Secretory activity of goat mammary glands during pregnancy and the onset of lactation

Changes in the concentrations of metabolites in milk at the onset and cessation of lactation in the goat

Changes in the concentrations of various metabolites in mammary secretions (organic acids, UDP-hexoses and nucleotides) have been observed at the onset and cessation of lactation. All metabolites, with the exception of galactose, increased dramatically in concentration just before and immediately after parturition. Galactose concentrations decreased with the onset of lactation. After regular milking ceased, the concentrations of these metabolites in milk contained within the udder fell, but the rates of decline varied.

Local production of prostaglandins in relation to mammary function at the onset of lactation in the goat

1. Arterial and mammary venous concentrations of prostaglandins F alpha (PGF alpha), E (PGE) and the PGF alpha metabolite, 13,14-dihydro-15-oxoPGF alpha (DHK-PGF alpha) were studied during late pregnancy and the onset of lactation in conscious goats. Mammary secretion concentrations of PGF alpha and DHK-PGF alpha were determined, and mammary blood flow, arterial plasma progesterone concentrations and milk composition were also studied. 2. A significant output of PGF alpha from the mammary gland into mammary venous blood was observed during late pregnancy; this output ceased near term. 3. Mammary output of DHK-PGF alpha into venous blood began about 6 days prepartum, suggesting an increasing capacity of the gland to metabolize PGF alpha. 4. The concentration of PGF alpha in mammary secretion increased from about 4 days pre-partum, that of DKH-PGF alpha from about 12 days pre-partum. 5. It is concluded that although total mammary output of PGF alpha decreases during late pregnancy and early lactation, the rate of mammary synthesis of PGF alpha increases and the PGF alpha is increasingly secreted into milk and metabolized to DHK-PGF alpha within the mammary gland. 6. Unilateral treatment of one mammary gland in goats with the PGF 2 alpha analogue, Cloprostenol, at two dose levels from 2-3 days pre-partum to 1-2 days post-partum prevented the changes in milk [Na] that occur at term in untreated glands. At the higher dose, the normal rise in milk [citrate] was abolished and milk yield was reduced; these effects persisted after cessation of treatment. 7. It is suggested that PGF alpha may play a local inhibitory role in mammary gland function during late pregnancy. It is further suggested that PGF alpha could be the factor, or one of the factors, proposed by Linzell & Peaker (1974) to be responsible for local control of mammary epithelial permeability and possibly also for secretory rate.

Lactose synthesis: the possibilities of regulation

Elucidation of the details of lactose synthesis, in particular its dependence upon alpha-lactalbumin and its location within the lumen of the Golgi apparatus, now allows one to ask useful questions pertaining to its regulation. Attention is directed towards galactosyltransferase itself (EC 2.4.1.22), which appears to be rate-limiting in the uridine nucleotide cycle that supports lactose synthesis, and to those factors that may affect its activity. In laboratory animals alpha-lactalbumin appears to be the major agent of regulation during lactogenesis but is not necessarily limiting at other times, whereas the increase in amount of galactosyltransferase seems largely to account for the rising yield of lactose during lactation. Studies with pinched-off Golgi membrane vesicles, together with measurements of intracellular chemical concentrations, suggest that beta-glucose and uridine diphosphategalactose do not saturate lactose synthesis and are, therefore potentially regulatory features of this process. Further aspects of lactose synthesis that may offer points of regulation include calcium ions, generation of protons within the Golgi lumen, and the generally rate-limiting nature of the Golgi membrane.

Changes in mammary function at the onset of lactation in the goat: correlation with hormonal changes

1. Changes in mammary function and plasma hormone concentrations during late pregnancy and the onset of lactation have been studied in conscious goats.2. Mammary blood flow, oxygen consumption and glucose uptake increased markedly and significantly between 2 days and 0.5-1 day pre-partum.3. The increase in mammary glucose uptake was relatively greater than that of oxygen consumption or blood flow.4. The concentration of citrate in mammary secretion increased; the first significant change was apparent 0.5-1 day pre-partum but the main rise occurred after this time.5. It is concluded that the marked increase in mammary glucose uptake 0.5-1 day before parturition indicates the time of onset of copious milk secretion.6. The first significant increase in the concentration of unconjugated oestrogens in arterial plasma occurred 3 days pre-partum, whereas the first significant decrease in progesterone and increase in 13,14-dihydro-15-oxoprostaglandin F(2alpha) occurred 0.5-1 day pre-partum, thus coinciding with the mammary changes; there was also a peak in prolactin concentrations at the latter time.7. The hormonal changes are discussed in relation to current concepts of the initiation of parturition and the onset of copious milk secretion in this species. It is suggested that the fall in plasma progesterone concentrations triggers milk secretion at high rates.

