Influence of cationic amphiphilic drugs on the characteristics of ouabain-binding to cardiac Na⁺/K⁺-ATPase

The influence of 12 cationic amphiphilic compounds on the equilibrium and kinetic characteristics of the binding of tritium-labelled ouabain to the lipoprotein Na⁺/K⁺-ATPase present in a crude membrane suspension of guinea pig myocardium was investigated. The drugs, e.g. local anaesthetic, antiarrhythmic and psychotropic agents, inhibited specific binding of ouabain in a concentration-dependent manner by reducing its affinity without affecting the number of binding sites. In the presence of chlorpromazine, propranolol and dibucaine, the decreased affinity of ouabain was due to both a diminished association rate and an increased dissociation rate, while in the presence of the weakly potent procaine only the association rate of ouabain was found to be reduced. The different potency of the catamphiphilic drugs was well correlated to the degree of their hydrophobicity. Evidence is presented that the protonized form of the drugs is the effective one. Concerning the mode of action, the catamphiphilic drugs are proposed to interact with the phospholipid part of the lipoprotein Na⁺/K⁺-ATPase, thereby indirectly altering the conformation of the embedded protein moiety and thus reducing the proper fit between ouabain and its receptor.