Activity of the new histamine H2-receptor antagonist zolantidine at cardiac and gastric H2-receptors.

The effect of the new histamine H2-receptor antagonist zolantidine was studied in different cardiac and gastric H2-receptor assays in comparison with ranitidine. Zolantidine (0.1-10 mumol/l) competitively antagonized the positive effects of histamine in the spontaneously beating guinea pig atria and in the electrically stimulated guinea pig papillary muscle (pA2 values were 6.98 and 6.78, respectively). At the highest concentrations zolantidine also reduced basal heart rate and cardiac contractility. In the isolated rat gastric fundus zolantidine up to 100 mumol/l did not modify histamine-induced acid secretion; it was similarly ineffective against dimaprit-induced acid secretion in the gastric fistula of conscious cats (up to 3 mumol/kg i.v.) and against histamine in the anesthetized rat with lumen-perfused stomach (up to 30 mumol/kg i.v.). In all these gastric secretory models ranitidine, as expected, antagonized histamine H2-receptor-mediated responses, showing a potency comparable to that found in cardiac preparations (pA2 values were 6.84, 6.38 and 6.78 in the atria, papillary muscle and gastric fundus, respectively). These data clearly showed that zolantidine is a very peculiar histamine H2-receptor antagonist, capable of distinguishing between cardiac and gastric H2-receptors; however, it still has to be elucidated whether this depends on a true heterogeneity in the histamine H2-receptor population or on the physicochemical properties of the drug.