Synergistic and antagonistic pharmacodynamic interaction between ranitidine and cisapride: a study on the isolated rabbit intestine.

The present study examines the pharmacodynamic interaction between the H(2)-receptor antagonist ranitidine and the prokinetic agent cisapride on the isolated rabbit intestine. Ranitidine produced a concentration-dependent contractile effect on the duodenal, ileal and ascending colon preparations, with EC(50)values of 1.35 x 10(-4)M for the duodenum, 1.2 x 10(-4)M for the ileum and 1.15 x 10(-4)M for the ascending colon. The effect of cisapride on the ranitidine contractile effect was dependent on the cisapride concentration used. Thus, cisapride, at concentrations from 10(-10)up to 5 x 10(-7)for the duodenum and the ascending colon and up to 10(-6)M for the ileum, potentiated the contractile responses of the preparations to ranitidine. However, at higher concentrations cisapride produced a non-competitive inhibition of the intestinal contractile responses to ranitidine with IC(50)values of 4.2 x 10(-5)M for the duodenum, 1.65 x 10(-5)M for the ileum and 3.2 x 10(-6)M for the ascending colon. These data show that cisapride may modify the contractile responses of the isolated rabbit intestine to ranitidine, having a potentiating effect up to a certain concentration and an antagonistic one at higher concentrations. In conclusion, co-administration of the above drugs may lead to enhanced or reduced intestinal motility.