Choline acetyltransferase activity in skeletal muscles after denervation

The activity of choline acetyltransferase (ChAc) in rat hind-limb muscles started to decrease rapidly between 10 and 24 hr after transection of the mixed nerve supply. The time course of changes in the tibialis anterior, extensor digitorum longus, and soleus muscles was very similar. Semilogarithmic plots of ChAc changes vs time revealed that the changes in enzyme activity proceed in two steps, both of which have an exponential time course. A steep decline occurs during the first 72–96 hr after denervation; this is followed by a slow decline. During the period of steep decline the activity of ChAc in the muscles decreased by 79–92%. After 7 mo, the mean ChAc activity in denervated tibialis anterior was 3.4 and in extensor digitorum longus 3.3% of that in contralateral control muscles. It is suggested that the ChAc disappearing from the muscles during the period of steep decline is the enzyme present in the motor nerve terminals and intramuscular branches of the motor nerve, whereas the enzyme remaining in the muscle after 4 days of denervation constitutes a different pool (or different pools) of ChAc. It appears likely that this pool of ChAc is also present in the muscle before denervation. No secondary increase of ChAc at later stages of denervation was observed. The level of nerve transection affected the time course of the steep ChAc decline; a 50% decrease of ChAc was attained 9 hr earlier when the nerve was cut near the muscle than when it was cut 34 mm away from the muscle.