The utilization of glucose for the synthesis of milk components in the fed and starved lactating goat in vivo

Fluorometric determination of isocitrate dehydrogenase (EC 1.1.1.42; 1; NADP+ dependent) in ruminant milk

The enzyme isocitrate dehydrogenase (EC 1.1.1.42; 1; NADP⁺ dependent) located in the mammary cell cytosol mediates the synthesis of the majority of reducing equivalents for the energetically demanding milk fat and cholesterol synthesis in mammary cell cytosol. The present article presents a novel fluorometric method for quantification of the activity of this enzyme (IDH) in ruminant milk without pretreatment of the sample. Further, 493 goat milk samples - harvested before, during and after a nutritional restriction - were analysed for IDH activity i) with addition of extra substrate (isocitrate), and ii) with the intrinsic isocitrate solely. The IDH activity ranged from 0.22 to 15.4 units [nano moles product/(ml * min)] (un-supplemented) and from 0.22 to 45.6 units (isocitrate supplemented). The IDH activity increased considerably in milk during the nutritional restriction period concomitant with the increase in the metabolite isocitrate concentration and somatic cell count and returned to the initial level shortly after restriction period. The present 'high through-put' analytical method may be beneficial in future studies to phenotype modifications in mammary energy metabolism and milk fat synthesis, for which IDH activity may be a biomarker.

Limiting factors for milk production in dairy cows: perspectives from physiology and nutrition

Milk production in dairy cows increases worldwide since many decades. With rising milk yields, however, potential limiting factors are increasingly discussed. Particularly, the availability of glucose and amino acids is crucial to maintain milk production as well as animal health. Limitations arise from feed sources, the rumen and digestive tract, tissue mobilization, intermediary metabolism and transport, and the uptake of circulating nutrients by the lactating mammary gland. The limiting character can change depending on the stage of lactation. Although physiological boundaries are prevalent throughout the gestation-lactation cycle, limitations are aggravated during the early lactation period when high milk production is accompanied by low feed intake and high mobilization of body reserves. The knowledge about physiological constraints may help to improve animal health and make milk production more sustainably. The scope of this review is to address contemporary factors related to production limits in dairy cows from a physiological perspective. Besides acknowledged physiological constraints, selected environmental and management-related factors affecting animal performance and physiology will be discussed. Potential solutions and strategies to overcome or to alleviate these constraints can only be presented briefly. Instead, they are thought to address existing shortcomings and to identify possibilities for optimization. Despite a scientific-based view on physiological limits, we should keep in mind that only healthy animals could use their genetic capacity and produce high amounts of milk.

A Comparative Review of the Cell Biology, Biochemistry, and Genetics of Lactose Synthesis

Lactose is the primary carbohydrate in the milk of most mammals and is unique in that it is only synthesized by epithelial cells in the mammary glands. Lactose is also essential for the development and nutrition of infants. Across species, the concentration of lactose in milk holds a strong positive correlation with overall milk volume. Additionally, there is a range of examples where the onset of lactose synthesis as well as the content of lactose in milk varies between species and throughout a lactation. Despite this diversity, the precursors, genes, proteins and ions that regulate lactose synthesis have not received the depth of study they likely deserve relative to the significance of this simple and abundant molecule. Through this review, our objective is to highlight the requirements for lactose synthesis at the biochemical, cellular and temporal levels through a comparative approach. This overview also serves as the prelude to a companion review describing the dietary, hormonal, molecular, and genetic factors that regulate lactose synthesis.

A Comparative Review of the Extrinsic and Intrinsic Factors Regulating Lactose Synthesis

Milk is critical for the survival of all mammalian offspring, where its production by a mammary gland is also positively associated with its lactose concentration. A clearer understanding of the factors that regulate lactose synthesis stands to direct strategies for improving neonatal health while also highlighting opportunities to manipulate and improve milk production and composition. In this review we draw a cross-species comparison of the extra- and intramammary factors that regulate lactose synthesis, with a special focus on humans, dairy animals, and rodents. We outline the various factors known to influence lactose synthesis including diet, hormones, and substrate supply, as well as the intracellular molecular and genetic mechanisms. We also discuss the strengths and limitations of various in vivo and in vitro systems for the study of lactose synthesis, which remains an important research gap.

