Serotonin-containing enterochromaffin cells in the rabbit duodenal mucosa span the tissue contacting both the luminal and serosal sides. When the serosal surface is stimulated with carbachol in vitro, serotonin is secreted on the serosal side but not the mucosal side. Carbachol added to the luminal side is ineffective. Atropine but not hexamethonium blocks the effect of carbachol. Acetylcholine on the serosal surface also stimulates serotonin release on the serosal side. These findings indicate that enterochromaffin cells possess on their serosal surfaces muscarinic receptors that mediate vectorial release of serotonin when activated by cholinergic agonists.
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