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-mu-M) and bethanechol (10-mu-M) produced an immediate and marked ATP release, which was almost completely blocked by atropine (0.3-mu-M) and by the M3 muscarinic antagonist 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) (1-mu-M); release was only slightly affected by tetrodotoxin (0.6-mu-M). 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-mu-M. These findings suggest strongly that ATP is postjunctionally released from the ileal smooth muscles after stimulation of postsynaptic muscarinic receptors, presumably M3 receptors.