High acetylcholine concentrations cause rapid inactivation before fast desensitization in nicotinic acetylcholine receptors from Torpedo

By using both a 3 to 4 ms quenched-86Rb+ flux assay and native acetylcholine receptor (AChR) rich electroplaque vesicles on which 50–60% of acetylcholine activation sites were blocked with alpha-BTX, we determined apparent rates of agonist-induced inactivation in AChR from Torpedo under conditions where measured flux response was directly proportional to initial 86Rb+ influx rate. Inactivation kinetics with acetylcholine in both the activating range (10 microM-10 mM) and the self-inhibiting range (15–100 mM) were measured at 4 degrees C. In the presence of 10 microM-1 mM acetylcholine, inactivation is characterized by a single exponential rate constant, kd (fast desensitization). Plots of kd vs. acetylcholine concentration display maximum kds [kd(max)] of 6.6–8.0 s-1, half-maximal kd at 102 +/- 16 microM, and a Hill coefficient of 1.6 +/- 0.3, closely paralleling the initial ion flux response of AChR. Thus, fast desensitization probably occurs from a doubly-liganded preopen state or the open channel state. In the self-inhibiting acetylcholine concentration range, inactivation is biphasic. A "rapid inactivation" phase is complete within 30 ms, followed by fast desensitization at a rate close to kd(max). Both the rate and extent of rapid inactivation increase with acetylcholine concentration, indicating that acetylcholine binds to its self-inhibition site with apparent kon approximately equal to 10(3) M-1s-1 and koff approximately equal to 40 s-1. This slow kon suggests either hindered access to the inhibitory allosteric site or that a fast binding step is followed by a slower conformational change leading to channel inhibition. Overall, our data suggest that acetylcholine binds preferentially to its inhibitory site when the receptor is in the open-channel conformation and that fast desensitization can occur from all multiple-liganded states