The release of histamine by parasympathetic stimulation in guinea-pig auricle and rat ileum

Two preparations, a segment of rat ileum and the vagally innerved guinea-pig auricles, have been used in an analysis of the responses to vagal or to electrical field stimulation. The responses to parasympathetic stimulation were depressed by atropine and by tetrodotoxin, and potentiated by eserine. Supramaximal stimulation (10-20 Hz) resulted in increased release of acetylcholine and histamine, both in rat ileum and guinea-pig auricles. The release of histamine after parasympathetic stimulation did not exhibit tachyphylaxis, and it was not reproduced by non-parasympathetic stimuli. In both preparations, atropine produced a significant, dose-related reduction of histamine measured in the bath fluid after stimulation, while eserine increased histamine output. A significant diminution of mast cell granules metachromasia was observed in guinea-pig auricles and in rat intestine after parasympathetic stimulation. The possibility is discussed that acetylcholine released by parasympathetic stimulation would in turn evoke the secretion of histamine from tissue mast cells.