Mechanisms of steric and cooperative actions of alcuronium on cardiac muscarinic acetylcholine receptors.

Kinetics of the interactions between the neuromuscular blocker alcuronium, the specific muscarinic antagonist N-[methyl-3H] methyl scopolamine ([3H]NMS), and muscarinic receptors were investigated in homogenates of rat heart atria. Two effects of alcuronium on the binding of [3H]NMS could be distinguished. (a) Alcuronium concentration-dependently slowed the association of [3H]NMS with receptors and the dissociation of [3H]NMS from receptors so that, at high alcuronium concentrations, equilibrium binding could not be reached, even after 20 hr, without special precautions. (b) Alcuronium increased the affinity of receptors for [3H]NMS, which was manifested by a decrease of the apparent Kd (> 3-fold) with no change in the Bmax for [3H]NMS binding. The effects of alcuronium on the rates of [3H]NMS association and dissociation can be explained only by a reaction mechanism in which [3H]NMS binds only to free receptors (not occupied by alcuronium), whereas alcuronium binds both to free receptors and to receptors occupied by [3H]NMS. Similarly, [3H]NMS cannot dissociate from receptors as long as alcuronium is attached to them. Experimental data agree with corresponding mathematical models. It is proposed that alcuronium blocks entry to the pocket containing the [3H]NMS binding site. In addition to this blocking effect, alcuronium has a positive allosteric effect on [3H]NMS binding, presumably by inducing a conformational change of the orthosteric muscarinic binding site. Earlier observations suggesting that, at high concentrations, alcuronium also competes for [3H]NMS binding sites can be explained by insufficient equilibration of the system.