Abstract
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 M2receptors inhibited the relaxant effects of the various agonists. In these experiments, the ileum was first treated withN-(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.
Footnotes
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Send reprint requests to: Frederick J. Ehlert, Ph.D., Department of Pharmacology, College of Medicine, University of California, Irvine, Irvine, CA 92717.
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↵1 This work was supported by National Institutes of Health Grant NS30882.
- Abbreviations:
- AF-DX 116
- [[2-[(diethylamino)methyl]-1-piperidinyl]acetyl]-5,11-dihydro-6H-pyrido[2,3b][1,4]benzodiazepine-6-one
- cAMP
- cyclic AMP
- 4-DAMP mustard
- N-(2-chloroethyl)-4-piperidinyl diphenylacetate
- 5-HT
- 5-hydroxytryptamine
- KRB
- Krebs Ringer bicarbonate
- 5-MT
- 5-methoxytryptamine
- oxo-M
- oxotremorine-M
- PGE1
- prostaglandin E1
- PGE2
- prostaglandin E2
- PGI2
- prostaglandin I2
- VIP
- vasoactive intestinal peptide
- Received April 15, 1996.
- Accepted September 10, 1996.
- The American Society for Pharmacology and Experimental Therapeutics
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M2 Muscarinic Receptor Inhibition of Agonist-induced Cyclic Adenosine Monophosphate Accumulation and Relaxation in the Guinea Pig Ileum
