Abstract

Chlormadinone acetate, a hydroxyprogesterone derivative, reversibly inhibits both Na⁺,K⁺-ATPase activity and [³H]ouabain binding to the enzyme. Chlormadinone acetate substitutes for ouabain in "chasing" the label from the enzyme-[³H]ouabain complex. Chlormadinone acetate is a noncompetitive inhibitor of Na⁺,K⁺-ATPase with respect to ATP and is a competitive inhibitor with respect to potassium. The monovalent cation site which regulates ouabain binding to Na⁺,K⁺-ATPase (apparent K_D for potassium = 1.4 mM) also regulates chlormadinone binding to Na⁺,K⁺-ATPase (apparent K_D = 0.96 mM). The I₅₀ values for chlormadinone acetate inhibition of Na⁺,K⁺-ATPase isolated from sheep kidney, cat heart, and guinea pig heart were 3,6, and 30 µM, respectively. Similar differences in species sensitivity have been found for ouabain and ouabagenin. These data suggest that chlormadinone acetate binds to the ouabain binding site of isolated Na⁺,K⁺-ATPase. Concentrations of chlormadinone acetate up to 30 µM can be obtained in unstirred solutions in glass vessels. However, the highest concentrations that could be obtained in muscle baths containing isolated strips of guinea pig left atria, or strips of cat atria and ventricle, aerated with 95% O₂-5% CO₂ were only 0.5-4 µM, and these concentrations did not prevent ouabain from binding to its receptor in intact muscles. Thus, it appears that chlormadinone acetate is not capable of modifying the pharmacological action of ouabain or of producing an inotropic action independently under these experimental conditions because of its limited solubility.