Effect of metoclopramide and ranitidine on acetylcholine release from isolated rat stomach

Metoclopramide and ranitidine (10(-6)-10(-4) M) enhanced the electrical field stimulation-evoked contractions of isolated rat fundus and increased the gastric emptying in conscious rats. The enhancement of the fundus contractions by metoclopramide and ranitidine was abolished by atropine, but not by yohimbine, hexamethonium, propranolol or methysergide. The electrical field stimulation-evoked [3H]outflow from rat fundus strips, which has been preincubated with [3H]choline, was reduced by tetrodotoxin (10(-6) M) or in calcium-free medium, and potentiated by 4-amino-pyridine (3 X 10(-4) M), an acetylcholine (ACh)-releasing agent. Metoclopramide and ranitidine (10(-6)-10(-4) M) did not increase the [3H]outflow from the strips, in spite of causing a significant enhancement of their contractile response. However, both agents caused an increase in the ratio of [3H]acetylcholine/[3H]choline released into the superfusate during electrical field-stimulation. In rat fundus homogenates, metoclopramide and ranitidine showed a significant cholinesterase inhibition. These results seem to cast a doubt on the generally held ACh release hypothesis for the action mechanism of metoclopramide on one hand, and suggest, on the other hand, that cholinesterase inhibition contributes to some extent to the gastrokinetic effects of metoclopramide and ranitidine.