Conformational states of the nicotinic acetylcholine receptor from Torpedo californica induced by the binding of agonists, antagonists, and local anesthetics. Equilibrium measurements using tritium-hydrogen exchange

Abstract

The tritium.sbd.hydrogen exchange kinetics of Torpedo californica AChR, in native membrane vesicles at pH 7.4 and 0 .degree.C, have been analyzed in the presence of agonists, partial agonists, local anesthetics, and competitive antagonists. The agonists carbamylcholine (10 .mu.M .sbd. 1 mM) and suberyldicholine (10 .mu.M) and the partial agonists decamethonium (25 .mu.M and 1 mM) and hexamethonium (1 mM) have no effect on the exchange kinetics, although at lower concentration carbamylcholine may slightly accelerate exchange. Nondesensitizing local anesthetics do affect the exchange behavior, dependent on concentration. Procaine at 500 .mu.M moderately retards exchange while procaine at 10 mM and tetracaine at 5 mM slightly accelerate exchange. The competitive antagonist .alpha.-bungarotoxin retards exchange significantly, as does d-tubocurarine although to a lesser extent. These results suggest that the resting and desensitized conformations of the AChR are very similar in overall solvent accessibility and that at lower concentrations noncompetitive blockers such as procaine may stabilize a less solvent-accessible state of the AChR. The competitive antagonists .alpha.-bungarotoxin and d-tubocurare also stabilize a dynamically restricted, less solvent-accessible conformation of the acetylcholine receptor, demonstrating that a large conformational change accompanies binding of these toxins. Any change in conformation which may accompany desensitization is very different from these effects.