Mismatch of Glucose Allocation between Different Life Functions in the Transition Period of Dairy Cows

Immune cell functions such as phagocytosis and synthesis of immunometabolites, as well as immune cell survival, proliferation and differentiation, largely depend on an adequate availability of glucose by immune cells. During inflammation, the glucose demands of the immune system may increase to amounts similar to those required for high milk yields. Similar metabolic pathways are involved in the adaptation to both lactation and inflammation, including changes in the somatotropic axis and glucocorticoid response, as well as adipokine and cytokine release. They affect (i) cell growth, proliferation and activation, which determines the metabolic activity and thus the glucose demand of the respective cells; (ii) the overall availability of glucose through intake, mobilization and gluconeogenesis; and (iii) glucose uptake and utilization by different tissues. Metabolic adaptation to inflammation and milk synthesis is interconnected. An increased demand of one life function has an impact on the supply and utilization of glucose by competing life functions, including glucose receptor expression, blood flow and oxidation characteristics. In cows with high genetic merits for milk production, changes in the somatotropic axis affecting carbohydrate and lipid metabolism as well as immune functions are profound. The ability to cut down milk synthesis during periods when whole-body demand exceeds the supply is limited. Excessive mobilization and allocation of glucose to the mammary gland are likely to contribute considerably to peripartal immune dysfunction.

The effect of long term under- and overfeeding on the expression of genes related to glucose metabolism in the mammary tissue of goats

The glucose metabolism in the mammary tissue (MT) is controlled by several genes whose nutritional regulation is poorly documented. In this study, the effect of long-term under- and overfeeding on the expression of glucose transporter 1 (GLUT1), glucose transporter 3 (GLUT3), sodium glucose contransporter 1 (SGLT1), two isoforms of β- (1, 4) galactosyltransferase [β- (1, 4) GAT1 and β- (1, 4) GAT3], and α-lactalbumin (LALBA), related to glucose metabolism in goat MT, was examined by using real-time RT-PCR. Twenty- four lactating goats were divided into three homogenous subgroups and fed the same ration in quantities covering 70% (underfeeding), 100% (control) and 130% (overfeeding) of their energy and crude protein requirements, respectively. The results showed that the feeding level did not affect the GLUT1, GLUT3 and SGLT1 genes expression in goats MT. On the contrary, a significant reduction and a trend for reduction on mRNA of β- (1, 4) GAT1 and on β- (1, 4) GAT3, respectively, in the MT of underfed goats, compared with the overfed ones, was observed. Moreover, a significant decrease in the LALBA mRNA accumulation in the MT of underfed goats compared with the overfed was found. In conclusion, the MT of goats, unlike cows, adapt to changes in glucose or energy supply from different levels of feeding by changing the utilisation of glucose for the synthesis of lactose.

The effect of long term under- and over-feeding on the expression of genes related to glucose metabolism in mammary tissue of sheep

Glucose utilisation for lactose synthesis in the mammary gland involves expression of a large number of genes whose nutritional regulation remains poorly defined. In this study, the effect of long term under- and over-feeding on the expression of genes [glucose transporter 1: GLUT1, glucose transporter 3: GLUT3, Sodium glucose contransporter 1: SGLT1, two isoforms of β- (1,4) galactosyltransferase: β- (1,4) GAT1, β- (1,4) GAT3 and α-lactalbumin: LALBA] related to glucose metabolism in sheep mammary tissue (MT) was examined. Twenty-four lactating dairy sheep were divided into three homogenous sub-groups and fed the same ration in quantities which met 70% (underfeeding), 100% (control) and 130% (overfeeding) of their energy and crude protein requirements. The results showed a significant reduction on mRNA of GLUT1 and LALBA gene in the MT of underfed sheep, compared with the respective controls and overfed and a significant reduction on mRNA level of SGLT1 and β- (1,4) GAT1 in the MT of underfed sheep, compared with the overfed ones. A significant increase in the GLUT3 mRNA accumulation in the MT of both under- and over- fed sheep was found. Additionally, a trend of reduction on β- (1,4) GAT3 mRNA level in the MT of the underfed sheep, compared with the overfed, was observed. A close positive relationship was obtained between the mRNA transcripts accumulation of GLUT1, SGLT1, β- (1,4) GAT1 and LALBA gene with the milk lactose content and milk lactose yield respectively. In conclusion, feeding level and consequently nutrient availability, may affect glucose uptake and utilisation in sheep MT by altering the GLUT1, GLUT3, SGLT1, β- (1,4) GAT1 and LALBA gene expression involved in their metabolic pathways.

