Secretion of Calcium into Milk: Review

Abstract

Milk calcium exists in bound and ionized forms. Bound calcium is associated both with casein micelles and complexed to citrate and phosphate. Ionized calcium in milk is 1 to 4 millimolar, at least 1000 times its postulated concentration in the mammary alveolar cell. For this reason active transport mechanisms are necessary for transfer of this nutrient to the lumen of the mammary alveolus. Evidence that the major active transport system is a calcium adenosine triphosphatase residing in the membrane of the Golgi secretory vesicle is summarized. This adenosine triphosphatase appears to be activated by calcium concentrations in the micromolar range, to require magnesium ions, and to operate by phosphorylation of a 100,000 dalton enzyme intermediate. Metabolic processes are required to maintain a low concentration of calcium within the cytosol of the mammary alveolar cell. Because no evidence for sodium/calcium exchange could be found in the mammary gland of the lactating mouse, we suggest that these processes operate through a calcium adenosine triphosphatase in the basolateral membrane of the cell. Decreased calcium in the alveolar lumina decreased the integrity of the barrier between blood and milk. It is postulated from observations in other secretory systems that an increase in cystolic activity calcium may play a role in lactogenesis.

Calcium transport by mammary secretory cells: Mechanisms underlying transepithelial movement

“…The evidence in favour of model 1 is derived from biochemical, physiological and electron microscopical studies which were performed during the 1970s and 1980s [8,9]. However, some aspects of the model are supported by recently published molecular and immunohistochemical data from the laboratories whose work has given rise to Model 2.…”

“…Unfortunately, the study of calcium transport by the mammary gland is complicated by the very nature of the secreted product. Milk calcium exists in a variety of forms including calcium bound to casein, colloidal calcium phosphate/citrate and free ionized calcium [8]. The majority of calcium is associated with casein micelles allowing milk to maintain calcium at levels which, if fully ionized, would put mammary secretory cells under considerable osmotic stress.…”

“…The third step involves the movement of calcium across the apical membrane of the secretory epithelial cell and into milk. A model based on experiments conducted several decades ago (hereafter referred to as Model 1) was formulated to describe the movement of calcium across the lactating mammary gland [8] (see Fig. 1).…”

Effects of Calmodulin and Calmodulin Antagonists on Neutral Ca2+-ATPase in the Membrane Fraction of Human Milk

“…It is, therefore, still difficult to conclude that there is an interaction between calmodulin and Ca2+ -ATPase, which may mediate the calcium transport between cytoplasm and extracellular space in the human mammary gland. 23 ) …”

Avaliação do iogurte produzido com leite contendo diferentes níveis de células somáticas.

“…O cálcio no leite pode ser encontrado na forma ionizada, ligado às micelas de caseína ou associado ao citrato e fosfato. Durante a mastite, há redução na concentração de cálcio no leite (NEVILLE & WATTERS, 1983).…”