The action of chloroquine on the histamine receptors of the isolated guinea-pig ileum.

The action of chloroquine on the histamine receptors was investigated on the guinea-pig isolated ileum. The log dose response curve to histamine in the presence of chloroquine (0.75 mug/ml) was to the right and parallel to that in the absence of the chloroquine. This concentration of chloroquine did not modify the contractions of the preparation to acetylcholine or to transmural stimulation. It had not effect on the contractions of the isolated vas deferens from the guinea-pig to either transmural stimulation or to hypogastric nerve stimulation. The pA2-pA10 value for chloroquine using histamine as the agonist was 0.87. It was concluded that chloroquine, in low concentration, is a specific antagonist of histamine, by probably a competitive mechanism.