Two affinity states of M1 muscarine receptors

Abstract
The binding of oxotremorine-M to M1 muscarine receptors was examined by measuring competition between the agonist and3H-pirenzepine, using rabbit hippocampal membranes suspended in 20 mM Tris buffer containing 1 mM Mn2+. Both ligands interacted with a single class of receptors. The receptors could assume two affinity states for oxotremorine-M, with equal numbers of high-affinity (K H) and low-affinity (K L) sites. K H interconverted reversibly toK L in the absence of divalent cations and interconverted reversibly to a state similar toK L in the presence of guanyl 5′-yl imidodiphosphate. The results are compatible with a model in which a model in which a pair of receptor molecules can be stabilized by a guanine nucleotide-binding “G protein” and have one site each ofK H andK L affinity.