The effects of GABA (γ-aminobutyric acid) and homotaurine (3-aminopropane sufonic acid) on propulsive activity of the isolated segment, and on lomgitudinal and circular muscle strips were investigated in the guinea-pig distal colon.
GABA (0.1 and 1 mM) inhibited spontaneous propulsive activity with a reduction of longitudinal tension of the segment. Homotaurine (1 mM) slightly inhibited spontaneous propulsive activity.
GABA (0.01-1 mM) relaxed both longitudinal and circular muscle strips. Homotaurine (1 mM) slightly relaxed cicular muscle strip.
Desensitization to GABA and homotaurine was observed.
The inhibitory effects of GABA on both muscle strips were abolished by tetrodotoxin or atropine, but not by bicuculline. Carbachol-induced contractions on both muscle strips were not influenced by GABA or homotaurine.
These results suggest that GABA-induced inhibition of propulsive activity in the isolated colonic segment may result from activation of GABAB receptor on the cholinergic neurones in the wall, which in turn leads to reduction of release of transmitter acetylcholine.
