Bombesin potentiates the effect of acetylcholine on isolated strips of fish stomach.

The interaction between bombesin and acetylcholine acting on smooth muscle of the stomach wall was investigated in two species of teleost fish. Oncorhynchus mykiss (rainbow trout) and Gadus morhua (Atlantic cod). Acetylcholine or bombesin alone has an excitatory effect on the stomach muscle. The effect on contraction amplitude of acetylcholine (10(-6)-10(-5) M) alone is about 10-times greater than the effect of bombesin (10(-9)-10(-7) M). In molar terms however, bombesin is more potent than acetylcholine. Bombesin (10(-8)-10(-7) M) added 0.5-3 min prior to acetylcholine potentiates the effect of acetylcholine in a dose-dependent manner. The potentiation is most pronounced in circular muscle preparations, but is present also in longitudinal muscle preparations. Bombesin affects the response to carbachol (10(-6) M) with a similar potentiation, indicating that the potentiation is not caused by inhibition of choline esterase activity. Atropine (10(-6)-10(-5) M) abolishes the response to bombesin plus acetylcholine as well as the response to acetylcholine alone. Tetrodotoxin (10(-6) M) does not block the effect of acetylcholine, bombesin or the combination acetylcholine plus bombesin. Substance P (10(-9)-10(-7) M) which has a similar excitatory effect on the stomach muscle as bombesin, does not potentiate the effect of acetylcholine. Immunohistochemistry has shown the presence of strong bombesin-like immunoreactivity in stomach nerves of the cod and weak bombesin-like immunoreactivity in rainbow trout nerves. In addition, bombesin-like immunoreactivity was demonstrated in endocrine cells in the gastric and intestinal mucosa of both species. It is concluded that bombesin, contained either in nerve fibres or in mucosal endocrine cells, specifically potentiates the effect of acetylcholine in the fish stomach.