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.
