Effects of histamine H2 receptor agonists and antagonists on the isolated guinea pig gallbladder

— Histamine H2 receptor‐mediated responses were examined on cholecystokinin‐octapeptide (CCK‐8)‐precontracted guinea pig gallbladder in vitro, testing histamine and a series of specific histamine H2 receptor agonists and antagonists. Among the antagonists tested, zolantidine and compound SKF 92857 were previously shown to antagonize H2 receptor‐mediated responses in the heart, but not in the stomach. Histamine, in the presence of the H1 receptor antagonist mepyramine (10 μM), and the H2 receptor agonists dimaprit, impromidine and amthamine, inhibited CCK‐8 (3 nM)‐induced contractions in a concentration‐dependent fashion, with the following rank orders of potency: impromidine > amthamine > histamine > dimaprit (pD2 values were 6.73 ± 0.04, 5.44 ± 0.07, 4.64 ± 0.04 and 3.71 ± 0.05, respectively). The maximal relaxation produced by dimaprit was significantly lower than that produced by histamine, as well as by impromidine and amthamine, which behaved as full agonists. All the H2 receptor antagonists examined were able to inhibit amthamine‐induced relaxation. Famotidine (pA2 = 7.09 ± 0.26), zolantidine (pA2 = 6.54 ± 0.11), compound SKF 92857 (pA2 = 6.58 ± 0.13) and aminopotentidine (pA2 = 6.86 ± 0.06) competitively antagonised the amthamine‐induced effect, while iodoaminopotentidine produced surmountable antagonism only at low concentrations (1 μM, pA2 = 6.83 ± 0.21). Finally, the slowly‐dissociable antagonist loxtidine caused a non‐parallel displacement to the right of the concentration‐response curve to amthamine (pKB = 7.55 ± 0.24), with a significant depression of the maximal response to the agonist, even at the lowest effective concentration (0.3 μM). In conclusion, histamine H2 receptors in guinea pig gallbladder resemble those of the heart, as regards their sensitivity to zolantidine and compound SKF 92857, which, by contrast, were unable to antagonize histamine effects at gastric H2 receptors in different experimental models.