An Endogenous Digitalis-like Factor Derived from the Adrenal Gland: Studies of Adrenal Tissue from Various Sources

We have previously reported that the adrenal gland is the probable origin of the digitalis-like immunoreactive material (DLI) present in the plasma of rats and other species which have never received cardiac glycoside drugs. The present study demonstrates that adrenal glands removed from rats and then chopped release an immunoreactive digitalis-like material into a serum-free minimal incubation medium. HPLC studies indicated that this immunoreactivity was not homogeneous. Since such material may be a mammalian steroidal ligand for the glycoside receptor on the sodium pump, we investigated whether release of this material could be inhibited by antagonizing the conversion of cholesterol to pregnenolone through the addition of aminoglutethimide (AG) to the incubation medium. Our observations indicate that this manipulation successfully inhibited pregnenolone production during both of our 2-h serial incubation periods. However, in neither incubation period was AG able to inhibit the release of DLI into the medium; rather, during the first 2-h period, the release of this material was increased in the presence of AG compared to that in controls. We attempted to determine whether DLI was of adrenocortical or medullary origin. Extracts of whole beef adrenal gland, beef cortex, whole rat adrenal gland, and whole dog adrenal gland diluted in parallel in RIA, suggesting that the assay detected the same or similar material in each tissue sample. Medullary and cortical tissue samples were dissected from slices of fresh beef adrenals and extracted for assay. These data indicated that the cortex was the primary source of endogenous digitalis-like material. Efforts to enhance release of this material from the cortex of intact rats was performed by exposing animals to ether stress. After ether stress, the plasma concentration of digitalis-like material was lower than that in controls. Finally, extracts of incubation medium from chopped adrenal glands indicate that this medium possesses the ability to inhibit the binding of radiolabeled ouabain to human erythrocytes, suggesting that adrenal glands release material that has the ability to be recognized by both antidigoxin antibodies and the ouabain-binding site of erythrocyte membrane Na+,K+-ATPase.