Biology of glucose transport in the mammary gland

Glucose is the major precursor of lactose, which is synthesized in Golgi vesicles of mammary secretory alveolar epithelial cells during lactation. Glucose is taken up by mammary epithelial cells through a passive, facilitative process, which is driven by the downward glucose concentration gradient across the plasma membrane. This process is mediated by facilitative glucose transporters (GLUTs), of which there are 14 known isoforms. Mammary glands mainly express GLUT1 and GLUT8, and GLUT1 is the predominant isoform with a Km of ~10 mM and transport activity for mannose and galactose in addition to glucose. Mammary glucose transport activity increases dramatically from the virgin state to the lactation state, with a concomitant increase in GLUT expression. The increased GLUT expression during lactogenesis is not stimulated by the accepted lactogenic hormones. New evidence indicates that a possible low oxygen tension resulting from increased metabolic rate and oxygen consumption may play a major role in stimulating glucose uptake and GLUT1 expression in mammary epithelial cells during lactogenesis. In addition to its primary presence on the plasma membrane, GLUT1 is also expressed on the Golgi membrane of mammary epithelial cells and is likely involved in facilitating the uptake of glucose and galactose to the site of lactose synthesis. Because lactose synthesis dictates milk volume, regulation of GLUT expression and trafficking represents potentially fruitful areas for further research in dairy production. In addition, this research will have pathological implications for the treatment of breast cancer because glucose uptake and GLUT expression are up-regulated in breast cancer cells to accommodate the increased glucose need.

Seasonal changes in the fat composition and concentration of citrate and related metabolites in cows' milk

Seasonal variations were observed in the proportion of long-chain (C18 and over) and medium-chain (C6–14) fatty acids (FA) in milk fat. Long-chain FA were proportionately higher during the period June to October. Changes were also observed in the concentrations of citrate, isocitrate and 2-oxoglutarate in milk. Changes in [isocitrate]: [2-oxoglutarate] correlated positively with changes in the proportion of long-chain FA in milk fat and negatively with the proportion of medium-chain FA.

Metabolism and udder health at dry-off in cows of different breeds and production levels

