Data from several laboratories have demonstrated elevated levels of endogenous digoxin-like immunoreactive factors (DLIF) in serum from women with pregnancy induced hypertension (PIH). Evidence suggests the adrenal and the placenta are sources of DLIF and that these factors are endogenous inhibitors of ouabain-sensitive Na/K ATPase. ATPase is a modulator of vascular smooth muscle contraction in arterioles. Inhibition of Na/K ATPase enhances vascular smooth muscle contraction causing vasoconstriction which leads to systemic hypertension. Given these considerations it seems plausible that DLIF might be responsible for the vasoconstriction effects observed in pregnancy induced hypertension and hence play a role in the etiology of this disease. We propose a working hypothesis: DLIF are endogenous inhibitors of ouabain-sensitive sodium-potassium ATPase and by this mechanism they increase blood pressure in PIH. We will test the first part of this working hypothesis. Demonstrating that DLIF are inhibitors of ATPase requires purification and biological characterization of these endogenous factors isolated from pregnant women. Assays for measuring the total, protein-bound, and free components of DLIF in serum and urine have been developed.
The aim of this project is to use these assays to isolate, purify, and characterize the chemical nature of the immunoactive factors obtained from the serum and urine of pregnant women and from human placental tissue. Characterization will be done utilizing radioimmuno-, radioreceptor-, and sodium-potassium ATPase inhibition assays. To provide a specific immunoassay for DLIF, antibodies against this factor will be developed. Because the binding of DLIF to serum proteins may regulate its bioactivity, the binding characteristics of these factors to serum proteins will be studied. Levels and relative protein binding of bioactive DLIF in serum of third-trimester normal and hypertensive pregnant women will be determined. This research will provide the physical-biochemical characterization of DLIF and test the hypothesis that DLIF are endogenous inhibitors of sodium-potassium ATPase. The general hypothesis proposing a link between endogenous digoxin immunoactivity, endogenous ATPase inhibitors, and pregnancy induced hypertension can then be addressed in future studies. These endogenous compounds may prove useful in elucidating the mechanism of and/or provide a marker for this disease.
