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/1 (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/1) of lactation, decreasing to 4.5 ± 0.8 pmol/1 after 1 week (p < 0.05) and to 2.0 ± 0.3 pmol/1, 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/1, 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.