The secretion of citrate into milk

Genome-wide association studies for citric and lactic acids in dairy sheep milk in a New Zealand flock

The objectives of this study were to estimate genetic parameters for citric acid content (CA) and lactic acid content (LA) in sheep milk and to identify the associated candidate genes in a New Zealand dairy sheep flock. Records from 165 ewes were used. Heritability estimates based on pedigree records for CA and LA were 0.65 and 0.33, respectively. The genetic and phenotypic correlations between CA and LA were strong-moderate and negative. Estimates of genomic heritability for CA and LA were also high (0.85, 0.51) and the genomic correlation between CA and LA was strongly negative (-0.96 ± 0.11). No significant associations were found at the Bonferroni level. However, one intragenic SNP in C1QTNF1 (chromosome 11) was associated with CA, at the chromosomal significance threshold. Another SNP associated with CA was intergenic (chromosome 15). For LA, the most notable SNP was intragenic in CYTH1 (chromosome 11), the other two SNPs were intragenic in MGAT5B and TIMP2 (chromosome 11), and four SNPs were intergenic (chromosomes 1 and 24). The functions of candidate genes indicate that CA and LA could potentially be used as biomarkers for energy balance and clinical mastitis. Further research is recommended to validate the present results.

Impact of milking interval and time on milk spontaneous lipolysis and composition in dairy cows

Milk lipolysis is defined as the hydrolysis of triglycerides, the major component of milk fat, resulting in the release of short-chain fatty acids (FA) responsible for rancid flavor and partial glycerides that impair functional properties such as foaming and creaming abilities. Milk lipolysis is a complex phenomenon that depends on both animal parameters and breeding factors. Milk spontaneous lipolysis is known to be higher in milk from evening milkings than from morning milkings. This may be related to the longer length of overnight milking intervals or to the nycthemeral cycle. In this experiment, our objective was thus to study the impact of both milking intervals and time of day on milk spontaneous lipolysis in twice-daily-milking systems with one of 3 milking intervals: Short Day - Long Night (SD-LN, 6.30 a.m. and 4.30 p.m.,); Long Day - Short Night (LD-SN, 6:30 a.m. and 8:30 p.m.,); and Balanced Day and Night (BDN, 6:30 a.m. and 6:30 p.m.,). To achieve this goal, 21 multiparous dairy cows in mid-lactation were used in a 3 × 3 Latin square design over 3 periods. The experiment lasted 5 weeks, corresponding to 3 experimental periods of 6 d alternating with 8 d of milking with conventional hours (morning-evening gap of 10 h). We confirmed that milk spontaneous lipolysis was influenced by milking interval, but not the milking time. Indeed, we observed more lipolysis in SD-LN evening milk (+0.20 mEq/100 g fat) and LD-SN morning milk (+0.22 mEq/100 g fat), both of which corresponded to a 10 h interval between successive milkings. High lipolysis milk came from cows that produced less milk with a higher milk fat content. No significant difference between milkings was observed for BDN. Milk protein, total P and citrate contents increased according to the duration of mammary gland storage of milk (from 10 to 14 h). There was no effect of milking intervals on milk fat globule diameter. The milk Na+/K+ ratio, indicating an opening of tight junctions in the mammary gland, increased only in evening milkings with BDN and LD-SN. In conclusion, we found that the effect of milking intervals on lipolysis is stronger than that of the nycthemeral cycle.

Influence of Dairy Products on Bioavailability of Zinc from Other Food Products: A Review of Complementarity at a Meal Level

In this paper, we reviewed the role of dairy products in dietary zinc absorption. Dairy products can have a reasonable contribution for dietary zinc intake in Western diets, where dairy consumption is high. However, the co-ingestion of dairy products can also improve zinc absorption from other food products. Such improvements have been observed when dairy products (e.g., milk or yoghurt) were ingested together with food such as rice, tortillas or bread products, all of which are considered to be high-phytate foods with low inherent zinc absorption. For foods low in phytate, the co-ingestion of dairy products did not improve zinc absorption. Improved zinc absorption of zinc from high-phytate foods following co-ingestion with dairy products may be related to the beneficial effects of the citrate and phosphopeptides present in dairy products. Considering that the main dietary zinc sources in areas in the world where zinc deficiency is most prevalent are typically high in phytate, the inclusion of dairy products in meals may be a viable dietary strategy to improve zinc absorption.

