Some studies report that hormone melatonin can be found in human milk, but the daily variation in colostrum melatonin is not available. This study verified the effects of milk collection time (diurnal/nocturnal) on colostral melatonin levels and the ability of this hormone to modulate colostral phagocyte activity. Colostrum samples were collected from 30 mothers during the day and night, for a total of 60 samples. We determined melatonin levels in colostrum and superoxide release and bacterial killing by colostral phagocytes. Melatonin levels were higher in colostrum samples collected at night. Phagocytes in nocturnal colostrum samples increased spontaneous superoxide release. In diurnal colostrum samples, mononuclear (MN) phagocytes increased superoxide release when exposed to enteropathogenic Escherichia coli (EPEC), but not polymorphonuclear (PMN) phagocytes. Phagocytes exposed to both EPEC and melatonin had higher superoxide release, independent of phagocyte type and colostrum collection period. Phagocytosis rate was higher in colostrum samples collected at night. In diurnal samples, EPEC killing by MN phagocytes was lower than by PMN phagocytes. Phagocytosis increased significantly in the presence of melatonin in both MN and PMN cells, irrespective of colostrum collection period. In response to melatonin, MN phagocytes from both diurnal and nocturnal samples increased bactericidal activity, whereas colostral PMN phagocytes increased it only in diurnal samples. The melatonin increased in the intracellular Ca2+ levels. The highest intracellular Ca2+ release were found in MN phagocytes diurnal samples. These results confirm that melatonin levels in human colostrum follow a day-night variation and increase phagocytic activity of colostral cells against bacteria.

Institute of Biological and Health Science Federal University of Mato Grosso Barra do Garças MT Brazil

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