Functional characterization of muscarinic receptors in murine airways

Abstract

1 The effects of muscarinic receptor antagonists considered to be selective for Ml receptors (pirenzepine; PZ), M2 receptors (AFDX-1 16), and for M3 receptors (4-diphenyl acetoxy N-methylpiperidine (4-DAMP)) were used to investigate the existence of muscarinic receptor subtypes in murine airways. Atropine was used as a nonselective antagonist. The effects of these antagonists were studied upon tracheal contractions induced either by EFS (electric field stimulation) or by application of an exogenous cholinoceptor agonist (arecoline).2 The muscarinic receptor antagonists tested inhibited arecoline-induced tracheal contractions with the following rank order of potency: 4-DAMP = atropine> pirenzepine = AFDX-1 16. The rank order of potency of the muscarinic antagonists used in inhibiting EFS-induced tracheal contractions was: 4-DAMP = atropine> PZ> AFDX-1 16. The pA2 values for these antagonists were similar when compared to the pA2 values determined in guinea-pig and bovine airway smooth muscle. 3 In addition to in vitro studies, the effects of inhalation of the different muscarinic antagonists on lung function parameters in vivo were investigated. Inhalation of 4-DAMP induced a decrease in airway resistance and an increase in lung compliance. In contrast, inhalation of AFDX-116 induced an increase in airway resistance and almost no change in lung compliance. Apart from some minor effects of atropine on airway resistance, atropine, PZ, and pilocarpine failed to induce changes in lung mechanics as determined by in vivo lung function measurements. 4 The results provide evidence for the existence of M3 receptors on murine tracheae that are involved in the contraction of tracheal smooth muscle. This is in agreement with other animal species such as the guinea-pig and bovine. In vivo experiments also demonstrated that in the mouse, M3 receptors play an important role in bronchial smooth muscle contraction and thus in bronchoconstriction. Interestingly we have also demonstrated that M2 receptors can play a role in bronchodilatation. Inhalation of an M2 receptor antagonist induced an increase in airway resistance whereas inhalation of an M3 receptor antagonist induced a decrease in airway resistance. It is therefore likely that an M3/M2 receptor balance plays an important role in the regulation of airway function.

Characterization of muscarinic receptors mediating contractions of circular and longitudinal muscle of human isolated colon

“…In our study AF-DX 116 had at least a 100 fold lower affinity than 4-DAMP for muscarinic receptors in CM (7.36) and LM (6.44). The pA2 value in LM is similar to the low affinity of AF-DX 116 for the muscarinic receptor subtype in the guineapig ileum (Giachetti et al, 1986) but the higher value for CM is more similar to that observed in guinea-pig atrium (Giachetti et al, 1986). It is unclear at this time why there is a difference for AF-DX 116 between the CM and LM muscle coats as this was not observed with the other muscarinic antagonists.…”

“…AF-DX 116 has an affinity proffile of M2 > M 1> M4 > M3 in functional pharmacological studies (Hammer et al, 1986;Giachetti et al, 1986;Giraldo et al, 1987) and a similar profile in human cloned muscarinic receptors (Dorje et al, 1991). In our study AF-DX 116 had at least a 100 fold lower affinity than 4-DAMP for muscarinic receptors in CM (7.36) and LM (6.44).…”

“…Pirenzepine is a relatively high affinity antagonist for Ml muscarinic receptors with intermediate affinity for M4 muscarinic receptors and lower affinity for the M2 and M3 muscarinic receptors (Lambrecht et al, 1989;Caulfield & Brown, 1991;Dorje et al, 1991;Grimm et al, 1994). (11-[[[2 -(diethylamino)methyl] -l-piperidinyl]acetyl] -5,1 1-dihydro-6H-pyridol [2,3,-b][1,4]benzodiazepine-6-one) has high affinity for the M2 muscarinic receptor, but also has affinity for the Ml muscarinic receptor with lower affinity for the M3 and M4 muscarinic receptors (Birdsall & Hulme, 1983;Giachetti et al, 1986;Hammer et al, 1986). 4-DAMP has a relatively high affinity for M3 muscarinic receptors but also has affinity at Ml and M4 muscarinic receptor subtypes (Dodds et al, 1987;Eltze & Figala, 1988;Eltze et al, 1993;Grimm et al, 1994).…”

Effect of M2 and M3 Muscarinic Receptors on Airway Responsiveness to Carbachol in Bronchial-Hypersensitive (BHS) and Bronchial-Hyposensitive (BHR) Guinea Pigs.

“…This suggested that the response via M 3 muscarinic receptors and the number of M 3 muscarinic receptors might be greater in BHS than in BHR. After treatment with 4-DAMP (10 -5 M), no contraction was observed in either BHS or BHR, because it nonselectively blocked muscarinic receptors [8,11] and inhibited CCh-induced smooth muscle contraction.…”

Effects of neuraminidase on equine isolated bronchi

“…Moreover, also in other species, including humans, the comparison of the pre-and postjunctional potencies of the M1-, M2-and M3-selective antagonists has suggested that autoinhibition of acetylcholine release is mediated via an 'M2-like' receptors (22)(23)(24)(25). In any case, even thought the muscarinic autoreceptors may be a different receptor subtype to the classical M2, the results that we obtained with pilocarpine and methoctramine were not unexpected because these drugs are receptor selective rather the receptor specific (12).…”