M(2) MUSCARINIC RECEPTOR INHIBITION OF AGONIST-INDUCED CYCLIC ADENOSINE-MONOPHOSPHATE ACCUMULATION AND RELAXATION IN THE GUINEA-PIG ILEUM

The purpose of this study was to characterize the role of M(2) muscari nic receptors in inhibiting relaxant effects of drugs that stimulate c yclic 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 cel ls and tissues. Appreciable stimulation of cAMP (>50% over basal level s) was achieved with forskolin and maximally effective concentrations of isoproterenol, cicaprost, prostaglandin E(1), prostaglandin E(2) an d prostaglandin I-2, with the stimulation over basal levels of cAMP be ing 14.9-, 2.51-, 2.45-, 2.27-, 2.28- and 1.52-fold, respectively. Mod erate or no cAMP stimulation was observed with dopamine, 5-hydroxytryp tamine, 5-methoxytryptamine, dimaprit, vasoactive intestinal peptide, SKF-38393, 2-chloroadenosine, CGS-21680, prostaglandin D-2, secretin a nd vasopressin. Oxo-M (1 mu M) inhibited cAMP accumulation by 35% unde r basal conditions. Oxo-M inhibited specific agonist-stimulated cAMP l evels by 20 to 70%. However, oxo-M caused little or no inhibition of s pecific prostaglandin I-2- and cicaprost-stimulated cAMP levels (5 and 0%, respectively). In general, there was a correlation between the ab ilities of the various agonists to stimulate cAMP accumulation and to cause relaxation of the isolated ileum, with an exception being cicapr ost. Experiments were carried out with isolated ileum to determine whe ther activation of M(2) receptors inhibited the relaxant effects of th e various agonists. In these experiments, the ileum was first treated with N-(2-chloroethyl)-4-piperidinyl diphenylacetate to selectively in activate M(3) receptors. After this treatment phase, contractile respo nses to oxotremorine-M were measured in the presence of histamine and a given relaxant agent. These measurements were repeated in the presen ce of the M(2)-selective antagonist AF-DX 116. Analysis of the data sh owed that part of the contractile response to oxotremorine-M could be attributed to an M(2)-mediated inhibition of the relaxation. This M(2) component of the contractile response was greatest when forskolin or isoproterenol was used as the relaxant agent. In contrast, little or n o M(2) response was measured in the presence of dopamine and cicaprost .