Tiotidine (ICI 125211) is a potent and selective antagonist of histamine H2-receptor (pA2=7.3-7.8 for guinea-pig right atrium). Tiotidine has low affinity for both the H1 and the H3 receptors[2].In Vitro: Tiotidine competitively antagonizes the positive chronotropic action of histamine with an apparent dissociation constant of 30 μM (23-38 μM). Tiotidine abolishes H2-mediated increases in contractile force leaving H2-mediated negative inotropic responses unopposed. The actions of histamine at the A-V node, manifested by lengthening of the P-R interval and A-V block, are attenuated by 2.5 x 10(-7) M Tiotidine[2].
Tiotidine actually behaves as an inverse agonist in U-937 cells, diminishing basal cAMP levels.