Synthesis and secretion of A-type natriuretic peptide in the auricular cardiocytes during pregnancy and lactation in mouse

A-type (atrial) natriuretic peptide (ANP) levels in the auricular cardiocyt es and plasma were examined by immunohistochemistry, electron microscopy, a nd radioimmunoassay in pregnant and lactating mice. Additionally, the cardi ocyte ANP mRNA expression was measured by the polymerase chain reaction met hod. ANP-immunoreactivity (IR) and the number of ANP-granules in the cardio cytes on the 18th day of gestation were greater than those in virgin contro ls, but the plasma ANP concentration decreased on the 18th day of gestation . On the day of delivery, ANP-IR and the number of ANP-granules in the card iocytes were decreased compared to those during the pregnancy and to those in virgin controls, and then began to increase continually until the 15th d ay of lactation. Plasma ANP concentration after delivery was significantly higher than that during pregnancy, and than that in virgin controls, and co ntinued to increase until the 15th day of lactation. Cardiocyte ANP mRNA ex pression was highest on the day of delivery compared to that in all the oth er times. In conclusion, these results suggested that the circulating syste ms of ANP during pregnancy and lactation were regulated differentially.