The effects of milk yield at dry-off (DO), different calving intervals (CI; 12 and 15 mo) and breed on metabolism and udder health were studied in 56 primiparous and multiparous cows of the Swedish Red and White (SRB) and Swedish Holstein (SH) breeds. The cows were dried off 55 +/- 5 d prior to expected parturition. They were fed 4 kg of DM as silage and wheat straw ad libitum for 5 d, and were milked in the morning of d 2 and 5. Depending on their daily milk yield, the cows were divided into 3 numerically equal groups on 2 d during the week prior to DO: low (LY; 5.0 to 11.4 kg of milk/d, n = 19), medium (MY; 11.5 to 17.7 kg of milk/d, n = 19), and high (HY; 17.8 to 29.5 kg of milk/d, n = 18). The plasma cortisol concentration increased during DO only in MY and HY cows. Plasma nonesterified fatty acids increased during DO in all groups, but the maximum nonesterified fatty acid concentration was related to the milk yield prior to DO. The plasma glucose level during the DO period was not significantly affected by yield, but the insulin concentration decreased after DO, with a more pronounced drop in the HY group. The CI 15-mo group had a higher glucose level and tended to have a higher insulin level in plasma than the CI 12-mo group before DO. They also had a higher body condition than the CI 12-mo group. The results indicate that the CI 15-mo cows had a more positive nutrient balance. There were no effects of CI on milk production or composition during DO. The SRB and SH breeds did not differ in any of the measured plasma parameters or milk production. However, the lower somatic cell counts in SRB than in SH observed before and during DO, as well as after parturition, were attributed to being an effect of breed. The proportion of cows with intramammary infections (IMI) was significantly lower just after calving in the LY group than in the other yield groups. At 2 and 3 wk after DO, significantly fewer cows in the LY group had open teat canals compared with the HY and MY groups, respectively, but teat-end condition did not differ between yield groups. The yield before DO did not significantly influence the somatic cell counts during the first 4 wk after parturition or the presence of IMI 4 wk after parturition. We concluded that in the present study, higher milk yield prior to DO gave rise to a more pronounced metabolic response and a higher risk of contracting IMI during the dry period, at calving, or both, but yield at DO did not have any long-term effects on udder health. A prolonged CI did not facilitate a rapid decrease in milk production. The SRB and SH breeds responded equally in decreasing the milk production during DO, but the SRB breed had lower somatic cell counts.

Expression and regulation of glucose transporters in the bovine mammary gland

Glucose is the primary precursor for the synthesis of lactose, which controls milk volume by maintaining the osmolarity of milk. Glucose uptake in the mammary gland plays a key role in milk production. Glucose transport across the plasma membranes of mammalian cells is carried out by 2 distinct processes: facilitative transport, mediated by a family of facilitative glucose transporters (GLUT); and sodium-dependent transport, mediated by the Na+/glucose cotransporters (SGLT). Transport kinetic studies indicate that glucose transport across the plasma membrane of the lactating bovine mammary epithelial cell has a K(m) value of 8.29 mM for 3-O-methyl-D-glucose and can be inhibited by both cytochalasin-B and phloretin, indicating a facilitative transport process. This is consistent with the observation that in the lactating bovine mammary gland, GLUT1 is the predominant glucose transporter. However, the bovine lactating mammary gland also expresses GLUT3, GLUT4, GLUT5, GLUT8, GLUT12, and sodium-dependent SGLT1 and SGLT2 at different levels. Studies of protein expression and cellular and subcellular localizations of these transporters are needed to address their physiological functions in the mammary gland. From late pregnancy to early lactation, expression of GLUT1, GLUT8, GLUT12, SGLT1, and SGLT2 mRNA increases from at least 5-fold to several hundred-fold, suggesting that these transporters may be regulated by lactogenic hormones and have roles in milk synthesis. The GLUT1 protein is detected in lactating mammary epithelial cells. Its expression level decreases from early to late lactation stages and becomes barely detectable in the nonlactating gland. Both GLUT1 mRNA and protein levels in the lactating mammary gland are not significantly affected by exogenous bovine growth hormone, and, in addition, GLUT1 mRNA does not appear to be affected by leptin.

Effects of exogenous glucose on glucose metabolism in the lactating goat in vivo

1. Glucose turnover in fed and 48 h-starved lactating goats was determined during a glucose load of 500 μmol/min using a continuous infusion of [U-14C]- and [3-3H]glucose.

2. Endogenous rates of irreversible glucose turnover (i.e. total rates of irreversible glucose turnover minus the rate of exogenous glucose supply) were depressed during glucose loading by 14 and 62% in the fed and starved animals respectively.

3. Plasma glucose concentrations increased significantly by 57 and 88% in the fed and starved goats respectively. Plasma insulin concentrations increased by 108 and 128% in the fed and starved animals respectively.

4. Milk yields increased significantly (41%) in the starved animals during glucose loading, but were unaffected in fed animals.

5. In both the fed and 48 h-starved goats, mammary glucose metabolism via glycolysis and the pentose phosphate pathway appeared to be stimulated by glucose loading.

