Abstract

Competitive antagonists to nicotinic acetylcholine receptors are clinically used as muscle relaxants. Previously, we reported the kinetics of inhibition (in the absence of acetylcholine) by (+)-tubocurarine and pancuronium on embryonic receptors. Here, we examine cisatracurium, a commonly used muscle relaxant. Outside-out patches were equilibrated with cisatracurium before application of 300 μM acetylcholine. cisatracurium inhibited the initial peak current, but the decay of these currents displayed a pronounced biphasic behavior. The IC₅₀ value was 54 ± 2 nM and 115 ± 4 nM for adult and embryonic receptors, respectively. We designed a rapid perfusion system to apply or remove cisatracurium for various times before application of acetylcholine. We determined the association (embryonic, 3.4 ± 0.4 × 10⁸ M⁻¹ s⁻¹; adult, 1.8 ± 0.3 × 10⁸ M⁻¹ s⁻¹) and dissociation (embryonic, 34 ± 6/s; adult: 13 ± 5/s) rates for cisatracurium. Association was 2.9- and 1.3-fold greater than that of (+)-tubocurarine and pancuronium, respectively. Dissociation was 6- and 16-fold higher than (+)-tubocurarine and pancuronium, respectively. These measurements correspond to dissociation of cisatracurium from receptors in the absence of acetylcholine. Physiologically, acetylcholine interacts with receptors equilibrated with antagonist. We developed a mathematical technique that removes the effect of desensitization and determined dissociation (embryonic, 52 ± 9/s; adult, 33 ± 5/s) in the presence of acetylcholine. These data suggest that presence of acetylcholine on one binding site of the receptor increases the dissociation rate of antagonist from the other binding site. We incorporated all of these rates into a computer simulation of a comprehensive 11-state Markov model. There was excellent agreement (without curve fitting) between simulated and experimental currents.