Specific Heart Granules and Natriuretic Peptide in the Developing Myocardium of Fetal and Neonatal Rats and Hamsters

The ontogenesis of specific heart granules and of the related natriuretic peptide activity in heart muscle was studied in fetal and neonatal rats and golden hamsters by ultrastructural analysis including immunogold labelling for ANP-28 and by radioimmunoassay. In both species, immunoreactive granules first appear in the myocardial sleeve of the embryonic heart tube during the looping stages which precede chamber formation and the peptide becomes detectable by radioimmunoassay two or three days later by which time the chambers are identifiable. Granule density and ANP concentration in the rat are higher than in the hamster at all stages of development. Almost all atrial myocytes express ANP in fetal hearts whereas, in the ventricular wall, cells containing immunoreactive granules are scattered. The density of granules in atrial myocytes increases during further stages of fetal and neonatal development, while it decreases markedly even in those ventricular myocytes which are immunoreactive. Changes in the ultrastructural appearance of ventricular SHG suggest that the mode of production of ANP changes in ventricular myocytes after birth but does not change in atrial cells. There is no correlation between the distribution of immunoreactive ventricular myocytes and that of the conducting system. In both species, the concentration of ANP in the atrial well is higher than ventricular levels from the outset and the disparity becomes exaggerated with development till, in six months old adult animals, the atrial to ventricular concentration ratio is about 3 x 10(3):1 in the rat and 1.5 x 10(3): 1 in the hamster. In the hamster, a distinct gradient of ANP concentration between the right and left atria is already established in the early fetal period and it becomes enhanced in the neonatal period. In the rat, however, a slight difference becomes discernible only after birth.