title: Role of calcium in the pathway for milk protein secretion and possible relevance for mammary gland physiology.
abstract: In an attempt to define control points within the secretory pathway for casein synthesis and secretion, we have examined the role of both cytosolic and intra-organelle Ca2+ in the control of casein synthesis, phosphorylation and secretion. In addition, the possible role of cell volume changes in stretch-activation of Ca2+ signals was examined. Examination of the kinetics of casein secretion from freshly isolated lactating mouse mammary acini showed that a portion of the newly synthesized casein was secreted in a constitutive manner. A further portion remained within the cells, and this was released following elevation of the intracellular free calcium concentration ([Ca2+]i) using ionomycin, indicating the presence of a Ca(2+)-regulated pathway for casein release. An increase in [Ca2+]i occurred in response to hypotonic challenge to induce cell swelling, and this involved both Ca2+ entry and Ca2+ mobilization from intracellular stores. Experiments examining the effects of depletion of intra-organelle Ca2+ indicated that intra-organelle Ca2+ was required for maintained casein phosphorylation, but not its secretion. Depletion of Ca2+ from the endoplasmic reticulum led to a marked inhibition of casein synthesis. The possible significance of these control mechanisms for the physiology of the mammary gland is discussed.
authors.affiliation: Physiological Laboratory, University of Liverpool, U.K.
authors.forename: R D
authors.initials: RD
authors.lastname: Burgoyne
authors.forename: J S
authors.initials: JS
authors.lastname: Duncan
authors.forename: A W
authors.initials: AW
authors.lastname: Sudlow
mesh_terms: D000818:Animals; D001940:Breast / Q000502:physiology; D002118:Calcium / Q000502:physiology*; D003593:Cytoplasm / Q000378:metabolism; D005089:Exocytosis / Q000502:physiology; D005260:Female; D006801:Humans; D007774:Lactation / Q000502:physiology; D008321:Mammary Glands, Animal / Q000502:physiology* / Q000648:ultrastructure; D008894:Milk Proteins / Q000378:metabolism*
chemical_list: D008894:Milk Proteins; D002118:Calcium