[Endogenous digitalis-like substances in umbilical cord blood: analytical interference or new hormones?]

Endogenous substances with a digitalis-like action have been found in the newborn and in women in the first three months of pregnancy. These substances have three properties that are characteristic of cardiotonic heterosides: a positive inotropic effect; inhibition of Na+, K+-ATPase and binding with anti-digoxin antibodies. The result of this last property, when immunological methods are used to calculate the levels of digoxin, is that significant concentrations of "apparent digoxin" seem to be present although neither the mother nor the newborn child have received any digoxin. We describe the existence of concentrations of the "apparent digoxin" found in 37 cord sera using two radioimmunological methods. In the one, the concentrations were 0.88 +/- 0.20 nmol/L, and in the other 0.17 +/- 0.10 nmol/L. These results emphasize the significant differences obtained when the two methods are used for calculating levels (p less than 0.01). There was no significant variation as far as the geographical origin, the sex of the infant and associated therapy carried out at the delivery. The nature of the interference is not yet known. It could be due to natriuretic hormones or steroids. Its existence gives rise to numerous problems: the physiological role, the validity of pharmacokinetic studies and the control of treatment with digoxin in pregnant women and in the newborn.