Contribution of External Ca²⁺ to the Modulation by Prostaglandin E₂ of the Release of Acetylcholine from the Myenteric Plexus of Guinea Pig Ileum

Abstract

The effects of external concentration of [Ca²⁺] ([Ca²⁺]₀) on the modulation by indomethacin (IND) or prostaglandin E₂ (PGE₂) of the evoked release of acetylcholine (ACh) were investigated in the myenteric plexus of guinea pig ileum. An increase in the [Ca²⁺]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 PGE₂ did not modify the release of ACh induced by EFS and high K⁺, at any given [Ca²⁺]₀ tested. The nicotine-induced release of ACh was significantly inhibited by IND with no relation to [Ca²⁺]₀, and these inhibitory effects of IND were prevented by PGE₂. At 0.45 mM [Ca²⁺]₀, the extent of the recovery by PGE₂ was less potent than those at other [Ca²⁺]₀. IND inhibited the spontaneous release of ACh at lower [Ca²⁺]₀, and such inhibition was not observed in the presence of PGE₂. Thus, the modulatory actions of PGE₂ on the spontaneous and the nicotine-induced release of ACh may depend partially on [Ca²⁺]₀.