The occurrence of long openings in the purified cholinergic receptor channel increases with acetylcholine concentration.

To establish the relationship between cholinergic ligand occupancy and channel open states, we recorded single-channel currents activated by different acetylcholine (ACh) concentrations from Torpedo californica ACh receptors reconstituted in lipid bilayers. Inspection of single-channel records shows that the frequency of occurrence of long openings increases with ACh concentration. Analysis of the probability distribution of open dwell times indicates that the ACh receptor channel has two distinct channel open states, short- and long-lived. The frequency of occurrence of the long openings over the short increased with ACh concentration, whereas the corresponding time constants were virtually unaltered. The extent of agonist occupancy at the ACh-binding sites in the purified cholinergic receptor appears, therefore, to correlate with an increased probability of the long-lived open state. These results are consistent with the notion that the two open-channel states arise from different extents of ligand occupancy at the receptor molecule.