Like exocytotic secretion mechanisms found in other cells, proteins, oligosaccharides, and nutrients such as lactose and citrate are packaged into secretory vesicles within the Golgi that are then transported to the apical region of the cell, where they fuse with the apical plasma membrane, discharging their contents into the extracellular space. A unique feature of this pathway in the mammary gland is the presence of high concentrations of lactose, phosphate, citrate, and calcium within the vesicles. Lactose is synthesized in the Golgi from UDP-galactose and glucose, which have entered from the cytoplasm using specific transporters, by the enzyme /3-galactosidase, with a-lactalbumin acting as a cofactor. The high concentration of lactose present in the Golgi during lactation osmotically stimulates the influx of water that contributes to the fluidity of milk. Casein micelle formation begins in the terminal Golgi with condensation, and simultaneous phos-phorylation, of casein molecules. Addition of calcium, possibly in the secretory vesicle, leads to maturation of casein micelles into particles sufficiently dense to be seen in the electron microscope. This complex thus delivers an efficient package of protein, calcium, and phosphate that provides the nutrients necessary for bone growth, among other things. Calcium enters the cytoplasm from the plasma by a poorly defined transport process. Cyto-plasmic calcium is then transported into secretory vesicles by an ATP-dependent Ca2+ pump localized on Golgi and secretory membranes. The phosphate in secretory vesicles is derived from the hydrolysis of UDP-galactose during the synthesis of lactose. Citrate is generated endogenously within the cytoplasm of alveolar epithelial cells and transported into the Golgi lumen by an undefined process.
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