We have found that 3-(3-(ethoxycarbonyl) propionyl)-8-methoxy-4-((2-methylpheny1) amino) quin-oline (1, CP-113,411), a reversible inhibitor of gastric H+/K+-ATPase (IC50 10-20 mM), is also a potent inhibitor of bone resorption by osteoclasts in a bone slice assay at concentrations as low as M, with an IC50 of 2 mM. By contrast, the structurally related H+/K+- ATPase inhibitor 2 (3-(ethoxycarbonyl)-8-methoxy-4-((2-methylphenyl) amino) quinoline) ...