Identification of a vasoactive substance (vasopressin) in a brain extract containing an unknown inhibitor of Na,K-ATPase.

An endogenous inhibitor of the Na,K pump, postulated to be involved in the etiology of some hypertensive states, has been reported in extracts of mammalian brain. This encouraged us to test its effects on arterial muscles. An acid-acetone extract of guinea pig brain inhibited Na,K-ATPase derived from canine kidney and evoked responses in arterial strips similar to those produced by ouabain. Unlike ouabain, however, it did not prevent muscles in K-free solutions from relaxing when K was re-added. Bioassays on strips of arteries, uterus and portal vein indicated that the extract did not contain sufficient concentrations of norepinephrine, dopamine, serotonin, prostaglandins, angiotensin II, oxytocin or the Na,K-ATPase inhibitor to account for the observed vascular effects. This could not be said of vasopressin. Furthermore, vasopressin and the vasoactive component of the extract were equally sensitive to several peptidases, and conditions which cleave disulfide bridges. A radioimmunoassay verified that the extract contained sufficient vasopressin to cause contractions. Vasopressin did not inhibit the kidney Na,K-ATPase activity. Finally, the Na,K-ATPase inhibitor, but not the vasoactive substance, was present in extracts of vasopressin-deficient Brattleboro rat brains. Therefore, the Na,K-ATPase inhibitor and the vasoactive substance in these extracts were distinctly different.