Contrasting effects of carbachol, McN-A-343 and AHR-602 on Ca(2+)-mobilization and Ca(2+)-influx pathways in taenia caeci.

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 >> AHR-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 Ca(2+)-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 Ca(2+)-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.