Temperature-dependence and heterogeneity of muscarinic agonist and antagonist binding

The binding parameters of muscarinic agonists and antagonists in rat central (brain, cerebellum and striatum) and peripheral (heart and lung) tissues were determined at 2 degrees and 37 degrees from competitive binding experiments with (3H)-quinuclidinylbenzylate (QNB). Muscarinic ligands binding affinities for cerebellum, heart and lung were tightly correlated. These tissues were also characterized by their low concentration of muscarinic receptors with high affinity for agonists. In contrast, brain and striatum contained a higher concentration of muscarinic receptors with a lower affinity for agonists. The affinity of QNB was lower at 2 degrees whereas that of other ligands was higher. The temperature-dependent shifts of competition curves differed from tissue to tissue and from compound to compound. The shifts were highest with gallamine and carbachol. The binding isotherms of muscarinic ligands were tentatively studied with a two-site binding model. The percentage of high- and low-affinity binding components of agonists differed with the compound. Guanine nucleotides and the temperature increase lowered agonist affinity without changing the proportions of the high- and low-affinity binding components. These results corroborate that the binding heterogeneity of muscarinic ligands does not depend only on the presence of two distinct receptors. Neither guanine nucleotides nor temperature changes allow conversion between the different putative conformational states of the muscarinic receptors.