Diffusion and Binding Constants for Acetylcholine Derived from the Falling Phase of Miniature Endplate Currents

In previous papers we studied the rising phase of a miniature endplate current (MEPC) to derive diffusion and forward rate constants controlling acetylcholine (AcCho) in the intact neuromuscular junction. The present study derives similar values (but with smaller error ranges) for these constants by including experimental results from the falling phase of the MEPC. We find diffusion to be 4× 10-6cm2s-1, slightly slower than free diffusion, forward binding to be 3.3× 107M-1s-1, and the distance from an average release site to the nearest exit from the cleft to be 1.6 μ m. We also estimate the back reaction rates. From our values we can accurately describe the shape of MEPCs under different conditions of receptor and esterase concentration. Since we suggest that unbinding is slower than isomerization, we further predict that there should be several short ``closing flickers'' during the total open time for an AcCho-ligated receptor channel.