Diffusion and binding constants for acetylcholine derived from the falling phase of miniature endplate currents.

Abstract

The rising phase of a miniature endplate current (MEPC) was studied to derive diffusion and forward rate constants controlling acetylcholine (AcCho) in the intact neuromuscular junction. Similar values (but with smaller error ranges) for these constants [were studied in the Anolis carolinensis] by including experimental results from the falling phase of the MEPC. Diffusion was 4 .times. 10-6 cm2 s-1, slightly slower than free diffusion, forward binding was 3.3 .times. 107 M-1 s-1 and the distance from an average release site to the nearest exit from the cleft was 1.6 .mu.m. The back reaction rates were also estimated. From the values, the shape of MEPC under different conditions of receptor and esterase concentration can be described. Since it is suggested that unbinding is slower than isomerization, it is predicted that there should be several short closing flickers during the total open time for an AcCho-ligated receptor channel.