Endogenous digoxin immunoactivity is elevated in hypertensive pregnancy.

We have detected an endogenous digoxin-immunoreactive substance in the blood of third trimester pregnant women, of newborn infants and in amniotic fluids. In this report we present data showing an increase in blood levels of this endogenous substance in pregnant women with time of gestation (45 +/- 12, 125 +/- 32, and 195 +/- 27 pg/ml digoxin-equivalent for the 1st, 2nd, and 3rd trimesters, respectively, mean +/- SE). Preliminary evidence suggests the blood level of this substance is elevated in third-trimester hypertensive pregnant women (315 +/- 29 pg/ml). The immunoactive substance isolated from pregnant subjects is water soluble and reversibly bound to serum proteins. Heating the serum increases the measured immunoactivity. Exploration of the physiologic relevance of this endogenous substance or family of substances is in progress.