Evidence for a digitalis-like substance in the hypothalamo-pituitary axis in rats

The origin of an endogenous digitalis-like substance in rats was investigated. The tissue content of the substance measured by radio-immunoassay was highest in the pituitary gland, with a decreasing gradient through the hypothalamus, forebrain, cerebellum, brain stem, heart, liver and kidney. Sodium loading decreased the content in the hypothalamus and increased the urinary excretion of the substance. The urinary output of the substance decreased after electrical lesions of the anteroventral third ventricle in the brain. The content increased in the hypothalamus and decreased in the plasma when the axonal flow of neurosecretion was interrupted with intracerebroventricular injections of colchicine. These results suggest that the digitalis-like substance could be produced in the hypothalamus and secreted from the pituitary gland like vasopressin, and that sodium loading increases the turnover of the substance in the hypothalamus.