Effects of adenylate cyclase inhibitors, phosphodiesterase inhibitors, and dibutyryl cyclic AMP on spontaneous and various stimuli-induced acetylcholine release from guinea pig ileum myenteric plexus

Role of cyclic AMP in prostaglandin-induced modulation of acetylcholine release from the myenteric plexus of guinea pig ileum

Prostaglandins (PGs) have modulatory effects on spontaneous and nicotine-induced release of acetylcholine (ACh) from the myenteric plexus of guinea pig ileum. To determine whether cyclic AMP is involved in the mechanisms of these effects, we studied ACh release under conditions that inhibit PG synthesis. Indomethacin (IND), a cyclooxygenase inhibitor, inhibited ACh release concentration-dependently. The effect of the maximally inhibitory concentration of IND (2.8 microM) on nicotine-induced ACh release were reversed concentration-dependently by PGE2, forskolin, 3-isobutyl-1-methylxanthine (IBMX) and 8-bromo cyclic AMP. These compounds caused concentration-dependent reversal of the inhibition of spontaneous ACh release by IND, but their concentrations for restoration of spontaneous release were higher than those for restoration of nicotine-induced release. The effects of PGE2 and forskolin or IBMX were not additive in reversing the inhibition of nicotine-induced ACh release by IND. Neither forskolin nor 8-bromo cyclic AMP alone had any significant effect on either release. These results showed that increase in the level of cyclic AMP in myenteric cholinergic neurons restored ACh release from the tissue whose PG level had been lowered by IND and indicated that endogenous PGs may modulate the level of intraneuronal cyclic AMP.

Prostaglandin E2 induced the cyclic AMP-dependent release of acetylcholine in circular muscles of the isolated guinea pig ileum

Prostaglandin E2 (PGE2) induced a dose-dependent increase in tone of the circular muscles of guinea pig ileum in vitro. These actions of PGE2 were deleted in the cold-stored preparations and blocked by tetrodotoxin. Atropine reduced the effects of PGE2 and physostigmine potentiated the PGE2-induced contractions. The release of acetylcholine (ACh) by PGE2 was responsible for initiating this contraction. The effect of PGE2 was compared with that of an electrical stimulation which also initiated a non-receptor-mediated release of ACh. Hexamethonium abolished the effect of PGE2 but did not influence the actions of the electrical stimulations. Synaptosomal fractions of the circular muscles were prepared to study the release of [14C]ACh. However, PGE2 failed to evoke a marked increase in the efflux of radioactivity, even at the maximal concentration. Damage and/or removal of the myenteric plexus may be responsible for this result because electrical stimulations that exert a powerful spasmogenic effect on longitudinal muscles also induced an insensitive response. Alloxan and ethacrynic acid, inhibitors of adenylate cyclase, reduced the activity of PGE2 at a concentration insufficient to modify either the actions of ACh or the electrical stimulations. 3-Isobutyl-1-methylxanthine (IBMX) potentiated the responses to PGE2 at a dose sufficient to block the activity of phosphodiesterase (PDE). Imidazole, a stimulator of PDE, decreased the actions of PGE2 in a dose-dependent manner. IBMX, like imidazole, failed to modify the activities of both ACh and the electrical stimulations. These results indicate that PGE2 may function as a releaser of ACh in a cyclic AMP-dependent manner in the circular muscles of guinea pig ileum.

Picric acid functions as a releaser of [14C]acetylcholine in isolated ileal synaptosomal preparations of guinea-pig

The effect of picric acid on the release of [14C]acetylcholine has been investigated in isolated ileal synaptosomes of guinea-pig. Nicotine, high K-depolarization (50 mM KCl) and electrical field stimulation were employed to characterize the specificity of picric acid. Picric acid induced the release of labelled acetylcholine in a dose-dependent manner and this action was negated by the removal of calcium ions from the bathing medium. Tetrodotoxin (0.1 microM) abolished the actions of picric acid, nicotine or electrical field stimulation (0.1 Hz). It reduced but did not totally suppress the effect of high K-depolarization. Agents capable of affecting the content of cyclic AMP, such as forskolin and alloxan, modified the effects of picric acid or nicotine but did not influence the effects of high K-depolarization or electrical field stimulation. Indomethacin, at a concentration (1 microM) effective in inhibiting the synthesis of prostaglandins, reduced the release of acetylcholine evoked by picric acid or nicotine, but did not affect the responses to high K-depolarization or electrical field stimulation. [3H]5-hydroxytryptamine was also released by high K-depolarization at a concentration sufficient to induce the release of acetylcholine. Similar results were obtained when the frequency of electrical field stimulation was raised to 10 Hz. However, picric acid did not initiate the release of 5-hydroxytryptamine. These results suggest that picric acid functions as a releaser of acetylcholine through a mechanism different from that of other stimulants.

Effect of external Ca2+ on the spontaneous and the various stimuli-induced acetylcholine release from guinea-pig ileum myenteric plexus

Effect of external Ca2+ concentration ([Ca2+]o) on spontaneous and various types of stimuli-induced acetylcholine (ACh) release from guinea-pig ileum myenteric plexus was studied. Electrical field stimulation- or high-K+-induced ACh release increased with the increment of [Ca2+]o. On the other hand, the spontaneous and the nicotine-induced ACh release increased up to 0.45 mM [Ca2+]o and then declined progressively as [Ca2+]o was raised. Qualitatively similar results were obtained with dimethylphenylpirerazinium-, 5-hydroxytryptamine- and substance P-induced ACh release. These results were discussed in terms of the stabilizing effect of Ca2+.