Mechanisms of mammary gland ion transport

SLC34A2 Targets in Calcium/Phosphorus Homeostasis of Mammary Gland and Involvement in Development of Clinical Mastitis in Dairy Cows

The type II Na/Pi co-transporter (NaPi2b), encoded by the solute carrier (SLC) transporter 34A2 (SLC34A2), is responsible for calcium (Ca) and phosphorus (P) homeostasis. Unbalanced Ca/P metabolism induces mastitis in dairy cows. However, the specific role of SLC34A2 in regulating this imbalance in Holstein cows with clinical mastitis (CM) remains unclear. The aim of this study was to investigate the role of SLC34A2 and identify differentially expressed proteins (DEPs) that interact with SLC34A2 and are associated with Ca/P metabolism in dairy cows with CM. Immunohistochemical and immunofluorescence staining results showed that SLC34A2 was located primarily in the mammary epithelial cells of the mammary alveoli in both the control (healthy cows, Con/C) and CM groups. Compared to the Con/C group, the relative expression of the SLC34A2 gene and protein were significantly downregulated in the CM group. We identified 12 important DEPs included in 11 GO terms and two pathways interacting with SLC34A2 using data-independent acquisition proteomics. The PPI (protein-and-protein interaction) network results suggested that these DEPs were associated with ion metabolism and homeostasis, especially SLC34A2. These results demonstrate that SLC34A2 downregulation is negatively correlated with the occurrence and development of CM in Holstein cows, providing a basis for exploring the function and regulatory mechanism of SLC34A2 in Ca/P metabolism and homeostasis in Holstein cows with CM.

Mammary gland membrane transport systems

The secretion of milk depends on the activity of a large number of membrane transport systems located on the apical and basolateral membranes of mammary secretory cells. It follows that a thorough knowledge of individual mammary tissue membrane transport systems is required if we are to fully understand the process of milk secretion. The distribution of the transporters between the apical and basolateral poles of the mammary epithelium must be asymmetrical given that the mammary gland is capable of vectorial transport. This is particularly evident in the case of glucose and amino acid transport systems: the transport mechanisms for these compounds are predominantly situated in the blood-facing aspect of the secretory cells. In addition. it is apparent that there is a polarized distribution of transport systems (carriers and channels) which accept sodium, potassium, chloride, phosphate, and calcium as substrates.

Milking procedure alters the electrolyte composition of spontaneously hypertensive and normotensive rat milk

A number of groups, including our own, have shown that the severity of hypertension expressed by the adult spontaneously hypertensive rat (SHR), which is primarily considered to be a genetic model of hypertension, can be reduced as a result of exposure to the behavioural and nutritional environment provided by a normotensive foster mother. It has been suggested that the hypertensive influence of the SHR dam may involve increased sodium delivery to the pups and there have been some reports of elevated sodium concentrations in the milk of SHR dams. However, these studies used either a long (> or =6 h) dam-pup separation period before collecting milk or repeated milking of the same dams, both of which have been shown to alter the trace element content of rat milk. Therefore, we have compared the electrolyte content of milk collected by these methods with milk derived from SHR and normotensive Wistar-Kyoto (WKY) dams separated from their litters for 2 h prior to a single-milking session. Long separation and repeated milking resulted in variable effects on the electrolyte content of both SHR and WKY dams' milk, compared with milk collected after 2 h from dams which had not previously been milked. The most notable effects were the abolition of significant strain differences, observed following 2-h separation, for milk sodium (WKY 22.1+/-1.4 vs. SHR 27.5+/-2.1 mmol/liter, p < 0.05) and calcium (WKY 92.3+/-4.3 vs. SHR 69.4+/-2.9 mmol/liter, p < 0.05) when dams were separated for 6 h or were serially milked. These data suggest that the electrolyte content of SHR and WKY dams' milk can be altered by the collection procedure and it is recommended that dams be milked on only one occasion following a short separation period from their litter.

High affinity (Na(+) + Cl-)-dependent taurine transport by lactating mammary tissue

The transport of taurine by lactating rat mammary tissue has been examined. Taurine uptake was found to be dependent upon the presence of extracellular Na+ and Cl-, which is consistent with (Na(+) + Cl(-) + taurine) cotransport. The Ka and Vmax of taurine influx were respectively 43 microM and 37.5 mumol/kg cell water per 15 min. It is apparent that the mechanism responsible for taurine uptake is highly selective for beta-amino acids. Taurine efflux consisted of a fast extracellular component and a slow membrane-limited component. The slow component was relatively insensitive to temperature, suggesting that it may represent simple diffusion. Mammary tissue was found to contain a high level of intracellular taurine: 7.29-7.44 mmol/kg cell water. We suggest that taurine is taken up and concentrated by the mammary gland by a (Na(+) + Cl(-) + taurine) cotransport mechanism situated in the blood-facing aspect of the secretory epithelium and that a low outward permeability to taurine allows a high intra-to-extracellular concentration gradient to be maintained. Milk taurine may be derived from taurine diffusing from the cell cytosol across the apical membrane.

Interrelationships of the concentrations of some ionic constituents of human milk and comparison with cow and goat milks

1. Milk and milk ultrafiltrate samples from four women (Homo sapiens) during the first 16 weeks of lactation were analysed for the main salt constituents, protein and lactose and the interrelationships of these constituents were determined. 2. A good positive correlation was observed between ultrafiltrate calcium and citrate concentrations, as previously observed for cow (Bos taurus) and goat (Capra hircus) milk but the expected negative correlation of lactose with ultrafiltrate salts was poor. 3. An invariant ion activity product for milk calcium phosphate was found for a dicalcium phosphate stoichiometry but the milk serum was less supersaturated with respect to calcium phosphate and citrate salts than ruminant milks. 4. The observed interrelationships are discussed in relation to the constraints on human milk composition arising from a fixed osmotic pressure, overall charge neutrality and saturation with respect to calcium phosphate.

K+ and Cl- transport by mammary secretory cell apical membrane vesicles isolated from milk

The transport of K+ (Rb+) and Cl- by membrane vesicles isolated from bovine milk has been studied using ion-exchange column chromatography. K+ (Rb+) and Cl- accumulation by the vesicles was time-dependent and was almost abolished by 0.1% Triton X-100, suggesting that uptake represents 'real' transport rather than binding. K+ (Rb+) uptake was influenced by the anion in solution in a manner suggesting that influx is sensitive to changes in vesicle membrane potential. Similarly, Cl- uptake was found to be sensitive to vesicle electrical potential: Cl- influx was enhanced by inside positive potentials. Cl- uptake was not saturable with respect to external Cl-. The results suggest that K+ (Rb+) and Cl- cross the apical membrane by way of conductance pathways. The similarity between ion transport by skim milk membrane vesicles and that of the apical aspect of the intact mammary epithelium suggests that the former may be a good model to study solute transport by the apical membrane of mammary secretory cells.