EFFECT OF ATRIAL-NATRIURETIC-PEPTIDE ON VASCULAR PERMEATION IN THE OVINE FETUS

To study the effect of atrial natriuretic peptide (ANP) on vascular pe rmeation of albumin in the fetus, ANP (167-600 ng/min) was infused int o eight ovine fetuses and saline vehicle was infused into eight twin c ontrols (gestational age 127 +/- 3 d) over a 50-min period. Using two different radiolabeled albumin markers, we determined the tissue to bl ood isotope ratio (TBIR), an index of albumin permeation, and the albu min clearance. Although ANP had no hemodynamic effect, a marked increa se in the hematocrit was observed in ANT-infused fetuses compared with initial values (0.37 +/- 0.04 vs 0.42 +/- 0.04, p < 0.005) but was un changed in the twin fetuses receiving saline vehicle (0.35 +/- 0.03 ve rsus 0.35 +/- 0.02). TBIR and albumin permeation were increased in com bined tissues of ANP-infused fetuses compared with saline controls (TB IR: 1.49 +/- 0.58 versus 1.29 +/- 0.3, p < 0.001; albumin clearance: 1 091 +/- 1279 versus 827 +/- 1464 nL/g/min, p < 0.01). In individual ti ssues, TBIR was significantly increased in skin (2.88 +/- 0.67 versus 1.55 +/- 0.35, p < 0.02), muscle (1.6 +/- 0.27 versus 1.24 +/- 0.26, p < 0.02), adrenal (1.33 +/- 0.10 versus 1.13 +/- 0.15, p < 0.02), bone (1.67 +/- 0.45 versus 1.20 +/- 0.40, p < 0.02),kidney (1.52 +/- 0.25 versus 1.24 +/- 0.26, p < 0.03), and gut (1.69 +/- 0.20 versus 1.39 +/ - 0.34, p < 0.03). Albumin clearance was higher in most tissues but re ached statistical significance only in skin (2135 +/- 944 versus 775 /- 847 nL/g/min, p < 0.05) and bone (1012 +/- 1107 versus 428 +/- 482 nL/g/min, p < 0.05). We conclude that overall vascular filtration is h igher in the fetus than the adult. Infusion of ANP causes fetal hemoco ncentration, decreases blood volume, and enhances vascular permeation of albumin in most tissues, particularly fetal skin. We speculate that the cardiac atria, by secreting ANP, participate in blood volume regu lation by maintaining a critical balance between the intravascular and extravascular fluid compartments. Dysregulation of the ANP system mig ht result in fetal hydrops.