarotoxin (alpha-BGT) binding sites i.e. acetylcholine receptors (AchR) has been investigated and correlated with the distribution of intramembranous particles (approximately 15 nm in diameter) observed on the freeze-fractured face (P-face) of the non-synaptic sarcolemma of denervated muscle. By both light and electron microscopy autoradiography, randomly distributed clusters of alpha-BGT sites are visualized on the plasma membrane of the denervated muscle. Such distributions of the toxin binding sites correspond with that of the 15 nm particles on the P-face of the denervated muscle. Quantitative studies suggest that upon denervation the toxin binding sites increase approximately 60-fold in the non-synaptic sarcolemma. However, the density of these alpha-BGT sites is 4--5 times more than the density of the 15 nm particles. On the basis that there are two alpha-BGT binding sites per AchR molecule, these results suggest that each 15 nm particle is composed of more than one AchR receptor. In addition to the increase in toxin binding sites on the non-synaptic sarcolemma, a notable increase in the number of silver grains is observed in the peripheral sarcoplasm and is speculated to be part of the intracellular pool of receptors.