Transport of Milk Constituents by the Mammary Gland

Abstract

This review deals with the cellular mechanisms that transport milk constituents or the precursors of milk constituents into, out of, and across the mammary secretory cell. The various milk constituents are secreted by different intracellular routes, and these are outlined, including the paracellular pathway between interstitial fluid and milk that is present in some physiological states and in some species throughout lactation. Also considered are the in vivo and in vitro methods used to study mammary transport and secretory mechanisms. The main part of the review addresses the mechanisms responsible for uptake across the basolateral cell membrane and, in some cases, for transport into the Golgi apparatus and for movement across the apical membrane of sodium, potassium, chloride, water, phosphate, calcium, citrate, iodide, choline, carnitine, glucose, amino acids and peptides, and fatty acids. Recent work on the control of these processes, by volume-sensitive mechanisms for example, is emphasized. The review points out where future work is needed to gain an overall view of milk secretion, for example, in marsupials where milk composition changes markedly during development of the young, and particularly on the intracellular coordination of the transport processes that result in the production of milk of relatively constant composition at a particular stage of lactation in both placental and marsupial mammals.

Colostrum production by sows: variability of colostrum yield and immunoglobulin G concentrations

“…In a recent experiment, however, we concluded that in a herd with an average spontaneous gestation length of 114 days, farrowing can be induced at 113 days of gestation without detrimental effect on colostrum yield (Foisnet et al, 2010b). The transition from the first step of lactogenesis (before parturition) to copious secretion of colostrum and then of milk is characterised by two main events: a reduction of the permeability of the mammary epithelium due to the development of tight junctions between mammary epithelial cells and an increase in lactose synthesis and excretion into the mammary alveoli (Shennan and Peaker, 2000). A leaky epithelium is incompatible with copious milk secretion in many species (Shennan and Peaker, 2000) and copious production of colostrum in swine (Foisnet et al, 2010a).…”

Mammary gland secretion: hormonal coordination of endocytosis and exocytosis

“…The movement of sodium (Na 1 ), potassium (K 1 ) and chloride (Cl 2 ) across the basal membrane contributes to the gradients between interstitial and intracellular fluids (Linzell and Peaker, 1971;Shennan and Peaker, 2000). Based on histochemical localization, the presence of Na 1 -K 1 ATPases on the basolateral membrane supports the scheme of a functional Na 1 -K 1 pump on the blood-facing side of the mammary epithelium.…”

“…A Na 1 -K 1 -2Cl 2 co-transport system has also been described. In mammary explants and cell cultures, a long-term effect of PRL up-regulates K 1 uptake (Shennan and Peaker, 2000 and references therein). Moreover, in HC11 cells, PRL exerts an early effect (after 5 and 10 min of stimulation) on Cl 2 transport (Selvaraj et al, 2000) via phosphorylation of the Na 1 -K 1 -2Cl 2 co-transporter.…”

“…The glucose transporter (GLUT1) is present at the basolateral membrane of rodent MEC (Madon et al, 1990;Nemeth et al, 2000) and is the predominant glucose transporter in the lactating bovine mammary gland (Zhao and Keating, 2007). The expression of genes encoding the glucose transporters is under the control of lactogenic hormones in the mammary gland during pregnancy and lactation (Shennan and Peaker, 2000 and references therein; Zhao and Keating, 2007). In addition, the compartmentalization of GLUT1 between the cell surface and the intracellular site of lactose synthesis has been described in CIT3, a mouse mammary cell line.…”

“…Extensive overviews have summarized the major routes of secretion across the mammary epithelium (Linzell and Peaker, 1971;Shennan and Peaker, 2000;Boisgard et al, 2001). Transport of newly synthesized proteins proceeds in MECs according to the general pathway described in exocrine glands.…”