Secretory activation is reflected in dramatic modifications of the solute composition of milk and increased secretory volume, which in turn reflect the maturation of secretory mechanisms and transport pathways during this period. In women, there are three temporally distinct changes in milk composition at the onset of lactation. The earliest is a decrease in sodium and chloride concentrations and an increase in the lactose concentration of milk (Figure 4). These modifications occur immediately after delivery and are largely complete by 72 h post-partum. They precede increases in milk volume by at least 24 h and can be explained by closure of the tight junctions that block the paracellular pathway. Blocking this pathway prevents lactose, made by the epithelial cells, from passing from the lumen of the alveolus to the plasma, and it prevents sodium and chloride from directly entering the lumen from the interstitial space. These changes result in decreased concentrations of sodium and chloride and increased concentrations of lactose in the mammary secretion. The increased lactose concentration is reflective of decreased water entering the lumina as monovalent ion secretion decreases rather than an increase in the lactose secretion rate.
Secondarily, there are transient increases in the rates of secretion of sIgA and lactoferrin into milk of women soon after delivery. The concentrations of these two important protective proteins remain high, comprising
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