Effects of d-tubocurarine on rat cardiac muscarinic receptors: a comparison with gallamine.

Gallamine and d-tubocurarine inhibited (3H)N-methyl-scopolamine [3H)NMS) binding to rat cardiac muscarinic receptors with I50 values of 0.7 microM and 22 microM, respectively. They decreased the association and dissociation rates of the two ligands (3H)NMS and (3H)Oxotremorine M [3H)Oxo-M). Gallamine interaction with muscarinic receptors was markedly inhibited by (3H)NMS and (3H)Oxo-M binding to the receptors. We were unable to demonstrate (3H)NMS or (3H)Oxo-M binding to the muscarinic receptor-gallamine complex. By contrast, d-tubocurarine interaction with rat cardiac muscarinic receptors was facilitated by (3H)Oxo-M binding and only slightly inhibited by (3H)NMS binding to muscarinic binding sites. Furthermore, (3H)NMS and (3H)Oxo-M bound to the receptor-d-tubocurarine complex, indicating that the latter drug interacted with an allosteric site on cardiac muscarinic receptors but did not recognize the muscarinic binding site (at concentrations below 1 mM).