[Inhibition of microsomal enzyme activity of the liver by various H2 receptor antagonists]

The inhibition of 7-ethoxycoumarin deethylase activity by four different H2-receptor antagonists was studied using rat liver microsomes. The compounds tested represent two classes of H2-antagonists, i. e. structures with (cimetidine and oxmetidine) and without (ranitidine and SKF 93479) an imidazole ring. The microsomes were prepared from untreated and phenobarbital-treated animals. It was found that all four compounds, even those without an imidazole ring, inhibited deethylase activity. The compounds inhibited in the following order: SKF 93479 (90%) greater than cimetidine (58%) = oxmetidine (58%) greater than ranitidine (23%). In microsomes from phenobarbital-induced animals, the inhibitory activity of oxmetidine was increased 5-fold. Only the inhibitory potency of cimetidine was increased by preincubation of the H2-antagonist with the microsomes prior to the addition of the substrate.