Electron microscopic-cytochemical and biochemical studies of acetylcholinesterase activity in denervated muscle of rabbits.

Acetylcholinesterase (AChE) activity has been studied in normal, control and denervated muscle of rabbits by electron microscopic-cytochemistry and radiometric assay. A small amount of butyrylcholinesterase (BuChE) activity is also found in biochemical assay of unfixed muscle, but it is not demonstrable cytochemically in fixed specimens by the method used in this study. Both a soluble and particulate AChE activity are present in all specimens examined. The particulate activity is probably due to enzyme localized in the sarcotubular system and at the motor end-plate. Soluble AChE activity may represent those sites exhibiting random cytochemical end product, such as some areas of normal and denervated muscle and muscle nuclei, Schwann cells, and AChE-containing mononuclear cells in the connective tissue. There is a greater proportion of particulate than soluble AChE activity in normal and control muscle, a finding which is compatible with the well localized cytochemical sites. Four to six weeks post-denervation, there is a marked increase in extrajunctional AChE activity to peak values 15 to 30 fold above control values. The increase is accompanied by a reversal in the proportion of particulate to soluble enzyme, so that there is almost twice as much soluble as particulate AChE. There are also numerous "spots" of random cytochemical end product throughout extrajunctional muscle. The increase in levels of AChE activity, the change to predominantly soluble form, and the large numbers of new cytochemically active sites indicate that synthesis of new enzyme has taken place. Changes in AChE activity in denervated rabbit have been compared to those occurring in dystrophic mouse muscle. It has been suggested that there might be a relationship between the formation of new extrajunctional sarcoplasmic sites of AChE activity and the spread of alpha-bungarotoxin binding sites and chemosensitivity in developing and denervated muscle.