Pharmacological profile of new histamine H2-receptor antagonists related to cimetidine, ranitidine and lamtidine

New compounds structurally related to cimetidine, ranitidine and lamtidine have been prepared and tested for their histamine H2-receptor blocking activity on guinea-pig atria, rat perfused stomach and frog isolated gastric mucosa. These derivatives contain as a polar group, a diaminofurazan moiety, a 3-amino-4-methylfurazan or a 3-amino-4-phenylfurazan moiety. Ranitidine and lamtidine analogues display strong H2-antagonist activity in-vitro (KB on atria 0.037 microM and 0.0039 microM, respectively) and in-vivo on the lumen-perfused stomach of the anaesthetized rat (ID50 0.13 mumol kg-1 and 0.023 mumol kg-1 i.v., respectively). However, lamtidine analogues are ineffective in blocking the histamine-induced increase of H+ output in the frog isolated gastric mucosa. On the basis of the anomalous results in the frog, it is concluded that caution must be exercised in extrapolating information from amphibian to mammalian tissues with regard to the structure and the function of histamine receptors.