Flynn_1986_Mol.Pharmacol_30_193

The binding of carbachol, quinuclidinyl benzilate, pirenzepine, and scopolamine to muscarine receptors from the rabbit hippocampus and brainstem was examined in membranes and in digitonin solution, in order to determine whether the dispersion of receptor molecules altered the distinct ligand-binding profiles of the receptors of these tissues. The modification of receptor-effector complexes showing high affinity for carbachol in membranes, with guanyl 5'-yl imidodiphosphate (GppNHp), N-ethyl-maleimide (NEM), and ethylenediaminetetraacetate (EDTA), did not remove receptor heterogeneity. In addition to receptor heterogeneity, GppNHp-sensitive high affinity agonist binding to membranes was NEM sensitive in the brainstem but NEM insensitive in the hippocampus. Solubilization with digitonin in the presence of Mn2+ ions yielded GppNHp-sensitive, high affinity agonist-binding complexes from the brainstem but not the hippocampus. More extensive dissociation was achieved with solubilization in EDTA and NEM, and resulted in receptor populations which showed markedly lower and slightly different affinities for carbachol. However, the selectivities and affinities for three antagonists were little changed in solution from those found in membranes. These results support the view that there are differences in muscarine receptors as well as different receptor complexes with guanine nucleotide-binding regulatory proteins. The selective binding of carbachol appears largely dependent upon the association of receptors with other molecules, including effector molecules, whereas antagonists appear to recognize receptors irrespective of associated molecules.