Hyperosmolality elevates plasma trial natriuretic factor in the ovine fetus

This study was designed to explore the effect of hyperosmolality on fetal plasma atrial natriuretic factor (ANF) concentrations in chronically catheterized sheep fetuses averaging 133 days gestation. An isotonic solution of 0.9% NaCl or hypertonic solution of 2.5% NaCl, 13% mannitol, or 7% NaCl was infused intravascularly into the fetuses at 20 ml/kg over 10 min and simultaneously into their mothers. Fetal plasma osmolality changed by -2 ± 1 (SE) mosmol/kg in the isotonic group and by 16 ± 2, 20 ± 4, and 56 ± 3 mosmol/kg in the 2.5% NaCl, 13% mannitol, and 7% NaCl groups, respectively (P < 0.00001). Preinfusion fetal ANF levels were similar in all four groups and averaged 145 ± 7 (SE) pg/ml. With infusion, fetal plasma ANF increased significantly in the isotonic group by 28 ± 6%. In the 2.5% NaCl and 13% mannitol groups, the increment in plasma ANF was four times, whereas in the 7% NaCl group it was eight times that in the isotonic group (P < 0.01). Blood volume and venous pressure changes were similar in all groups. In the hypertonic groups, plasma ANF and venous pressure returned to control levels within 1 h after the start of the infusion, whereas plasma osmolalities remained elevated. Thus infusions of hypertonic solutions into the ovine fetus caused much greater increases in plasma ANF concentrations compared with those seen with isotonic infusion. The return of plasma ANF levels to control despite maintained hyperosmolality suggests that hyperosmolality stimulated ANF release either by a direct but transient mechanism or by potentiating the effects of vascular volume expansion.