Renal and cardiovascular effects of atrial natriuretic peptide in fetal sheep.

In the adult, atrial natriuretic peptide (ANP) has been postulated to regulate renal and cardiovascular function both when blood volume is expanded and when atrial pressure is increased. In late gestation fetal sheep, ANP has been detected in plasma concentrations higher than in pregnant adults; however, its function in the fetus is unclear. To assess the role of ANP in mediating changes in combined ventricular output and organ blood flow, GFR, urine flow rate, and urinary sodium excretion, we studied 11 chronically cannulated fetal sheep at 126 to 131 d gestation (term 145 d). We infused ANP in doses of 27 +/- 4 and 55 +/- 8 ng/kg/min into nine of the 11 fetuses and vehicle only into the remaining two fetuses. ANP increased hematocrit, Hb, and plasma protein concentrations, which suggests that blood volume decreased. Combined ventricular output and umbilical-placental blood flow, measured by the radionuclide-labeled microsphere technique, decreased from 458 +/- 158 to 344 +/- 59 ml/min/kg and 210 +/- 85 to 144 +/- 31 ml/min/kg, respectively and the calculated umbilical-placental vascular resistance increased from 0.20 +/- 0.10 to 0.32 +/- 0.09 mm Hg/ml/min/kg during the infusion of the high ANP dose. Blood flow to the gastrointestinal tract decreased with the high dose of ANP but blood flow to all other organs, mean arterial blood pressure, and heart rate did not change significantly. The effects of ANP on fetal renal function were minimal. Thus, ANP may play a role in maintaining fluid homeostasis in the fetus through its ability to decrease blood volume and to decrease combined ventricular output and umbilical-placental blood flow.