Changes in plasma and amniotic fluid endothelin levels during pregnancy: facts or artefacts?

To describe the evolution of immunoreactive endothelins (irETs) in maternal peripheral plasma and in amniotic fluid at different stages of pregnancy using two different radioimmunoassays.

Peripheral blood samples were obtained from ten non-pregnant and eighty-four pregnant patients at different stages of pregnancy, but not in labor. Amniotic fluid samples were obtained in ten patients during the second trimester of pregnancy and in twenty-two patients at term. Endothelin concentrations were assayed using two different kits: by antigenic cross-reactions, the RPA 545 assay allowed the detection of ET-1, ET-2 and big ET-1; the RPA 555 assay allowed the detection of ET-1, ET-2 and ET-3.

Using the RPA 555 kit, no differences were observed in irET plasma levels between non-pregnant and pregnant patients whatever the gestational age. With the RPA 545 kit, the irET levels were significantly lower in pregnant patients during the first and second trimesters of pregnancy when compared to non-pregnant patients. Immunoreactive ET levels then increased significantly in the last month of pregnancy when compared to mid-pregnancy levels. In amniotic fluid, irET levels were significantly higher at term than during the early second trimester, without any difference between the two RIA kits.

Our results are indicative of a differential evolution in ET isoforms during pregnancy in maternal peripheral blood. The increase in irET observed towards the end of pregnancy in maternal plasma and in amniotic fluid suggests that ET could play a role in the onset of parturition.