Swelling of Golgi vesicles in mammary secretory cells and its relation to the yield and quantitative composition of milk.

A quantitative description is given of some cellular secretory mechanisms in the mammary epithelium, that can be used in the interpretation of changes in milk yield and composition. The volume of the aqueous phase is considered to depend on a flux of vesicles derived from the Golgi apparatus and on the degree of vesicle swelling prior to exocytosis. Milk proteins secreted by the Golgi route are diluted during vesicle swelling and secreted fat globules are diluted to their final, milk, concentration in the alveolar lumen. Swelling of Golgi vesicles is considered to occur through an osmotic flow of water induced by the intravesicular synthesis of lactose and by the net transport of ions coupled to lactose synthesis. A hypothetical model is given of ion transport mechanisms across the Golgi vesicle membrane to illustrate how the coupling of lactose synthesis to salt transport might occur.