Bufalin radioimmunoassays: in search of the endogenous digitalis-like substance

The existence of a mammalian natriuretic substance similar to plant digitalis, which inhibits Na,K-ATPase, has been speculated about, but as yet no definite substance has been found. Digoxin-like immunoreactive substance (DLIS) has been reported in various clinical states including new born infants. Using bufalin (a cardioactive substance of animal origin) as antigen, four polyclonal antisera have been produced from 2 separate rabbits and characterised for cross-reactivity with 32 compounds. One antiserum showed a marked change in its cross-reactivity after resting the animal for a year. Of the endogenous substances tested, progesterone was found to be the most cross-reactive. Radioimmunoassay of foetal cord sera with different antisera, gave different levels of bufalin-like immunoactivity. However, after a novel "affinity-immunoassay" procedure, this apparent bufalin-like immunoactivity disappeared. It is concluded that bufalin-like immunoactivity in the cord blood is caused by the cross-reaction of endogenous steroids with bufalin antiserum, and the same may be true for DLIS.