The Impact of Seasonality in Pasture-Based Production Systems on Milk Composition and Functionality

Seasonal calving, pasture-based dairy systems are widely practiced in countries with a temperate climate and plentiful rainfall such as Ireland and New Zealand. This approach maximizes milk production from pasture and, consequently, is a low-cost, low-input dairy production system. On the other hand, the majority of global milk supply is derived from high input indoor total mixed ration systems where seasonal calving is not practiced due to the dependence on ensiled silages, grains and concentrated feeds, which are available year-round. Synchronous changes in the macro and micronutrients in milk are much more noticeable as lactation progresses through early, mid and late stages in seasonal systems compared to non-seasonal systems-which can have implications on the processability and functionality of milk.

Metabolomics Approach Explore Diagnostic Biomarkers and Metabolic Changes in Heat-Stressed Dairy Cows

In the present experiment, we investigated the impact of heat stress (HS) on physiological parameters, dry matter intake, milk production, the metabolome of milk, and blood plasma in lactating Holstein dairy cows. For this purpose, 20 Holstein lactating cows were distributed in two groups in such a way that each group had 10 cows. A group of 10 cows was reared in HS conditions, while the other group of 10 cows was reared in the thermoneutral zone. The results of the experiment showed that cows subjected to HS had higher respiration rates (p < 0.01) and greater rectal temperature (p < 0.01). Results of milk production and composition explored that HS lowered milk production (p < 0.01) and milk protein percentage (p < 0.05) than cows raised in a thermoneutral place. Furthermore, HS increased the concentrations of N-acetyl glycoprotein, scyllo-inositol, choline, and pyridoxamine in milk, while HS decreased the concentrations of O-acetyl glycoprotein, glycerophosphorylcholine, citrate, and methyl phosphate in milk. Moreover, HS enhanced plasma concentrations of alanine, glucose, glutamate, urea, 1-methylhistidine, histidine, and formate in cows, while the plasma concentration of low-density lipoprotein, very-low-density lipoprotein, leucine, lipid, and 3-hydroxybutyrate decreased due to HS. Based on the findings of the current research, it is concluded that HS alters the milk and blood plasma metabolites of lactating Holstein dairy cows. Overall, in the current experiment, HS altered eight metabolites in milk and twelve metabolites in the plasma of lactating Holstein dairy cows. Furthermore, the current study explored that these metabolites were mainly involved in proteolysis, gluconeogenesis, and milk fatty acid synthesis and could be potential biomarkers for dairy cows undergoing HS.

Effects of abomasal infusion of essential fatty acids together with conjugated linoleic acid in late and early lactation on performance, milk and body composition, and plasma metabolites in dairy cows

Rations including high amounts of corn silage are currently very common in dairy production. Diets with corn silage as forage source result in a low supply of essential fatty acids, such as α-linolenic acid, and may lead to low conjugated linoleic acid (CLA) production. The present study investigated the effects of abomasal infusion of essential fatty acids, especially α-linolenic acid, and CLA in dairy cows fed a corn silage-based diet on performance, milk composition, including fatty acid (FA) pattern, and lipid metabolism from late to early lactation. Rumen-cannulated Holstein cows (n = 40) were studied from wk 9 antepartum to wk 9 postpartum and dried off 6 wk before calving. The cows were assigned to 1 of 4 treatment groups. Cows were abomasally supplemented with coconut oil (CTRL, 76 g/d), linseed and safflower oil (EFA, 78 and 4 g/d; linseed/safflower oil ratio = 19.5:1; n-6/n-3 FA ratio = 1:3), Lutalin (CLA, 38 g/d; BASF SE, Ludwigshafen, Germany; isomers cis-9,trans-11 and trans-10,cis-12 each 10 g/d) or EFA+CLA. Milk composition was analyzed weekly, and blood samples were taken several times before and after parturition to determine plasma concentrations of metabolites related to lipid metabolism. Liver samples were obtained by biopsy on d 63 and 21 antepartum and on d 1, 28, and 63 postpartum to measure triglyceride concentration. Body composition was determined after slaughter. Supplementation of CLA reduced milk fat concentration, increased body fat mass, and improved energy balance (EB) in late and early lactation, but EB was lowest during late lactation in the EFA group. Cows with CLA treatment alone showed an elevated milk citrate concentration in early lactation, whereas EFA+CLA did not reveal higher milk citrate but did have increased acetone. Milk protein was increased in late lactation but was decreased in wk 1 postpartum in CLA and EFA+CLA. Milk urea was reduced by CLA treatment during the whole period. After calving, the increase of nonesterified fatty acids in plasma was less in CLA groups; liver triglycerides were raised lowest at d 28 in CLA groups. Our data confirm an improved metabolic status with CLA but not with exclusive EFA supplementation during early lactation. Increased milk citrate concentration in CLA cows points to reduced de novo FA synthesis in the mammary gland, but milk citrate was less affected in EFA+CLA cows, indicating that EFA supplementation may influence changes in mammary gland FA metabolism achieved by CLA.

