Hypothalamic Sodium-transport Inhibitor is a High-affinity Reversible Inhibitor of Na+-K+-ATPase

Bovine hypothalamus contains a stable, low molecular weight substance with ouabain-like properties. To further study its mechanism of action and potential physiological importance we examined its effects on purified Na+-K+-ATPase in a kinetic coupled-enzyme assay. Under optimal conditions up to 95% of Na+-K+-ATPase activity could be inhibited by the factor. Mg2+ is required for maximal inhibitory activity, but ligand requirements for optimal activity are otherwise distinct from those of both ouabain and vanadate. Inhibition is reversed by high concentrations of sodium chloride plus EDTA. Kinetic analysis yielded a Ki = 1.4 nM. The hypothalamic factor is a high-affinity reversible inhibitor of Na+-K+-ATPase, being at least as potent as the cardiac glycoside ouabain and may be a circulating inhibitor of sodium transport, which appears to be associated with experimental volume-expanded hypertension and human essential hypertension.