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 competitively antagonizes the positive chronotropic action of histamine with an apparent dissociation constant of 30 μM (23-38 μM). . Tiotidine actually behaves as an inverse agonist in U-937 cells, diminishing basal cAMP levels[3].