Relationship between energy balance and metabolic profiles in plasma and milk of dairy cows in early lactation

Negative energy balance in dairy cows in early lactation is related to alteration of metabolic status. However, the relationships among energy balance, metabolic profile in plasma, and metabolic profile in milk have not been reported. In this study our aims were: (1) to reveal the metabolic profiles of plasma and milk by integrating results from nuclear magnetic resonance (NMR) with data from liquid chromatography triple quadrupole mass spectrometry (LC-MS); and (2) to investigate the relationship between energy balance and the metabolic profiles of plasma and milk. For this study 24 individual dairy cows (parity 2.5 ± 0.5; mean ± standard deviation) were studied in lactation wk 2. Body weight (mean ± standard deviation; 627.4 ± 56.4 kg) and milk yield (28.1 ± 6.7 kg/d; mean ± standard deviation) were monitored daily. Milk composition (fat, protein, and lactose) and net energy balance were calculated. Plasma and milk samples were collected and analyzed using LC-MS and NMR. From all plasma metabolites measured, 27 were correlated with energy balance. These plasma metabolites were related to body reserve mobilization from body fat, muscle, and bone; increased blood flow; and gluconeogenesis. From all milk metabolites measured, 30 were correlated with energy balance. These milk metabolites were related to cell apoptosis and cell proliferation. Nine metabolites detected in both plasma and milk were correlated with each other and with energy balance. These metabolites were mainly related to hyperketonemia; β-oxidation of fatty acids; and one-carbon metabolism. The metabolic profiles of plasma and milk provide an in-depth insight into the physiological pathways of dairy cows in negative energy balance in early lactation. In addition to the classical indicators for energy balance (e.g., β-hydroxybutyrate, acetone, and glucose), the current study presents some new metabolites (e.g., glycine in plasma and milk; kynurenine, panthothenate, or arginine in plasma) in lactating dairy cows that are related to energy balance and may be of interest as new indicators for energy balance.

The application of NMR-based milk metabolite analysis in milk authenticity identification

BACKGROUND

Milk is an important food component in the human diet and is a target for fraud, including many unsafe practices. For example, the unscrupulous adulteration of soymilk into bovine and goat milk or of bovine milk into goat milk in order to gain profit without declaration is a health risk, as the adulterant source and sanitary history are unknown. A robust and fit-for-purpose technique is required to enforce market surveillance and hence protect consumer health. Nuclear magnetic resonance (NMR) is a powerful technique for characterization of food products based on measuring the profile of metabolites. In this study, 1D NMR in conjunction with multivariate chemometrics as well as 2D NMR was applied to differentiate milk types and to identify milk adulteration.

RESULTS

Ten metabolites were found which differed among milk types, hence providing characteristic markers for identifying the milk. These metabolites were used to establish mathematical models for milk type differentiation. The limit of quantification (LOQ) of adulteration was 2% (v/v) for soymilk in bovine milk, 2% (v/v) for soymilk in goat milk and 5% (v/v) for bovine milk in goat milk, with relative standard deviation (RSD) less than 10%, which can meet the needs of daily inspection.

CONCLUSION

The NMR method described here is effective for milk authenticity identification, and the study demonstrates that the NMR-based milk metabolite analysis approach provides a means of detecting adulteration at expected levels and can be used for dairy quality monitoring. © 2016 Society of Chemical Industry.

Milk metabolome relates enteric methane emission to milk synthesis and energy metabolism pathways

