Muscarinic antagonists induce different receptor conformations in rat adenohypophysis.

We have employed a method based on ligand competition experiments, which is capable of detecting interactions among ligand-occupied binding sites, to study the interactions between rat adenohypophysis muscarinic receptors occupied by several muscarinic antagonists. In this method, one examines the binding of a labeled ligand (the primary ligand) in the absence and presence of a competing ligand. The inhibition of binding of the primary ligand by the competing ligand shows significant deviations from that expected assuming a population of noninteracting, heterogeneous binding sites. The deviations seen in the case of competition between N-methyl-4-piperidyl benzilate (4NMPB) and (-)-N-methyl scopolamine (a benzilate and tropate) are more pronounced than in the case of 4NMPB and (-)-3-quinuclidinyl benzilate (two benzilate derivatives). The occurrence of such deviations suggests the existence of site-site interactions among rat adenohypophysis muscarinic receptors. On the other hand, no deviations were observed in competition experiments in homogenates of rat cortex and medulla-pons. This finding correlates with the linear Scatchard plots (with no indications for site-site interactions or heterogeneity) obtained for the binding of muscarinic antagonists in these brain regions. A mathematical analysis demonstrates that the deviations from the expectations of the site-heterogeneity model observed in the rat adenohypophysis system (which shows similar binding patterns for all ligands employed) can occur only if the primary and competing ligands induce different conformational transitions upon binding to the receptor. It is concluded that different muscarinic antagonists can lead to different isomerization states of the receptor in the system.