[3H]Acetylcholine Release from the Guinea-pig Distal Colon: Comparison with Ileal [3H]Acetylcholine Release and Effect of Adrenoceptor Stimulation

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Abstract
To study cholinergic function in the guinea-pig colon, resting and electrically evoked 3H release after preincubation with [3H] choline has been compared in colonic and ileal myenteric plexus preparations. Fractional spontaneous colonic 3H release was significantly higher than ileal 3H release, while the reverse was true for electrically evoked 3H outflow. Electrically evoked 3H outflow in the colon was linearly related to stimulation frequency (0·2–3 Hz range) and current intensity (300–600 mA range), while 3H outflow per pulse was inversely related to stimulation frequency. Electrically evoked 3H outflow was prevented in Ca2+-free solution, indicating that it probably mirrored neuronal exocytotic [3H]acetylcholine release. Both noradrenaline and clonidine concentration-dependently inhibited electrically evoked 3H outflow, Clonidine being more potent but less efficacious than noradrenaline. For both noradrenaline and Clonidine, the potency and efficacy for inhibition of 3H outflow were close to the values previously reported for the inhibition of electrically evoked endogenous acetylcholine output from colonic preparations. In conclusion, these data indicate that 3H release after incubation with [3H]choline is a valid alternative to measurement of endogenous acetylcholine output to study colonic cholinergic neuronal function in the guinea-pig.

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