Evidence for postjunctional release of ATP evoked by stimulation of muscarinic receptors in ileal longitudinal muscles of guinea pig.

The origin of the ATP release evoked by muscarinic agonists and veratridine from longitudinal muscles of the guinea pig was assessed with muscarinic antagonists. Acetylcholine (ACh) (1 microM) and bethanechol (10 microM) produced an immediate and marked ATP release, which was almost completely blocked by atropine (0.3 microM) and by the M3 muscarinic antagonist 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) (1 microM); release was only slightly affected by tetrodotoxin (0.6 microM). The bethanechol-evoked release of ATP was partly, but not significantly, inhibited by pirenzepine, a M1 muscarinic antagonist. Veratridine, an ACh releaser, also elicited a delayed ATP release, in a concentration-related manner. This ATP release was greatly antagonized by atropine and by Ca++ removal from the medium, implying mediation by endogenous ACh released after depolarization. In contrast, electrically evoked ACh release was enhanced by atropine and 4-DAMP. Bethanechol, unlike veratridine, failed to elicit measurable ACh release from the tissue. The contraction evoked by bethanechol was notably inhibited by atropine and 4-DAMP, but not by pirenzepine and AFDX-116, a M2 muscarinic antagonist, at a concentration of 0.3 microM. These findings suggest strongly that ATP is postjunctionally released from the ileal smooth muscles after stimulation of postsynaptic muscarinic receptors, presumably M3 receptors.