Propylbenzilylcholine mustard is selective for rat heart muscarinic receptors having a low affinity for agonists

Propylbenzilylcholine mustard (PrBCM), an irreversible muscarinic antagonist, inactivated receptors with a low affinity for agonists faster than those with a high affinity in rat heart membranes. This result was obtained using either: (a) a low ionic strength buffer (allowing heterogeneity among antagonist binding sites, (b) the same buffer enriched with GTP, or (c) a high ionic strength buffer (where antagonists showed similar binding characteristics to all receptors). These data suggest either that PrBCM is a 'selective' antagonist which detects conformational differences between low and high affinity receptors, or that the agonist affinity of cardiac muscarinic receptors is determined, in part, by the relative concentrations of receptor and GTP binding protein.