Methane (CH4) emission of dairy cows contributes significantly to the carbon footprint of the dairy chain; therefore, a better understanding of CH4 formation is urgently needed. The present study explored the milk metabolome by gas chromatography-mass spectrometry (milk volatile metabolites) and nuclear magnetic resonance (milk nonvolatile metabolites) to better understand the biological pathways involved in CH4 emission in dairy cattle. Data were used from a randomized block design experiment with 32 multiparous Holstein-Friesian cows and 4 diets. All diets had a roughage:concentrate ratio of 80:20 (dry matter basis) and the roughage was grass silage (GS), corn silage (CS), or a mixture of both (67% GS, 33% CS; 33% GS, 67% CS). Methane emission was measured in climate respiration chambers and expressed as CH4 yield (per unit of dry matter intake) and CH4 intensity (per unit of fat- and protein-corrected milk; FPCM). No volatile or nonvolatile metabolite was positively related to CH4 yield, and acetone (measured as a volatile and as a nonvolatile metabolite) was negatively related to CH4 yield. The volatile metabolites 1-heptanol-decanol, 3-nonanone, ethanol, and tetrahydrofuran were positively related to CH4 intensity. None of the volatile metabolites was negatively related to CH4 intensity. The nonvolatile metabolites acetoacetate, creatinine, ethanol, formate, methylmalonate, and N-acetylsugar A were positively related to CH4 intensity, and uridine diphosphate (UDP)-hexose B and citrate were negatively related to CH4 intensity. Several volatile and nonvolatile metabolites that were correlated with CH4 intensity also were correlated with FPCM and not significantly related to CH4 intensity anymore when FPCM was included as covariate. This suggests that changes in these milk metabolites may be related to changes in milk yield or metabolic processes involved in milk synthesis. The UDP-hexose B was correlated with FPCM, whereas citrate was not. Both metabolites were still related to CH4 intensity when FPCM was included as covariate. The UDP-hexose B is an intermediate of lactose metabolism, and citrate is an important intermediate of Krebs cycle-related energy processes. Therefore, the negative correlation of UDP-hexose B and citrate with CH4 intensity may reflect a decrease in metabolic activity in the mammary gland. Our results suggest that an integrative approach including milk yield and composition, and dietary and animal traits will help to explain the biological metabolism of dairy cows in relation to methane CH4 emission.

Ethanol Production by Selected Intestinal Microorganisms and Lactic Acid Bacteria Growing under Different Nutritional Conditions

To gain some specific insight into the roles microorganisms might play in non-alcoholic fatty liver disease (NAFLD), some intestinal and lactic acid bacteria and one yeast (Anaerostipes caccae, Bacteroides thetaiotaomicron, Bifidobacterium longum, Enterococcus fecalis, Escherichia coli, Lactobacillus acidophilus, Lactobacillus fermentum, Lactobacillus plantarum, Weissella confusa, Saccharomyces cerevisiae) were characterized by high performance liquid chromatography for production of ethanol when grown on different carbohydrates: hexoses (glucose and fructose), pentoses (arabinose and ribose), disaccharides (lactose and lactulose), and inulin. Highest amounts of ethanol were produced by S. cerevisiae, L. fermentum, and W. confusa on glucose and by S. cerevisiae and W. confusa on fructose. Due to mannitol-dehydrogenase expressed in L. fermentum, ethanol production on fructose was significantly (P < 0.05) reduced. Pyruvate and citrate, two potential electron acceptors for regeneration of NAD(+)/NADP(+), drastically reduced ethanol production with acetate produced instead in L. fermentum grown on glucose and W. confusa grown on glucose and fructose, respectively. In fecal slurries prepared from feces of four overweight volunteers, ethanol was found to be produced upon addition of fructose. Addition of A. caccae, L. acidophilus, L. fermentum, as well as citrate and pyruvate, respectively, abolished ethanol production. However, addition of W. confusa resulted in significantly (P < 0.05) increased production of ethanol. These results indicate that microorganisms like W. confusa, a hetero-fermentative, mannitol-dehydrogenase negative lactic acid bacterium, may promote NAFLD through ethanol produced from sugar fermentation, while other intestinal bacteria and homo- and hetero-fermentative but mannitol-dehydrogenase positive lactic acid bacteria may not promote NAFLD. Also, our studies indicate that dietary factors interfering with gastrointestinal microbiota and microbial metabolism may be important in preventing or promoting NAFLD.

Correlations between milk and plasma levels of amino and carboxylic acids in dairy cows

The objective of this study was to investigate the relationship between the concentrations of 19 amino acids, glucose, and seven carboxylic acids in the blood and milk of dairy cows and their correlations with established markers of ketosis. To that end, blood plasma and milk specimens were collected throughout lactation in two breeds of dairy cows of different milk yield. Plasma concentrations of glucose, pyruvate, lactate, α-aminobutyrate, β-hydroxybutyrate (BHBA), and most amino acids, except for glutamate and aspartate, were on average 9.9-fold higher than their respective milk levels. In contrast, glutamate, aspartate, and the Krebs cycle intermediates succinate, fumarate, malate, and citrate were on average 9.1-fold higher in milk than in plasma. For most metabolites, with the exception of BHBA and threonine, no significant correlations were observed between their levels in plasma and milk. Additionally, milk levels of acetone showed significant direct relationships with the glycine-to-alanine ratio and the BHBA concentration in plasma. The marked decline in plasma concentrations of glucose, pyruvate, lactate, and alanine in cows with plasma BHBA levels above the diagnostic cutoff point for subclinical ketosis suggests that these animals fail to meet their glucose demand and, as a consequence, rely increasingly on ketone bodies as a source of energy. The concomitant increase in plasma glycine may reflect not only the excessive depletion of protein reserves but also a potential deficiency of vitamin B6.

