Correlation between endogenous digoxin-like immunoreactivity and 3H-ouabain displacement on erythrocyte membranes in extracts of human plasma

The existence of endogenous cardiac glycoside-like compounds and their property of being recognized by anti-digoxin antibodies is still a matter of controversy. In order to investigate this problem, endogenous digoxin-like immunoreactivity (measured by RIA) and digitalis-like radioreceptor activity (measured by displacement of 3H-ouabain from erythrocyte membranes) were assessed in plasma extracts of normal adults, pregnant women and newborns. These three groups were chosen because of their known widely different levels of digoxin-like immunoreactivity. Compared to adults, newborns and pregnant women had significantly higher levels not only of immunoreactivity but also of displacement of 3H-ouabain binding, the latter being due, according to Scatchard analysis, to a decrease of the affinity of ouabain to its cellular receptor rather than to its maximal binding capacity. Furthermore, immunoreactivity and binding displacement correlated significantly. Our data indicate that one (or more) compounds with cardiac glycoside-like properties (both immunological and at the receptor level) are present in the plasma of newborns and pregnant women, and confirm the idea that radioimmunological methods may be useful in studying endogenous inhibitors of the sodium pump.