Release of endogenous and labeled GABA from isolated guinea pig ileum.

The release of gamma-aminobutyric acid (GABA) was studied by comparing the properties of labeled GABA released from preloaded preparations to those of endogenous GABA released from the isolated guinea pig ileum. The spontaneous release of endogenous GABA was 4.46 +/- 0.10 pmol X min-1 X g wet wt-1, and the fractional rate of endogenous GABA release was much lower. The ratio of evoked to spontaneous release of endogenous GABA was high compared with that of labeled GABA. The electrical transmural stimulation-evoked release of labeled and endogenous GABA was inhibited by superfusion with tetrodotoxin and Ca2+-free Krebs-Ringer solution containing 1 mM ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid. Thus the nature of the stimulation-evoked release of labeled GABA was similar to that of endogenous GABA. These results indicate that the released GABA is neuronal in origin and provide additional evidence for the presence of GABA-ergic neurons in the guinea pig ileum.