Changes in mammary uptake and metabolic fate of glucose with once-daily milking and feed restriction in dairy cows

The aim of this review is to better understand the regulation of milk yield in response to once-daily milking and feed restriction. Glucose is the principal precursor for the synthesis of lactose (a major osmotic agent in milk), and participates in determining the milk volume produced. When applying these two breeding factors, reductions in milk yield are associated with a reduction in milk lactose yield and in the arterial flow of glucose, due to a decrease in the mammary blood flow. The ability of the udder to extract glucose is altered with once-daily milking but not necessarily with feed restriction. Lactose synthesis is down-regulated in response to once-daily milking and feed restriction but the percentage of the extracted glucose which is converted into lactose is differently affected in response to treatments. No marked change is observed with once daily milking whereas this would be increased with feed restriction and in contrast, depressed with fasting.

Bovine glucose transporter GLUT8: cloning, expression, and developmental regulation in mammary gland

GLUT8 is a newly identified member of the facilitative glucose transporter family, which characteristically exhibits high-affinity glucose transport activity. The expression of GLUT8 has been shown to depend on gonadotropin secretion in human testes and to be regulated by insulin in the blastocyst. To characterize GLUT8 and investigate its role in normal mammary gland function, we cloned and sequenced the full-length cDNA of bovine GLUT8. The 2073-base-pair cDNA sequence is predicted to encode a protein of 478 amino acids, with a molecular weight of approximately 51 kDa. The deduced amino acid sequence of bovine GLUT8 is 90%, 84%, 84% and 58% identical to human, mouse, rat and chicken GLUT8, and is 26%, 27% and 24% identical to bovine GLUT1, GLUT3 and GLUT4, respectively. Bovine GLUT8 retains the characteristic structural features of GLUT8 proteins previously identified from other species including membrane spanning helices, glucose transporter motifs, an N-linked glycosylation site on loop 9 and a putative dileucine internalization motif. The major in vitro transcription and translation product of bovine GLUT8 cDNA migrated at an apparent molecular weight of 38 kDa similar to the sizes reported for GLUT8 from other mammalian species. In the presence of canine microsomal membranes, the translation product increased to 40 kDa suggesting glycosylation. Transient transfection studies using a FLAG epitope tagged construct in COS-7 cells revealed that bovine GLUT8 is localized to the cytoplasm in non-stimulated conditions. A 2.1-kb GLUT8 mRNA transcript was detected at high levels in bovine testes, at moderate levels in lactating bovine mammary gland, lung, kidney, spleen, intestine and skeletal muscle, and at low levels in bovine liver. GLUT8 mRNA expression in bovine mammary gland increased about 10-fold (P<0.001) during late pregnancy and early lactation, similar to the pattern of change in GLUT1 mRNA and more dramatic than the increase seen in mouse mammary gland. These results suggest that GLUT8 expression may be regulated by lactogenic hormones and that GLUT8 may play a role in glucose uptake in the lactating mammary gland.

Adaptations of glucose metabolism during pregnancy and lactation

Increased glucose requirements of the gravid uterus during late pregnancy and even greater requirements of the lactating mammary glands necessitate major adjustments in glucose production and utilization in maternal liver, adipose tissue, skeletal muscle, and other tissues. In ruminants, which at all times rely principally on hepatic gluconeogenesis for their glucose supply, hepatic glucose synthesis during late pregnancy and early lactation is increased to accommodate uterine or mammary demands even when the supply of dietary substrate is inadequate. At the same time, glucose utilization by adipose tissue and muscle is reduced. In pregnant animals, these responses are exaggerated by moderate undernutrition and are mediated by reduced tissue sensitivity and responsiveness to insulin, associated with decreased tissue expression of the insulin-responsive facilitative glucose transporter, GLUT4. Peripheral tissue responses to insulin remain severely attenuated during early lactation but recover as the animal progresses through mid lactation. Specific homeorhetic effectors of decreased insulin-mediated glucose metabolism during late pregnancy have yet to be conclusively identified. In contrast, somatotropin is almost certainly a predominant homeorhetic influence during lactation because its exogenous administration causes specific changes in glucose metabolism (and many other functions) of various nonmammary tissues which faithfully mimic normal adaptations to early lactation.

