The release of gamma-aminobutyric acid was confirmed in isolated cat colon loaded with tritiated gamma-aminobutyric acid. Thirty to 180 minutes after loading the spontaneous efflux of tritium appeared to fit a single exponential curve with an efflux rate coefficient of 0.002 per minute. Electrical stimulation produced frequency-dependent increases in the tritium efflux and in the contractions. Even 120 minutes later over 91 percent of the total radioactivity in the superfusates was attributable to tritiated gamma-aminobutyric acid. The acid release and the contractions induced by electrical transmural stimulation were inhibited by tetrodotoxin and by a calcium-free medium. Release of the acid was not significant during contractions elicited by nicotine and acetylcholine. These findings indicate that gamma-aminobutyric acid is released from the terminals of neurons in the myenteric plexus of the colon.

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