Tiotidine and ICIA 5165 inhibit central and peripheral H2 receptors with equal potency.

Histamine, acting through H2 receptors, increases cAMP formation in homogenates of guinea pig hippocampus. The activities of two H2 antagonists, tiotidine and ICIA 5165, as inhibitors of the histamine-induced increase were investigated. ICIA 5165 and tiotidine were potent antagonists of histamine with a pKi of 8.0 and 7.6, respectively. Similarly, the positive chronotropic response of isolated guinea pig atria to histamine was antagonized by tiotidine and ICIA 5165 with a pKB of 7.8 and 7.9, respectively. These studies suggest that histamine H2 receptors in the CNS and atria are similar.