Vasopressin concentration in amniotic fluid as an index of fetal hypoxia: mechanism of release in sheep.

Hypoxia is a potent stimulus to the release of vasopressin in fetal sheep, and plasma concentrations of the hormone correlate inversely with fetal oxygenation. Since the fetal kidney contributes to vasopressin clearance, we propose that measurement of increased amounts of vasopressin in amniotic fluid would be indicative of fetal hypoxia. Therefore, we measured concentrations of vasopressin in amniotic fluid under resting conditions, during and after fetal hypoxia, and with intravenous and intra-amniotic administration of vasopressin in 15 chronically instrumented fetal lambs between 111 and 141 days gestation. In the resting state, mean (+/- SE) vasopressin concentrations in amniotic fluid (1.6 +/- 0.3 pg ml-1) did not differ from those in maternal (1.4 +/- 0.4 pg ml-1) or fetal (1.8 +/- 0.2 pg ml-1) plasma. Following exposure of the ewe to 10% O2 or partial occlusion of the umbilical cord, vasopressin concentrations in fetal plasma increased significantly (P less than 0.001) to 200 +/- 59 pg ml-1 with a delayed increase in amniotic fluid concentrations (P less than 0.03) to 15.8 +/- 4.5 pg ml-1. This rise in concentration of vasopressin in amniotic fluid was sustained for at least 24 h and levels at that time were highly correlated with peak plasma concentrations (r = 0.83; P less than 0.001). Intravenous infusion of vasopressin into the fetus was accompanied by an equally significant (P less than 0.02) and sustained increase of vasopressin in amniotic fluid. Following intra-amniotic injection of vasopressin, levels remained increased for at least 24 h.(ABSTRACT TRUNCATED AT 250 WORDS)