Potentiation by phenolphthalein of the responses of guinea-pig ileum and rat stomach strip to PGE2 and other agonists.

Phenolphthalein was examined for its effect on the activity of isolated muscle (guinea-pig ileum and colon, rat stomach), and on the tissue responses to PGE2, histamine and 5-HT. The ileal circular muscle and both muscle layers of the colon were unaffected by phenolphthalein. In contrast, the laxative potentiated the responses of the longitudinal muscle of guinea-pig isolated ileum and the rat stomach strip to the agonists, particularly PGE2. This potentiation was reduced by indomethacin in vivo, but mepyramine or methysergide had little or no effect. Augmentation of muscle activity by phenolphthalein, particularly in the response to PGE2, may contribute to the laxative effect.