Localization and gene expression of glucose transporters in bovine mammary gland

Glucose uptake in the mammary gland is a rate-limiting step in milk synthesis. To study glucose transporters in the bovine mammary gland, the erythrocyte-type glucose transporter (GLUT1) and the insulin-responsive glucose transporter (GLUT4) proteins were assessed by Western blotting and immunohistochemical staining, using polyclonal antibodies against the C-terminal peptide of GLUT1 and GLUT4. Our results demonstrated that the bovine mammary gland expressed a relatively high level of GLUT1 protein, whereas GLUT4 protein was not detected in the mammary gland of either lactating or dry cows. The absence of GLUT4 may indicate that glucose transport is not regulated by insulin in the lactating and dry bovine mammary gland. The anti-GLUT1 antibody strongly stained the single layer of epithelial cells of mammary alveoli. The expression of GLUT1 mRNA was similar in the mammary gland of late lactation and non-lactating cows. However, a smaller molecular weight species (38 kDa) of GLUT1 protein was detected in the mammary gland of non-lactating cows where its abundance in crude membrane preparation was 80% higher than in lactating animals. There were no significant differences in GLUT1 mRNA in bovine mammary gland at 118 d and 181 d postpartum, however, GLUT1 protein expression tended to be greater at 118 d postpartum.

Effects of graded amounts of duodenal infusions of lysine on the mammary uptake of major milk precursors in dairy cows

Effects of providing a limiting AA on milk secretion and mammary nutrient uptake were investigated using four lactating dairy cows given duodenal infusions of graded amounts of L-Lys-HCl (0, 9, 27, and 63 g/d). Infusions were administered over 4-d periods according to a Latin square design. Diets consisted of a 70:30 corn silage:concentrate ratio supplemented with 1.5 kg of DM/d as dehydrated alfalfa. The basal diet met requirements for energy and protein and was deficient in Lys. Cows were fitted with indwelling catheters inserted into the left carotid and left subcutaneous abdominal veins, and a transit-time flow probe was implanted around the left external pudic artery. Milk, fat, and protein yields were unaffected by infusions of Lys. Milk protein content increased to a maximum with the third treatment, and fat content showed opposite variation. Although arterial concentrations and arteriovenous differences of Lys increased to a plateau with the third treatment, the relationship between arteriovenous differences and arterial concentrations was curvilinear. Infusions of Lys tended to increase the efficiency of N utilization and the mammary extraction rates of AA to synthesize more milk proteins. Initially, Lys was extracted in a direct ratio to its milk output and then taken up in excess by the secretory cells, suggesting that Lys was no longer a limiting factor in milk protein synthesis.

[The effect of the main protein source in rations of ewes and the time of blood collection on the glucose and triacylglycerol levels in blood at the beginning of lactation]

The effect of the protein source of the ration (soybean meal, cottonseed cake, corn gluten and fish meal) and the time (period) of blood sample taking was examined on the content of glucose and triacylglycerols in the blood during the initial lactation period. Thirty-six ewes of the Thessaloniki crossbred type were randomly allocated to 4 groups. The ewes of each group were fed ad libitum with one of the 4 different rations, respectively. From each ewe 4 blood samples were taken in different times. The experimental design was factorial 4x4 with 9 replicates with main factors the main protein source (ration), as well as the time of blood sample taking. The protein source effect on glucose and triacylglycerol concentration in blood was not significant, while that of time of blood sample taking was significant. The interaction "ration" X "time" of sampling for the glucose and TGC concentration was not significant.

