Influence of ligand choice on the apparent binding profile of gallamine to cardiac muscarinic receptors. Identification of three main types of gallamine-muscarinic receptor interactions

The binding profile of the positively charged muscarinic antagonist, gallamine, was studied in rat heart homogenates. A proportion of the binding sites labeled by the tertiary muscarinic ligands {[³H]quinuclidinyl benzilate (QNB) and [³H]atropine} were inaccessible to their quaternary analogs {[³H]N-methyl-QNB (NMeQNB) and [³H]-N-methylscopolamine (NMS)} or gallamine. Whereas gallamine displaced the binding of [³H]NMeQNB with high affinity, biphasic competition curves were observed using [³H]NMS only at higher ligand concentrations. The rank order of potency of gallamine in allosterically decelerating ligand dissociation kinetics was: [³H]NMS > [³H]atropine > [³H]NMeQNB > [³H]QNB. Our calculations demonstrate that the displayed heterogeneity of gallamine binding sites detected using [³H]NMS, but not the tertiary ligands, might be accounted for by the allosteric modification of the binding of this ligand by gallamine. Based on these findings, the exhibited binding profile of gallamine to muscarinic receptors is influenced strongly by ligand choice, and also by the ligand concentration used in the binding experiment. Furthermore, it is concluded that gallamine binds to three major sites on the muscarinic receptor, thereby revealing an apparent heterogeneity of its binding sites, even in a tissue which presumably possesses one major muscarinic receptor subtype such as the heart. According to several lines of evidence, gallamine binds competitively and with high affinity to NMS-accessible sites on the receptor. Under certain experimental conditions, it also appears to identify another low-affinity site, either due to its binding to NMS-inaccessible sites or through its differential ability to alter the binding of ligands to the main binding domain on the receptor in an allosteric fashion.