Concentration of calcium and other elements at a subcellular level in the lactating epithelium of rat

Unfixed, freeze-dried and uncoated sections of lactating mammary tissue from rat were analysed for element concentration using a scanning electron microscope fitted with energy-dispersive X-ray analytical equipment. The subcellular concentration (mmol/kg dry weight) of calcium and several other elements (Na, Mg, P, S, Cl and K) was measured in: the basal-lateral cytoplasm, the nucleus, the apical cytoplasm, and the alveolar lumen in the lactating epithelium. The element concentration data was correlated to the ultrastructural features of the microprobed areas. Ca was concentrated, 4- to 5-fold, in the apical cytoplasm in comparison to other parts of the cell. The lumen contents show a 10-fold increase over the apical cytoplasm. The apical cytoplasm includes the Golgi apparatus and secretory vacuoles containing dense granules or casein micelles. The latter are numerous in the lumen. Because most of the CA in milk is known to be bound to the casein micelles, our data support the idea that Ca is sequestered in the golgi region, and transported within the secretory vacuoles to the plasma membrane for exocytosis to the lumen.