Intracellular acetylcholinesterase of adult rat myofibers is more concentrated in endplate than non-endplate regions.

The intracellular distribution of acetylcholinesterase (AChE) was determined in adult rat anterior gracilis muscles. Echothiophate iodide (ECHO), a water-soluble cholinesterase inhibitor, was applied to muscles in situ to eliminate extracellular and/or extracellularly oriented enzyme. Control and ECHO-treated muscles were either cut into 1-mm segments and assayed for AChE activity or cytochemically stained for AChE. Subsequent analysis by light and electron microscopy showed that the AChE stain inside myofibers was highly localized and clearly visible only in the zone immediately underlying the point of nerve-muscle contact. Biochemical assay of muscle segments showed intracellular AChE to be most highly concentrated in regions containing large numbers of endplates (approximately twice the activity of endplate-free areas). Since such "endplate-rich" segments are in fact mostly extra-synaptic tissue, we conclude that intracellular AChE of adult rat gracilis myofibers, although present along the length of the cell, is more than two times as concentrated in sub-synaptic areas as compared to extra-synaptic areas. This result must be carefully considered when attempting to identify "endplate-specific" AChE activity of mammalian muscle, and further points to the importance of neural influences on AChE metabolism/regulation.