Cibenzoline, an ATP-sensitive K⁺ channel blocker, binds to the K⁺-binding site from the cytoplasmic side of gastric H⁺, K⁺-ATPase

1 Cibenzoline, (±)-2-(2,2-diphenylcyclopropyl-2-imidazoline succinate, has been clinically used as one of the Class I type antiarrhythmic agents and also reported to block ATP-sensitive K⁺ channels in excised membranes from heart and pancreatic β cells. In the present study, we investigated if this drug inhibited gastric H⁺, K⁺-ATPase activity in vitro. 2 Cibenzoline inhibited H⁺, K⁺-ATPase activity of permeabilized leaky hog gastric vesicles in a concentration-dependent manner (IC₅₀: 201 μM), whereas no effect was shown on Na⁺, K⁺-ATPase activity of dog kidney (IC₅₀: > 1000 μm). Similarly, cibenzoline inhibited H^-, K⁺-ATPase activity of HEK-293 cells (human embryonic kidney cell line) co-transfected with rabbit gastric H⁺, K⁺-ATPase α- and β-subunit cDNAs (IC₅₀: 183 μM). 3 In leaky gastric vesicles, inhibition of H⁺, K⁺-ATPase activity by cibenzoline was attenuated by the addition of K⁺ (0.5-5 mM) in a concentration-dependent manner. The Lineweaver-Burk plot of the H⁺, K⁺-ATPase activity shows that cibenzoline increases K_m value for K⁺ without affecting V_max, indicating that this drug inhibits H⁺, K⁺-ATPase activity competitively with respect to K⁺. 4 The inhibitory effect of H⁺, K⁺-ATPase activity by cibenzoline with normal tight gastric vesicles did not significantly differ from that with permeabilized leaky gastric vesicles, indicating that this drug reacted to the ATPase from the cytoplasmic side of the membrane. 5 These findings suggest that cibenzoline is an inhibitor of gastric H⁺, K⁺-ATPase with a novel inhibition mechanism, which inhibits gastric H⁺, K^--ATPase by binding its K⁺-recognition site from the cytoplasmic side.