Distribution of mammalian facilitative glucose transporter messenger RNA in bovine tissues

1. The complementary DNA for five human facilitative glucose transporters (GLUT1, GLUT2, GLUT3, GLUT4 and GLUT5) were used to determine the distribution of facilitative glucose transporter mRNA in bovine tissues by Northern blotting. Under high stringency hybridization conditions, a single 2.8 kb transcript of GLUT1 was seen in all bovine tissues examined except liver. Mammary gland had the highest abundance of GLUT1 mRNA. 2. Four GLUT2 transcripts of 6.3, 3.8, 2.2 and 1.6 kb were observed to be most abundant in liver, with lower abundance in kidney and duodenum. 3. Only a very low level of GLUT3 mRNA was detected in the mammary gland, skeletal muscle and duodenum. 4. Skeletal muscle contained the greatest abundance of GLUT4 mRNA, which was barely detectable in omental fat, kidney and mammary gland. 5. Transcripts of GLUT5 mRNA were detected in relatively high abundance in liver and kidney, and in lower abundance in the duodenum and mammary gland. 6. With the exception of the mammary gland, for which human data have not been reported, the distributions of GLUT1, 2 and 4 mRNA for the bovine tissues examined are similar to that reported for humans. On the other hand, the distributions of GLUT3 and 5 mRNA in the bovine differ from those reported for humans.

Changes in the concentration of metabolites in milk from cows fed on diets supplemented with soyabean oil or fatty acids

Cows were fed on diets supplemented with soyabean oil or soyabean fatty acids which in some cases were protected from rumen hydrogenation. The fat-containing diets reduced the output of short- and medium-chain fatty acids in milk. Associated with this fall in short- and medium-chain fatty acids was a decrease in the concentration of 2-oxoglutarate and an increase in that of isocitrate and citrate. Protection of polyunsaturated fat from rumen hydrogenation had no significant effect. Milk yields were unaffected by diet, but the variation in milk yield among cows correlated positively with the concentration of glucose in milk.

The effects of concanavalin A on milk secretion and mammary metabolism in the goat

The effects of concanavalin A on the rate of milk secretion and the concentration of metabolites in milk were studied following intramammary injection of the lectin via the teat canal into one mammary gland of lactating goats. Concanavalin A decreased milk secretion from the treated gland, reduced the concentrations of phosphoenolpyruvate, nucleoside diphosphate and 2-oxoglutarate in milk and increased the concentrations of glucose, galactose, glycerol, L-lactate, pyruvate, isocitrate and citrate. The changes in the concentrations of the metabolites in milk are discussed in relation to biochemical changes occurring in the mammary gland during the suppression of milk secretion. It is suggested that, when lactose synthesis and secretion is decreased, substantial metabolism of glucose via glycolysis occurs.

Glucose production and utilization in non-pregnant, pregnant and lactating ewes

By using continuous infusions of 3H- and 14C-labelled substrates, three-pool models, incorporating rumen propionate, plasma glucose and blood carbon dioxide were constructed to determine the contribution of propionate to glucose in non-pregnant, pregnant (mid and late) and lactating hill ewes. Although the intakes of non-pregnant and pregnant ewes were the same (1200 g dried grass/d) and resulted in similar levels of propionate production (33 g C/d), glucose production rate (GPR) increased from 44 g C/d in the non-pregnant ewes to 62 g C/d in the ewes carrying twins in late pregnancy. In lactating ewes given 2500 g dried grass/d, propionate production increased to 56 g C/d and GPR increased to 93 and 104 g C/d in ewes suckling single and twin lambs respectively. There was an increase in the percentage of the propionate resource which was diverted to glucose, from 37% in the non-pregnant ewes and ewes in mid-pregnancy, to 55% in late pregnancy and 60% in lactation. In spite of this apparent metabolic adaptation to the additional requirements for glucose, approximately 55% of the glucose-C was supplied by metabolites other than propionate and CO2. From the determination of plasma glycerol concentrations it was estimated that the maximum possible contribution of glycerol-C to glucose was only 8-12 g C/d. The remaining 40% of the glucose-C could not be accounted for and could have been derived from non-essential amino acids (NEAA). In the non-pregnant and pregnant ewes only 62% of the GPR was oxidized to CO2. In the lactating ewes only 49 and 30% of the GPR was oxidized to CO2 in the ewes suckling single and twin lambs respectively. In the majority of cases there was a marked similarity between the amounts of glucose-C apparently derived from NEAA and the amount of glucose-C which was not oxidized to CO2.

