Presence of atriopeptin-like immunoreactivity in human and rat milk.

Maternal milk is a significant source of hormones and other bioactive substances. They might be involved either in the control of mammary gland function or in the regulation of growth and development of the neonate. Atriopeptin (atrial natriuretic factor, ANF) is a peptide with strong diuretic, natriuretic and vasorelaxant actions, and it has been suggested to play an important role in the circulatory adaptation to extrauterine life. The aim of this study was to determine whether ANF is present in maternal milk, using radioimmunological analysis. The levels of ANF-like substance in human milk were found to be in the range of 0.3-3.0 pg/ml, those in rat milk between 37-117 pg/ml. The measured concentrations of ANF were proportional to the volume of the extracted milk. Serial dilutions of the extracts yielded curves which were not totally parallel to the human alpha-ANF standard curve. Our data indicate that, during the first days after delivery, ANF levels in human milk are higher than those in later periods of lactation. This pilot study provides the first description of the presence of atriopeptin in milk. Though a detailed characterization of milk ANF-like immunoreactivity is needed, a biological significance of present findings seems possible.