Vol. 280, Issue 1, 189-199, 1997

M₂ Muscarinic Receptor Inhibition of Agonist-induced Cyclic Adenosine Monophosphate Accumulation and Relaxation in the Guinea Pig Ileum¹

Rennolds S. Ostrom and Frederick J. Ehlert

Department of Pharmacology, College of Medicine, University of California, Irvine, Irvine, California

The purpose of this study was to characterize the role of M₂ muscarinic receptors in inhibiting relaxant effects of drugs that stimulate cyclic AMP (cAMP) accumulation in the guinea pig ileum. We investigated the ability of oxotremorine-M (oxo-M) to inhibit cAMP accumulation in the presence of agonists that stimulate adenylyl cyclase in other cells and tissues. Appreciable stimulation of cAMP (>50% over basal levels) was achieved with forskolin and maximally effective concentrations of isoproterenol, cicaprost, prostaglandin E₁, prostaglandin E₂ and prostaglandin I₂, with the stimulation over basal levels of cAMP being 14.9-, 2.51-, 2.45-, 2.27-, 2.28- and 1.52-fold, respectively. Moderate or no cAMP stimulation was observed with dopamine, 5-hydroxytryptamine, 5-methoxytryptamine, dimaprit, vasoactive intestinal peptide, SKF-38393, 2-chloroadenosine, CGS-21680, prostaglandin D₂, secretin and vasopressin. Oxo-M (1 µM) inhibited cAMP accumulation by 35% under basal conditions. Oxo-M inhibited specific agonist-stimulated cAMP levels by 20 to 70%. However, oxo-M caused little or no inhibition of specific prostaglandin I₂- and cicaprost-stimulated cAMP levels (5 and 0%, respectively). In general, there was a correlation between the abilities of the various agonists to stimulate cAMP accumulation and to cause relaxation of the isolated ileum, with an exception being cicaprost. Experiments were carried out with isolated ileum to determine whether activation of M₂ receptors inhibited the relaxant effects of the various agonists. In these experiments, the ileum was first treated with N-(2-chloroethyl)-4-piperidinyl diphenylacetate to selectively inactivate M₃ receptors. After this treatment phase, contractile responses to oxotremorine-M were measured in the presence of histamine and a given relaxant agent. These measurements were repeated in the presence of the M₂-selective antagonist AF-DX 116. Analysis of the data showed that part of the contractile response to oxotremorine-M could be attributed to an M₂-mediated inhibition of the relaxation. This M₂ component of the contractile response was greatest when forskolin or isoproterenol was used as the relaxant agent. In contrast, little or no M₂ response was measured in the presence of dopamine and cicaprost.