Activity, molecular forms, and cytochemistry of cholinesterases in developing rat diaphragm.

Acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) were studied in the diaphragm of early postnatal rats, using radiometric determination of enzyme activities, velocity sedimentation for separation of molecular forms, and electron microscopic cytochemistry to localize enzymes. AChE activity did not change significantly during the first 2 months after birth. The level of 16 S AChE was high at early stages, but decreased to adult levels between days 19 and 32 after birth. In newborn and 8-day-old rats, 16 S and 13 S AChE were present in both endplate and extrajunctional muscle. In newborn muscle, BuChE activity was higher than AChE activity, but decreased 6-fold by day 32. Newborn muscle contained 16 S, 10 S, and 4 S BuChE. The principal form throughout development was 4 S BuChE, but all forms diminished as total BuChE activity decreased with maturation. At early postnatal neuromuscular junctions, end product of both AChE and BuChE was present in the cleft, as well as in Schwann cells. More Schwann cells wee present in early stages than in later stages, and this might account for part of the later decrease of BuChE activity measured biochemically.