Multidisciplinary analysis of muscarinic receptors in guinea-pig isolated ileum, atria and uterus in vitro

Abstract The classification of muscarinic receptor subtypes is hampered by an unavailability of highly selective, competitive antagonists. A combination of approaches, encompassing classical pharmacology, radioligand binding, biochemistry and molecular biology, may provide a more robust definition of the muscarinic receptor population in various tissues. This has been used, in the present study, to characterize muscarinic receptors in guinea-pig ileum, atria and uterus. The results show a high degree of congruence between the techniques. In ileum, M 1 , M 2 and M 3 receptors are present with the latter mediating contraction and inositol phospholipid hydrolysis. The function of M 2 receptors, while in the majority, is unknown although speculations include inhibition of β adrenoceptor mediated relaxation. M 1 receptors appear to be located prejunctionally and modulate cholinergic outflow. In atria, M 2 receptors are the only subtype present and act mediate negative and positive inotropy, due to coupling to distinct G proteins. In uterus, M 2 receptors probably mediate contraction and also couple to distinct G proteins. Additional studies using more selective antagonists are required to address this point. The uterus appears to represent the only smooth muscle identified to date which lacks, from binding, functional and northern blot analysis, M 3 receptors. It is concluded that use of a range of techniques provides more precise definition of muscarinic receptor subtypes. This approach may also have general applicability in instances where discriminative ligands are unavailable.