Contractile role of M-2 and M-3 muscarinic receptors in gastrointestinal smooth muscle

Muscarinic agonists elicit contraction through M-3, receptors in most isola ted preparations of gastrointestinal smooth muscle, and not surprisingly, s everal investigators have identified M-3, receptors in smooth muscle using biochemical, immunological and molecular biological methods. However, these studies have also shown that the M-2, receptor outnumbers the M-3, by a fa ctor of about four in most instances. In smooth muscle, M-2 receptors media te phosphoinositide hydrolysis and Ca2+ mobilization, whereas M, receptors mediate an inhibition of cAMP accumulation. The inhibitory effect of the M- 2, receptor on cAMP levels suggests an indirect role for this receptor; nam ely, an inhibition of the relaxant action of cAMP-stimulating agents. Such a function has been rigorously demonstrated in an experimental paradigm whe re gastrointestinal smooth muscle is first incubated with 4-DAMP mustard to inactivate M, receptors during a Treatment Phase, and subsequently, the co ntractile activity of muscarinic agonists is characterized during a Test Ph ase in the presence of histamine and a relaxant agent. When present togethe r, histamine and the relaxant agent (e.g., isoproterenol or forskolin) have no net contractile effect because their actions oppose one another. Howeve r, under these conditions, muscarinic agonists elicit a highly potent contr actile response through the M-2 receptor, presumably by inhibiting the rela xant action of isoproterenol or forskolin on histamine-induced contractions . This contractile response is pertussis toxin-sensitive, unlike the standa rd contractile response to muscarinic agonists, which is pertussis toxin-in sensitive. When measured under standard conditions (i.e., in the absence of histamine and without 4-DAMP mustard-treatment), the contractile response to muscarinic agonists is moderately sensitive to pertussis toxin if isopro terenol or forskolin is present. Also, pertussis toxin-treatment enhances t he relaxant action of isoproterenol in the field-stimulated guinea pig ileu m. These results demonstrate that endogenous acetylcholine can activate M-2 , receptors to inhibit the relaxant effects of ss-adrenoceptor activation o n M-3, receptor-mediated contractions. An operational model for the interac tion between M-2, and M-3, receptors shows that competitive antagonism of t he interactive response resembles an M-3, profile under most conditions, ma king it difficult to detect the contribution of the M-2, receptor.