If small doses of histamine are alternated with small doses of acetylcholine on the isolated guinea-pig ileum in Tyrode solution, the response to histamine tends to diminish and may disappear. When the response to the small doses of histamine has disappeared, a contraction may be elicited by larger doses. The diminution is reversible, the response returning if the preparation is left unstimulated by drugs for varying periods of time. This phenomenon also occurs when the loops are suspended in Tyrode solution containing cocaine or atropine.
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