Changes in the activity of lactose synthetase in the goat udder during pregnancy

The lactose synthetase activity of homogenates and particulate fractions prepared from the udders of goats killed at various stages of pregnancy were determined. Both components of the enzyme, galactosyltransferase and alpha-lactalbumin, became detectable about half way through the gestation period, at the time when rapid proliferation of mammary epithelial tissue commences. Throughout the second half of pregnancy the udder possessed a high level of lactose synthetase activity though the lactose content of the tissue did not increase. These findings are discussed in relation to the endocrine control of lactogenesis in the goat and possible mechanisms for the induction of milk production at parturition.

Lactose and major milk proteins are present in secretory vesicle-rich fractions from lactating mammary gland

Preparations enriched in apparently intact secretory vesicles were isolated from homogenates of lactating rat and bovine mammary tissue by differential and density gradient centrifugation in isoosmotic media. Morphologically these preparations consisted nearly entirely of vesicles of varying sizes, at least some of which contained casein micelles. Endoplasmic reticulum vesicles, Golgi apparatus cisterna and dictyosomes, mitochondria, peroxisomes, lysosomes, and nuclei were not observed in secretory vesicle-rich fractions. Vesicle preparations were enriched in lactose relative to total membrane fractions from mammary gland. The galactosyltransferase of lactose synthase (UDPgalactose: D-glucose 4 beta-galactosyl-transferase, EC 2.4.1.22) was also present in secretory vesicle preparations, alphas1- and beta-caseins, alpha-lactalbumin, and beta-lactoglobulin, the major secretory proteins of differentiated mammary epithelial cells, were identified as constituents of vesicle-rich fractions from bovine mammary gland. These observations suggest that the major carbohydrate and major proteins of milk are compartmentalized into secretory vesicles and are secreted by exocytotic fusion of secretory vesicles with the apical plasma membrane.

Mammary function and its control at the cessation of lactation in the goat

1. The changes in mammary function following cessation of milking during declining lactation have been studied in conscious goats. 2. No significant changes in the rate of milk secretion, mammary blood flow or metabolism occurred in the first 24 h after cessation of milking. After then, secretory rate, mammary blood flow, oxygen consumption, glucose uptake and acetate uptake decreased markedly over the next 3 days. Up to the time of maximum udder distension on day 3, there were no major changes in milk composition. 3. It was found that the rate of milk secretion declined when the calculated pressure within the alveoli became positive. 4. After 3 days, mammary volume and intramammary pressure decreased, and the composition of milk changed slowly to resemble that of extracellular fluid, i.e. [Na+], [Cl-], [HCO3-] and pH increased while [K+], [lactose] and [citrate] decreased. During this time [lactose] and [K+] were positively correlated, and [lactose] and [Na+], and [lactose] and [Cl-] negatively correlated. 5. It is suggested that the changes in milk composition, the decreases in mammary volume and in intramammary pressure after day 3 are due to the loss of integrity of the mammary epithelium. 6. By about 7 weeks after the cessation of milking the udder volume was less than the empty udder volume before milking was stopped, indicating a loss of mammary tissue as well as the resorption of fluid. 7. When milking of an autotransplanted gland was stopped, while milking of the control gland in situ was continued, the rate of secretion in the transplant fell while that of the control did not change. 8. In goats milked normally but in which a volume of isosmotic lactose equal to the volume of milk removed at that milking was injected into the lumen of one gland at each milking, the rate of secretion of that gland, but not that of the other, decreased.

The concentration of lactate in relation to other components of bovine mammary secretion during premature regression and after resumption of milking

An experiment lasting 42 d was performed in 4 consecutive stages on 6 healthy Friesian cows during mid-lactation. Mean values for the different components of mammary secretion during normal lactation were established. Milking was then suspended on all quarters from d 1–14. The mean values for lactate increased 20- to 30-fold over the mean value for normal lactation. Over the same period the leucocyte count (polymorphonuclear leucocytes and lymphocytes) also increased, whereas the mean values for glucose and the estimated redox potential decreased. From d 15–28 milking was resumed on one half of the udder and from d 29–42 milking was also resumed on the other udder-half. When milking was resumed the above-mentioned changes were reversed, taking 5–7 d to reach values obtained during normal lactation in the udder-half which had not been milked for 14 d, but at least 14 d in the udderhalf which had not been milked for 28 d. The changes in the levels of lactate, glucose, leucocyte count and estimated redox potential are discussed in relation to changes in the secretory activity of the mammary gland (lactose, β-lactoglobulin and epithelial cell count), permeability changes of the mammary epithelium (Na+, K+and serum albumin) and the immune defence mechanism in the udder (immunoglobulins). The results indicate that lactate is formed during anaerobic glycolysis by the leucocytes in the mammary secretion, most probably due to reduced blood flow to the udder and the accumulation of secretion in the gland and teat cisterns on cessation of milking.

The secretion of citrate into milk

1. The time course of changes in specific activities of citrate, lactose and fatty acids in milk during frequent milking, following the I.V. 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 activites of fatty acids were reached at 5-7 hr. 2. Following short I.A. infusions of 24Na, 36Cl, 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 secretory 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 sucretory 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.