Acetylcholinesterase density and turnover number at frog neuromuscular junctions, with modeling of their role in synaptic function.

Acetylcholinesterase (AChE) density at the neuromuscular junction of frog cutaneous pectoris muscle was determined by electron microscope autoradiography and biochemistry to be approximately 600 sites micron-2 of postsynaptic area, approximately 4-fold lower than all previous reports (mouse), whereas the hydrolytic turnover number was 9,500 s-1, well within the range (2,000-16,000 s-1) for AChE from other species. Monte Carlo computer simulations of miniature endplate currents showed that for vertebrate neuromuscular junctions with different morphologies, an AChE density of only approximately 400 sites microns-2 and a turnover number of only approximately 1,000 s-1 are sufficient for normal quantal currents. Above these critical lower limits, miniature endplate currents were essentially insensitive to AChE density and turnover number values up to 5,000 sites microns-2 and 16,000 s-1, respectively.