POSITIVE ALLOSTERIC ACTION OF ALCURONIUM ON SOLUBILIZED CARDIAC MUSCARINIC RECEPTORS

Alcuronium was found earlier to increase the binding of (H-3)methyl-N- scopolamine [(H-3)NMS] to muscarinic receptors in membranes of rat hea rt atria by increasing the receptors' affinity for (H-3)NMS, without a ltering the number of (H-3)NMS binding sites. We have now investigated how the interaction of alcuronium with muscarinic receptors is affect ed by solubilization. In experiments on pig heart atria, alcuronium ha d a positive allosteric effect on (H-3)NMS binding both to unsolubiliz ed receptors and to receptors solubilized by digitonin and deoxycholat e. The cooperativity coefficient alpha was 0.39 +/- 0.01 for unsolubil ized and 0.57 +/- 0.01 for solubilized receptors. Under the conditions used for solubilization, muscarinic receptors did not interact with G proteins, as indicated by experiments with the binding of carbachol i n the absence and presence of a stable GTP analogue. Alcuronium slowed down the rates of (H-3)NMS association with, and dissociation from, s olubilized receptors; the dissociation rate constant was diminished mo re than 300 times by 30 mu M alcuronium. The results show that the pos itive allosteric action of alcuronium on cardiac muscarinic receptors and its effect on the kinetics of radioligand binding are preserved af ter receptor solubilization and do not depend on the interaction of re ceptors with G proteins.