Cellular localization of cytochemically stained acetylcholinesterase activity in adult rat skeletal muscle.

Cytochemically stained acetylcholinesterase (AChE) activity in endplate regions of adult rat gracilis muscles was studied after in situ treatment with AChE inhibitors which differ in lipid solubility and hence in their ability to penetrate cell membranes. Control preparations showed intense AChE staining over junctional infoldings and within myofibres, but little enzymatic reaction product in nerve terminals and Schwann cells. Echothiophate (poorly lipid soluble) drastically reduced only extracellular AChE activity, whereas sequential treatment with BW284C51 (poorly lipid soluble) and diisopropylfluorophosphate (lipid soluble) primarily eliminated intracellular AChE. Extracellular AChE activity (associated with the synaptic basal lamina) was predominantly composed of asymmetric enzymatic forms. Intracellular AChE (associated with the sarcoplasmic reticulum of the myofibres) primarily contained globular forms and a small proportion of asymmetric forms. Little or no external AChE activity was detected in non-endplate muscle regions and the internal enzyme was confined to a restricted subcellular region close to the point of innervation. These results establish the validity of using the above in situ pharmacological treatments to demonstrate intracellular and extracellular pools of AChE in adult skeletal muscles. In addition, they are consistent with the idea that motor neurons play an essential role in the mechanisms which determine the subcellular distribution of AChE.