Acetylcholine receptor site density affects the rising phase of miniature endplate currents.

Abstract

The relationship between acetylcholine receptor (AcChoR) site density (.sigma.) and the rising phase of the miniature endplate current was determined in esterase-inactivated lizard [ANOLIS-CAROLINENSIS] intercostal neuromuscular junctions. The currents were recorded by using a voltage clamp. The receptor site density was determined by EM autoradiography after labeling with 125I-labeled .alpha.-bungarotoxin in normal endplates and in those partially inactivated with nonradioactive .alpha.-bungarotoxin. As .sigma. was decreased the rise time increased and the amplitude decreased. The saturating disk model, which implies a quantum of acetylcholine (AcCho) acts on a small postsynaptic area at saturating concentration is compatible. In the normal neuromuscular junction the most likely number of AcCho molecules needed to open an ion channel apparently is 2 and the 20-80% rise time of < 100 .mu.s is influenced by the .sigma.-dependent factors, e.g., diffusion and binding of AcCho to AcChoR, and the .sigma.-independent time delays, e.g., the conformation change time to open the ion channels. The lower limits to the forward rate constant of AcCho binding to AcChoR are .gtoreq. 3 .times. 107 M-1 s-1, and the diffusion constant for AcCho in the cleft is .gtoreq. 4 .times. 10-6 cm2/s.