The secretion of calcium and phosphorus into milk.

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Abstract
The time course of appearance of radioactivity in milk was studied following close-arterial infusion of labeled P, Ca or leucine in to the mammary artery of lactating goats. Maximum activities were reached at 1.5 h in all milk fractions including soluble P, colloidal P, casein P, soluble Ca, protein-associated C and casein. At 0.5 h, labeling of the soluble and colloidal P fractions was significantly higher than that of the casein P. Recovery of 32P or 47Ca 3 or more h after infusion into the cistern of the mammary glands was 98% or greater, indicating that the mammary epithelium is virtually impermeable to [32P]phosphate and 47Ca in the milk to blood direction. Ca and P failed to enter milk in excess of the normal secretion rate when the milk was diluted with isosmotic sucrose given by intraductal injection. Milk Ca and phosphate in their various forms are evidently secreted, like protein and lactose, by exocytosis of Golgi vesicles. Unless a paracellular pathway is present, as in oxytocin-treated animals, the milk concentrations are maintained by virtue of the impermeability of the mammary epithelium to these substances.