ELECTRON MICROSCOPE RADIOAUTOGRAPHY AS A QUANTITATIVE TOOL IN ENZYME CYTOCHEMISTRY : II. The Distribution of DFP-Reactive Sites at Motor Endplates of A Vertebrate Twitch Muscle

The distribution of diisopropylfluorophosphate (DFP)-sensitive enzyme sites at the neuromuscular junction was determined quantitatively by electron microscope radioautography after incubation of muscle fragments in DFP-³H. Most of the sensitive sites were located in the subneural apparatus at a concentration of 90,000 sites per µ³ of cleft tissue or 12,000 sites per µ² of postjunctional membrane surface area. A considerable concentration is also present in the teloglial cap. It has previously been demonstrated (Rogers et al., 1966) that one-third of the DFP-sensitive sites at the endplate can be reactivated by pyridine-2-aldoxime methiodide (2-PAM)—a compound which selectively reactivates phosphorylated acetylcholinesterase. In the present study, it was found that this ratio of 1:2 holds also on a fine-structural level. Muscle mast cells were found to have a heavy concentration of bound DFP.