Mechanism of inactivation (desensitization) of acetylcholine receptor. Investigations by fast reaction techniques with membrane vesicles.

Exposure of the acetylcholine receptor to acetylcholine, or its stable analogue carbamylcholine, inactivates (desensitizes) the receptor. Inactivation of receptor-controlled ion (86Rb+) flux in the presence of different concentrations of carbamylcholine (12.5 microM to 28 mM) was measured in the millisecond to minute time region, using a quench flow technique and membrane vesicles prepared from the electric organ of Electrophorus electricus. Three different kinetic measurements were made to establish the relationship between carbamylcholine concentration and the ion translocation process: (i) the rate of inactivation of the ion translocation process; (ii) the rate of recovery of the inactivated receptor upon removal of carbamylcholine; and (iii) the rate of the ion flux mediated by equilibrium mixtures of active and inactive receptor forms. The kinetics of these three processes follow single-exponential rate laws, and simple analytical expressions for their ligand concentration dependence could be used. Therefore, it was possible to determine the value of the rate constants in a scheme relating the ligand binding steps to ion translocation, and to predict the dependence of these rate constants on carbamylcholine concentration over the 200-fold range investigated.