The blood-milk barrier (BMB) forms at parturition when the gland switches form a non-lactating state to one of copious milk production and becomes leaky again when milk removal ceases and mammary involution is initiated. In this review the importance of the BMB in milk production and, in particular, its hormonal regulation is explored. Tight junctions (TJ) between adjacent mammary epithelial cells form a barrier to the two-directional paracellular movement of small molecules between the blood and milk and are responsible for establishing and maintaining the BMB. They form part of the cell's junctional complex and consist of transmembrane proteins that are linked to the mammary cell's cytoskeleton. This means that when, during lactation, TJ become “leaky” the resulting perturbation of the cytoskeleton interferes with the cell's secretory function. As such, TJ are involved in regulating and maintaining milk production. Mammary TJ are under hormonal control, with progesterone, glucocorticoids, prolactin, parathyroid hormone-related peptide (PTHrP) and serotonin (5-HT) being the key hormones. Progesterone prevents closure of TJ and the immediate prepartum drop in its concentration is a prerequisite for TJ closure. A simultaneous increase in the levels of glucocorticoids and prolactin is necessary for full TJ closure and initiation and maintenance of lactation. Both PTHrP and 5-HT are important hormones in maintaining extracellular calcium concentrations, a requirement for maintaining TJ integrity. Whereas PTHrP reduces TJ permeability, necessary for establishing and maintaining milk production, 5-HT has an opening effect on TJ. The latter may help speed up mammary involution and facilitate the movement of immune factors into the gland, preventing intramammary infections. In summary, mammary TJ make up the BMB and play a role in establishing and maintaining milk production and are under hormonal control, with progesterone, glucocorticoids, PTHrP and 5-HT being key regulatory hormones and prolactin likely playing a supporting role.

The ATP-binding cassette transporter G2/breast cancer resistance protein (ABCG2/BCRP) is an efflux protein involved in the bioavailability and milk secretion of endogenous and exogenous compounds, actively affecting milk composition. A limited number of physiological substrates have been identified. However, no studies have reported the specific effect of this polymorphism on the secretion into milk of compounds implicated in milk quality such as vitamins or endogenous compounds. The bovine ABCG2 Y581S polymorphism is described as a gain-of-function polymorphism that increases milk secretion and decreases plasma levels of its substrates. This work aims to study the impact of Y581S polymorphism on plasma disposition and milk secretion of compounds such as riboflavin (vitamin B2), enterolactone, a microbiota-derived metabolite from the dietary lignan secoisolariciresinol and uric acid. In vitro transport of these compounds was assessed in MDCK-II cells overexpressing the bovine ABCG2 (WT-bABCG2) and its Y581S variant (Y581S-bABCG2). Plasma and milk levels were obtained from Y/Y homozygous and Y/S heterozygous cows. The results show that riboflavin was more efficiently transported in vitro by the Y581S variant, although no differences were noted in vivo. Both uric acid and enterolactone were substrates in vitro of the bovine ABCG2 variants and were actively secreted into milk with a two-fold increase in the milk/plasma ratio for Y/S with respect to Y/Y cows. The in vitro ABCG2-mediated transport of the drug mitoxantrone, as a model substrate, was inhibited by enterolactone in both variants, suggesting the possible in vivo use of this enterolignan to reduce ABCG2-mediated milk drug transfer in cows. The Y581S variant was inhibited to a lesser extent probably due to its higher transport capacity. All these findings point to a significant role of the ABCG2 Y581S polymorphism in the milk disposition of enterolactone and the endogenous molecules riboflavin and uric acid, which could affect both milk quality and functionality.

The mammary epithelium coordinates the uptake of milk precursors and the transport of milk components in order to produce milk of relatively constant composition at a particular stage of lactation, as long as the mammary gland is healthy. The mammary epithelial cell controls the uptake of blood-borne molecules at its basal side and the release of products into milk at its apical side, through mechanisms of internalization (endocytosis) and mechanisms of release (exocytosis). These events are strictly dependent on the physiological stage of the mammary gland. This review addresses the mechanisms responsible for these processes and points out new questions that remain to be answered concerning possible interconnections between them, for an optimal milk secretion.

A database consisting of 35291 milking records from 83 cows was built over a period of 10 months with the objectives of studying the effect of teat cup attachment failures and milking interval regularity on milk production with an automated milking system (AMS). The database collected records of lactation number, days in milk (DIM), milk production, interval between milkings (for both the entire udder and individual quarters in case of a teat cup attachment failure) and average and peak milk flows for each milking. The weekly coefficient of variation (CV) of milking intervals was used as a measure of milking regularity. DIM, milking intervals, and CV of milking intervals were divided into four categories coinciding with the four quartiles of their respective distributions. The data were analysed by analysis of variance with cow as a random effect and lactation number, DIM, the occurrence of a milking failure, and the intervals between milkings or the weekly CV of milking intervals as fixed effects. The incidence of attachment failures was 7·6% of total milkings. Milk production by quarters affected by a milking failure following the failure was numerically greater owing to the longer interval between milkings. When accounting for the effect of milking intervals, milk production by affected quarters following a milking failure was 26% lower than with regular milkings. However, the decrease in milk production by quarters affected by milking failures was more severe as DIM increased. Average and peak milk flows by quarters affected by a milking failure were lower than when milkings occurred normally. However, milk production recovered its former level within seven milkings following a milking failure. Uneven frequency (weekly CV of milking intervals >27%) decreased daily milk yield, and affected multiparous more negatively than primiparous cows.