Plasma levels of atrial natriuretic peptides during pregnancy and post partum in the rat.

Plasma concentrations of atrial natriuretic peptides (ANP) in female Wistar rats were measured by radioimmunoassay at oestrus, during pregnancy, during parturition and between 3 h and 4 days post partum. Concentrations of ANP in rats on days 10, 15 and 17 of pregnancy were not significantly different from those in non-pregnant animals in oestrus (32.5 +/- 2.2 pmol/l; mean +/- S.E.M., n = 9), but levels near term (days 20 and 21 of pregnancy) were reduced by approximately 50%. However, plasma concentrations of ANP at 6, 12 and 24 h post partum were approximately twice those of non-pregnant animals in oestrus, but returned to normal levels within 4 days after parturition. Maternal plasma volume increased significantly during pregnancy, and fell 15-20% 6-24 h post partum. These results suggest that the relationship between plasma volume and the plasma concentration of ANP is reset during pregnancy and changes rapidly post partum. The results do not necessarily, however, imply any changes in the relationship between atrial pressure and the concentration of ANP.