Contrasting effects of carbachol, McN-A-343 and AHR-602 on Ca²⁺-mobilization and Ca²⁺-influx pathways in taenia caeci

Abstract

1. We compared the binding profiles and contractile mechanisms of putative muscarinic M₁ agonists McN-A-343 and AHR-602 with those of carbachol in smooth muscle of guinea-pig taenia caeci.

2. McN-A-343 and AHR-602, as well as carbachol, completely displaced the atropine-sensitive binding of [³H]-quinuclidinyl benzilate to muscarinic receptors present in the membrane preparation. The potency order for the affinity of these agents for muscarinic receptors was carbachol>McN-A-343>4AHR-602.

3. In the presence of 2.2 mM extracellular Ca²⁺, McN-A-343 and AHR-602 induced contraction corresponding to 79 and 85%, respectively, of the maximal contraction to 0.1 mM carbachol. Contractions induced by these agents were mediated via activation of the muscarinic receptor subtype that had a high affinity for 4-DAMP (M₃ selective) but a low affinity for pirenzepine (M₁ selective) and AF-DX 116 (M₂ selective). These contractions were inhibited by an L-type Ca²⁺ channel blocker, verapamil.

4. In Ca²⁺-free solution containing 2 mM EGTA, carbachol elicited a transient contraction whereas no contraction was observed in response to McN-A-343 and AHR-602. Application of McN-A-343 or AHR-602 inhibited the carbachol-induced contraction in Ca²⁺-free solution, and this inhibition was surmounted by a higher concentration of carbachol.

5. The EC₅₀ value for carbachol-induced contraction in the presence of extracellular Ca²⁺ was approximately 175 times lower than that in the absence of Ca²⁺. After treatment with propylbenzilylcholine mustard, carbachol induced contraction only in the presence of extracellular Ca²⁺.

6. The results suggest that in the taenia caeci there is a greater receptor reserve for muscarinic M₃ receptor-mediated Ca²⁺ influx than for M₃ mediated Ca²⁺ release. The compounds McN-A-343 and AHR-602 are agonists of the Ca²⁺ influx pathway, but do not appear to stimulate the Ca²⁺ release pathway.

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