Comparison of the effects of some muscarinic agonists on smooth muscle function and phosphatidylinositol turnover in the guinea‐pig taenia caeci

1 The effects of the muscarinic agonists acetylcholine (ACh), carbachol (CCh), AHR‐602, and McN‐A‐343 on contractility and on inositol phosphate accumulation in the presence of lithium were compared in the taenia of the guinea‐pig caecum. 2 Compared to CCh, ACh was a full agonist for contraction but AHR‐602 and McN‐A‐343 were partial agonists producing 80–85% of the maximal response to CCh. Similar to previous findings with CCh, tonic contractions produced by AHR‐602 and McN‐A‐343 were less sensitive to inhibition by nifedipine or verapamil than tonic contractions to ACh. 3 CCh and ACh produced similar increases in inositol phosphate accumulation and the effect of CCh (0.1 mm) was inhibited by atropine (IC50 8.5 nm) and pirenzepine (IC50 450 nm). The accumulation of inositol phosphates in the presence of AHR‐602 or McN‐A‐343 was not significantly different (P > 0.05) from basal levels. 4 A concentration of 0.2 mm AHR‐602 produced a parallel shift of the concentration‐response curve to CCh on inositol phosphate accumulation. The IC50 value for inhibition of CCh (0.1 mm) was > 50 fold higher than the EC50 value for contraction produced by the partial agonist. McN‐A‐343 (20 μm) produced a flattening of the concentration‐response curve to CCh for inositol phosphate accumulation. 5 The results suggest that the increase in phosphatidylinositol turnover produced by muscarinic agonists, like the contractile response, involves an M2‐muscarinic receptor. AHR‐602 and McN‐A‐343 are partial agonists for the contractile response and while producing no significant increase in phosphatidylinositol turnover inhibit the response to CCh.