Endogenous ouabain and its binding globulin: Effects of physical exercise and study on the globulin's tissue distribution

Ouabain, that has been isolated from bovine adrenals and hypothalamus, is a new cardiotonic steroid hormone, which is either synthesized in the adrena ls or stored there after it has absorbed from the diet. Little is known in vivo which events may lead to the release of ouabain into blood. Moreover, a binding protein for cardiotonic steroids exists in blood, which binds car diac glycosides with high affinity. It may affect the action of endogenous ouabain on heart and circulation, but the physiological function of this pr otein is unclear. To realize, which physiological stimuli in vivo may affec t blood concentrations of endogenous ouabain and which function the cardiot onic binding protein may have in modulating ouabain effects, the effect of physical exercise on endogenous ouabain was studied and the tissue distribu tion of its binding protein was investigated. We found that endogenous ouab ain changes rapidly in blood upon physical exercise and behaves like expect ed for a hormone of circulation. The cardiotonic steroid binding globulin s hows the highest concentration in the kidney, which suggests that sodium pu mps of the kidney are protected against its inhibition by ouabain which wou ld lead not only to natriuresis but also to a deleterious loss of glucose, amino acids and phosphate.