Control of fatty acid synthesis in lactation

Lactation results not only in an increased rate of fatty acid synthesis in the mammary gland but also in a decreased rate of fatty acid synthesis in adipose tissue and, in the rat at least, an increased rate of hepatic fatty acid synthesis. Progesterone (during pregnancy), prolactin and (in ruminants) GH are implicated in the regulation of the reciprocal changes in fatty acid synthesis in mammary gland and adipose tissue. Progesterone and prolactin, at least, appear to influence the rate of fatty acid synthesis by modulating the insulin-binding capacities of the tissues, but it is clear that steps in the mechanism of action of insulin subsequent to its binding to the receptor are also changed in adipose tissue during lactation.

Glucose production measured by tracer and balance data in conscious miniature pig

The rate of glucose production was estimated in the conscious, unrestrained miniature pig during metabolic adaptation to starvation (up to 120 h) by the simultaneous use of three different techniques: 1) the isotopedilution technique, 2) the arteriohepatovenous catheter technique, and 3) the urinary nitrogen balance. During the experimental period 1) whole-body glucose turnover decreased, whereas the amount of glucose recycling increased; 2) splanchnic glucose output decreased, whereas the rate of splanchnic precursor extraction increased up to 48 h, followed by a decrease; 3) gut glucose consumption amounted to about 30% of splanchnic glucose output; and 4) urinary nitrogen excretion declined continuously. The comparison of the different methods revealed that during starvation 1) tracer-determined glucose production rate was within the range (+/- 10%) of the splanchnic glucose output; 2) mean hepatic glucose output overestimated the tracer data by about 30-40%; 3) splanchnic glucose output underestimated hepatic glucose production by the amount of gut glucose consumption; 4) tracer-determined glucose recycling corrected for isotope dilution and amino acid contribution was within the range of splanchnic gluconeogenic precursor extractions. Considering the limitations (e.g., gut glucose consumption, gut lactate and alanine release, blood flow measurement) and methodological problems of the different approaches applied, it is evident that each method reflects different events. It is suggested that the versatile tracer technique combined with nitrogen balance should be preferred for measurement of endogenous glucose production.

Metabolic significance of milk glucose

The free glucose concentration in the aqueous phase of samples of goat, sheep, cow, rat and rabbit milk was about 0.1-0.3 mM, while that in human milk was about 2mM. During starvation the glucose concentration of goat milk fell considerably (by about 80% in 2 d) in parallel with the decreased rate of lactose production. With rats fed ad lib., glucose concentration in the milk was greater at 12.00 h than at 18.00 h, when lactose synthesis has been shown to decrease. 3-O-Methyl-D-glucose injected into the goat mammary gland via the teat canal specifically entered the blood. These findings support the idea that glucose equilibrates across the apical membrane of mammary secretory cells, so that milk concentrations reflect intracellular glucose concentratioins.

Changes in the concentrations of the minor constituents of goat's milk during starvation and on refeeding of the lactating animal and their relationship to mammary gland metabolism

1. Changes in the concentrations of the minor constituents of goat's milk were observed during 48 h starvation and on refeeding. 2. The concentrations of hexose phosphate and UDP-hexoses increased during starvation and decreased on refeeding. 3. The concentrations of phosphoenolpyruvate and glycerate 3-phosphate decreased during starvation and increased on refeeding. 4. Isocitrate:2-oxoglutarate increased during starvation and decreased on refeeding. 5. Changes in the minor constituents of milk can be explained in terms of the metabolic changes occurring in the mammary gland during starvation. It is proposed that changes in the concentrations of these metabolites in milk reflect changes in their concentrations in the cytosol or Golgi vesicles of the mammary gland.