We have recently isolated two endogenous digitalis-like factors (EDLFs) from human urine. The polar EDLF (ouabain-displacing compound 1; ODC-1) fulfills the criteria for the putative natriuretic and vasoactive digitalis-like factor. In this study, we examined the effects of ODC-1 and ouabain on endothelin-1 (ET-1) secretion from cultured endothelial cells. ODC-1 was partially purified from human urine with use of reverse-phase HPLC based on the inhibitory effects on [H-3]ouabain binding to intact human erythrocytes. A serial dilution curve of the extract of the serum-free culture medium from CPAE (bovine pulmonary artery endothelium) cells was parallel to that of standard ET-1. Immunoreactive (ir-) ET-1 was secreted in a linear fashion over 5 h from CPAE cells. ODC-1 stimulated ET-1 secretion in a dose-dependent manner. In contrast, ouabain did not affect ET-1 secretion at the concentration of 10(-9)-10(-5) M. These results indicate that human urine-derived ODC-1 is distinct from plant-derived ouabain and acts as a stimulator of ET-1 secretion, and also suggest that ODC-1 may play an important role as a direct and indirect modulator of vascular tone.
