The effects of external concentration of [Ca
2+] ([Ca
2+]
0) on the modulation by indomethacin (IND) or prostaglandin E
2 (PGE
2) of the evoked release of acetylcholine (ACh) were investigated in the myenteric plexus of guinea pig ileum. An increase in the [Ca
2+]
0 from 0.45 to 3.6 mM depressed both the spontaneous and the nicotine-induced release of ACh, while the releases of ACh induced by electrical-field stimulation (EFS) or by high K
+ were augmented. IND and PGE
2 did not modify the release of ACh induced by EFS and high K
+, at any given [Ca
2+]
0 tested. The nicotine-induced release of ACh was significantly inhibited by IND with no relation to [Ca
2+]
0, and these inhibitory effects of IND were prevented by PGE
2. At 0.45 mM [Ca
2+]
0, the extent of the recovery by PGE
2 was less potent than those at other [Ca
2+]
0. IND inhibited the spontaneous release of ACh at lower [Ca
2+]
0, and such inhibition was not observed in the presence of PGE
2. Thus, the modulatory actions of PGE
2 on the spontaneous and the nicotine-induced release of ACh may depend partially on [Ca
2+]
0.
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