The search for a hypothalamic Na+,K+-ATPase inhibitor.

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Abstract
Accumulating experimental evidence suggests that natriuresis in response to intravascular volume expansion is promoted by an endogenous regulator of Na+,K+-adenosine triphosphatase (ATPase). Efforts to purify this substance by a number of laboratories have as yet been unsuccessful. The properties of partially purified inhibitors from plasma, urine, and tissue often fail to possess the characteristics thought to be consistent with those of a physiological regulator. These include potency (Ki of approximately 1 nM), reversibility of inhibition, specificity for Na+,K+-ATPase, and responsiveness to relevant physiological stimuli. Two rather different candidate substances, extracted from urine and hypothalamus, have been purified to a high degree. Neither is a peptide, and both are of low molecular weight and resistant to acid hydrolysis. The substance from urine is rather nonpolar and interacts with digoxin-specific antibodies, while that from hypothalamus is polar and does not appear to share epitopes with the cardiac glycosides. On the serosal surface of the toad urinary bladder, the hypothalamic substance causes a reversible inhibition of Na+ transport, inhibits rubidium uptake in red blood cells by acting on the membrane's exterior surface, inhibits binding of ouabain to purified Na+,K+-ATPase, and reversibly inhibits hydrolysis of adenosine 5'-triphosphate by the enzyme with a Ki of 1.4 nM. The hypothalamic inhibitor may be differentiated from ouabain by their respective ionic requirements for optimal inhibition of enzymatic activity, and although both ouabain and the hypothalamic inhibitor fix Na+,K+-ATPase in its E2 conformation, the hypothalamic inhibitor does not promote phosphorylation of the enzyme by inorganic phosphate in the presence of Mg2+. Ionic requirements for inhibition also differentiate the hypothalamic inhibitor from vanadate ion, as does the inhibitor's activity in the presence of norepinephrine. Further enzymological and physiological studies will be facilitated by structural characterizations of the inhibitory substances and by the availability of a method to measure their concentrations in physiological fluids.