Fetal hypoxia elevates plasma atrial natriuretic factor concentration

Acute hypoxia in the fetus is associated with a reduction in fetal blood volume. We hypothesized that atrial natriuretic factor in the fetal circulation may be one of the factors that mediate this blood volume decrease. Thus the present study was designed to determine the effects of hypoxia on circulating concentrations of atrial natriuretic factor in fetal sheep and correlate these changes with fetal blood volume. Hypoxia was induced in chronically catheterized sheep fetuses by infusing nitrogen containing CO2 into the trachea of the ewe for 30 minutes. Fetal arterial PO2 decreased by 10.2 +/- 1.3 (SE) mm Hg. Plasma atrial natriuretic factor concentration rose concurrently with the fall in PO2 such that atrial natriuretic factor increased to 565 +/- 196 pg/ml from a basal level of 127 +/- 13 pg/ml (p less than 0.001). Fetal blood volume was reduced by 7.2% +/- 2.1% and was significantly related to changes in atrial natriuretic factor levels (p less than 0.0001). At the termination of hypoxia, PO2 returned to normal levels before plasma concentrations of atrial natriuretic factor fell to baseline values. Therefore fetal hypoxia appears to be a potent stimulus for elevating plasma concentration of atrial natriuretic factor in the fetus, and this rise in atrial natriuretic factor in the circulation may be partially responsible for the reduction in fetal blood volume observed during hypoxia.