Dehydration increases the renal response to atrial natriuretic peptide in fetal sheep

In sheep, maternal water deprivation results in urinary natriuresis in spite of suppression of plasma atrial natriuretic factor levels. Near-term fetal sheep also have a urinary natriuresis without change in plasma atrial natriuretic factor during maternal dehydration. This study was designed to explore the role of plasma atrial natriuretic factor levels in fetal dehydration-natriuresis.

Eight chronically instrumented preterm (113 +/- 1 days) ovine fetuses received two atrial natriuretic factor infusions (3 and 15 ng/kg/min) in a euhydrated state and after 48 +/- 1 hours of maternal water deprivation.

Dehydration significantly increased maternal plasma osmolality (302 +/- 2 to 313 +/- 2 mOsm/kg water), sodium (148.1 +/- 0.8 to 154.3 +/- 0.4 mEq/L), chloride (112.4 +/- 0.6 to 116.8 +/- 0.9 mEq/L), and arginine vasopressin (4.2 +/- 1.2 to 23.0 +/- 4.0 pg/ml) and significantly decreased plasma atrial natriuretic factor (36 +/- 6 to 19 +/- 4 pg/ml) concentrations. Fetal plasma osmolality (296 +/- 1 to 308 +/- 2 mOsm/kg), atrial natriuretic factor (128 +/- 16 to 241 +/- 36 pg/ml), and arginine vasopressin (3.5 +/- 0.8 to 12.3 +/- 4.8 pg/ml) concentrations and urine osmolality (170 +/- 10 to 253 +/- 10 mOsm/kg), osmolar clearance (0.80 +/- 0.02 to 0.14 +/- 0.02 ml/kg/min), and fractional sodium excretion (3.3% +/- 1.7% to 8.5% +/- 2.1%) increased significantly with dehydration, whereas the plasma atrial natriuretic factor clearance decreased from 127 +/- 27 to 63 +/- 10 ml/kg/min. Dehydration had no effect on fetal hematocrit, vascular pressures, glomerular filtration rate, urine flow, or free water clearance. In euhydrated fetuses plasma atrial natriuretic factor increased from 128 +/- 16 to 287 +/- 46 pg/ml with sequential atrial natriuretic factor infusion, and no significant increases were observed in urine flow, fractional sodium excretion, and glomerular filtration rate. In contrast, atrial natriuretic factor infusion to dehydrated fetuses significantly increased urine flow (0.17 +/- 0.03 to 0.32 +/- 0.07 ml/kg/min), osmolar clearance (0.14 +/- 0.02 to 0.28 +/- 0.06 ml/kg/min), and fractional sodium excretion (8.5% +/- 2.1% to 14.8% +/- 4.0%).

These results demonstrate that in the fetus at 113 days' gestation plasma atrial natriuretic factor levels increase with dehydration, probably a result of decreased plasma atrial natriuretic factor clearance, and the fetal renal responsiveness to atrial natriuretic factor infusion increases during maternal dehydration.