Inhibition of a Cl-/HCO3(-)ATPase in the avian salt gland by furosemide and ethacrynic acid

An ouabain-insensitive, Mg2+-dependent, chloride- and bicarbonate-stimulated ATPase is present in salt gland homogenates of domestic ducks (Anas platyrhynchos). Furosemide and ethacrynic acid are non-competitive inhibitors of this enzyme in vitro. The inhibitor constant of furosemide is Ki = 2.5 mM and of ethacrynic acid Ki = 1.9 mM. In contrast, under the same conditions the salt gland Na+/K+-ATPase activity is not inhibited by furosemide, whereas ethacrynic acid becomes inhibitory only at higher concentrations (10 mM).