Figure 4 Changes in milk composition and volume in women during secretory activation and early lactation. (Reproduced with permission from McManaman JL and Neville MC (2003) Advanced Drug Delivery Reviews 55: 630-641.)
as much as 10% of milk, for the first 48 h after birth. The concentration of each protein decreases rapidly after day 2, both from dilution as milk volume secretion increases and from actual decreases in their rates of secretion, particularly of immunoglobulins. Although both these proteins are found at high concentrations in colostrum, they are likely to be secreted by different mechanisms; lactoferrin, an endogenous protein of alveolar cells, is secreted by the exocytotic pathway (pathway I), whereas sIgA, a plasma-derived protein, is secreted by receptor-mediated transcytosis (pathway III). In addition, the peak secretion rate of lactoferrin occurs at the same time as that of lactose and the major milk proteins, whereas sIgA secretion peaks 1 day earlier, indicating the possibility that the exocytotic and transcytosis pathways are regulated differently during early lactation.
The third phase occurs approximately 36 h post-partum and is associated with massive and concerted increases in milk volume and the rates of synthesis and/or secretion of almost all the components of mature milk, including, but not limited to, lactose, protein (mainly casein), lipid, calcium, sodium, magnesium, potassium, citrate, glucose, and free phosphate. Considering that the secretion of these substances involves the actions of several distinct transport pathways and biosynthetic processes, such tightly synchronized increases imply the presence of a common activation switch for coordinating their activities.
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