Atrial natriuretic peptide and endothelin concentrations in human milk during postpartum lactation

Milk samples from lactating women (n = 24) were examined for atrial natriuretic peptide (ANP) and endothelin-1 (ET-1) content throughout the first 3 months postpartum. Samples were collected at the beginning (foremilk) and towards the end (hindmilk) of nursing. ANP was detected in the milk samples. A value of 9.4 +/- 1.8 pmol/l (mean +/- SEM) was observed on the third day postpartum. No significant variations in concentrations were observed during the 3 months examined. Foremilk and hindmilk ANP concentrations were also similar. ET-1, as previously reported, is present in the milk of lactating women. According to our observations, the concentration of ET-1 varies during the first 3 postpartum months. The highest concentrations were observed on the third day (10.2 +/- 1.8 pmol/l) of lactation, decreasing to 4.5 +/- 0.8 pmol/l after 1 week (p < 0.05) and to 2.0 +/- 0.3 pmol/l, at 1 month postpartum (p < 0.05), this level being maintained for up to 3 months postpartum. Foremilk samples on the third day postpartum contained significantly higher concentrations of ET-1, compared to hindmilk samples (10.2 +/- 1.8 vs 7.7 +/- 1.2 pmol/l, p < 0.05). The stable levels of ANP and the initial high and subsequently decreasing levels of ET-1 in human milk, during the first 3 months postpartum, suggest that these peptides might be of importance either in the lactating mammary gland or in the suckling newborn.