The secretion of citrate into milk.

Abstract

SUMMARY1. The time course of changes in specific activities of citrate, lactose and fatty acids in milk during frequent milking, following the ixv. administration of labelled glucose, acetate and chylomicrons in goats has been studied. Peak specific activities of lactose and citrate in milk were reached at 2-3 hr, while peak specific activities of fatty acids were reached at 5-7 hr.2. Following short I.A. infusions of 24Na, 3"Cl and 42K, peak specific activities in milk were reached in 1 hr or less.3. The mammary epithelium of lactating goats was found to be virtually impermeable to labelled citrate in both directions. 4. Labelled citrate had an apparent volume of distribution in lactating guinea-pigs mammary slices in vitro similar to that of extracellular space markers.5. Treatment of goats with large doses of oxytocin markedly increased the permeability of the secretary epithelium to labelled citrate.6. In the goat mammary gland, citrate, protein and calcium failed to enter milk which had been diluted with isosmotic lactose by intraductal injection, whereas Na, K and Cl did enter, thus tending to restore the concentrations of these ions to normal.7. It is suggested that citrate, which is formed within the secretary cell, enters milk not by passage across the apical cell membrane but, in common with lactose and milk protein, by exocytosis of Golgi vesicles. It appears that citrate is held at high concentrations in milk by virtue of the impermeability of the mammary epithelium to the forms in which it occurs in milk.

Ionized calcium in milk and the integrity of the mammary epithelium in the goat.

“…While substantial dilution of milk in vivo with isosmotic sucrose or lactose solutions does not lead to the changes observed with citrate included (Linzell et al 1976;Peaker, 1977;Neville & Peaker, 1979), it seems likely that Ca2+ could be replenished by re-equilibration of reactions between Ca2±, calciocitrate and protein-associated calcium under these circumstances. If Ca2+ in milk is important for epithelial integrity, the question arises of whether calcium movements or binding control milk composition when the proposed epithelial paracellular pathway through leaky junctions (see Linzell & Peaker, 1971;Peaker, 1978) opens or closes.…”

“…2). However, it is difficult to interpret changes at this milking because of four complicating factors: (i) the presence of the additional volume of liquid in the gland would tend to depress concentrations by a dilution effect; (ii) the presence of additional Na in the citrate-sucrose solution, although when this is allowed for the Na concentration was still increased; (iii) the tendency for Na, K and Cl to pass into the diluted milk from the cell as a result of altering the concentration gradients (Linzell, Mepham & Peaker, 1976); (iv) the additional citrate could affect anion movements by a Donnan or other effect. Therefore, interpretation of the effects of the solution is not based on changes at this milking.…”

Mechanism of milk secretion: milk composition in relation to potential difference across the mammary epithelium

“…However, earlier studies showed that the secretion of fat, protein, Ca and citrate continues when the rate of lactose and water secretion is very low (Hardwick et al 1961;). Since there is evidence that protein (see , Ca (Baumrucker & Keenan, 1975) and citrate (Linzell, Mepham & Peaker, 1976) are secreted, like lactose, by exocytosis of vesicles arising from the Golgi apparatus, it is clear that the normal secretary processes of exocytosis and lipid droplet extrusion continue in these circumstances.…”

The effect of raised intramammary pressure on mammary function in the goat in relation to the cessation of lactation.

“…In all cases sterile 300 mM-sucrose solution at approximately 37 0C was pumped into the cistern via a catheter in the teat canal. The validity of this procedure in terms of failure to induce osmotic water movements is discussed by Linzell, Mepham & Peaker (1976) and Peaker (1977).…”