Crude glycerol as glycogenic precursor in feed; effects on milk coagulation properties and metabolic profiles of dairy cows

As grain prices rise, the search for alternative glycogenic precursors in animal feed becomes increasingly important, and this study was conducted to determine if the replacement of starch with glycerol, as an alternative glycogenic precursor, affects the milk metabolic profile and milk coagulation ability, and therefore the quality of the milk. Eight primiparous mid-lactation Holstein cows were fed during a replicated 4 × 4 Latin square trial with four different isoenergetic rations: (1) control (T0) fed a total mixed ration (TMR) with barley meal; (2) group T1, decreased barley content, replaced isoenergetically with 1 kg crude glycerol; (3) group T2, the barley meal was replaced with 2 kg of crude glycerol; and (4) group T3 the barley meal was replaced with 3 kg of crude glycerol. Rumen, blood and milk samples were collected at the end of every 21-d treatment period. Rumen samples were analysed for proportion of total volatile fatty acid (VFA), blood samples for insulin and glucose, and milk for metabolites (e.g. citric-acid cycle compounds). The change in glycogenic precursors had a positive effect on rumen VFA proportions; the proportion of propionic acid increased (P < 0·001). Milk protein (P < 0·001) and curd firmness (P < 0·001) both increased. The increase in milk protein concentration may have been due to an increase in microbial protein. Regarding the milk metabolic profiles, different signals were positively associated with coagulation ability and change in the diet. Based on this study, changing the glycogenic precursor in animal diet in this way is possible, and may have no immediate deleterious consequences on milk quality or cow health. Indeed, there is evidence for benefits from this substitution.

Comparison of effects of dietary coconut oil and animal fat blend on lactational performance of Holstein cows fed a high-starch diet

Dietary medium-chain fatty acids (C(8:0) through C(12:0)) are researched for their potential to reduce enteric methane emissions and to increase N utilization efficiency in ruminants. We aimed to 1) compare coconut oil (CNO; ~60% medium-chain fatty acids) with a source of long-chain fatty acids (animal fat blend; AFB) on lactational responses in a high-starch diet and 2) determine the effect of different dietary concentrations of CNO on dry matter intake (DMI). In experiment 1, the control diet (CTRL) contained (dry basis) 40% forage (71% corn silage, and alfalfa hay and haylage), 26% NDF, and 35% starch. Isonitrogenous treatment diets contained 5.0% of AFB (5%-AFB), CNO (5%-CNO), or a 1-to-1 mixture of AFB and CNO (5%-AFB-CNO) and 0.8% corn gluten meal in place of corn grain. Thirty-two multiparous dairy cows (201 ± 46 d postpartum; 42.0 ± 5.5 kg/d 3.5% fat-corrected milk yield) were adapted to CTRL, blocked by milk yield, and randomly assigned to 1 of 4 treatment diets for 21 d with samples and data collected from d 15 through 21. Treatment 5%-CNO decreased DMI markedly and precipitously and was discontinued after d 5. In wk 3, 5%-AFB and especially 5%-AFB-CNO lowered total-tract NDF digested vs. CTRL (2.6 vs. 1.8 vs. 3.1 kg/d, respectively), likely because fat treatments reduced DMI and 5%-AFB-CNO impaired total-tract NDF digestibility. Milk fat concentrations were 3.10% (CTRL), 2.51% (5%-AFB), and 1.97% (5%-AFB-CNO) and correlated negatively to concentrations of C(18:2 trans-10,cis-12) in milk fat. Additionally, 5%-AFB and 5%-AFB-CNO tended to lower milk yield and decreased yields of solids-corrected milk and milk protein compared with CTRL. Fat treatments decreased milk lactose concentration, but increased milk citrate concentration. Moreover, cows fed 5%-AFB-CNO produced less solids-corrected milk than did cows fed 5%-AFB. In experiment 2, diets similar to CTRL contained 2.0, 3.0, or 4.0% CNO. Fifteen multiparous cows (219 ± 42 d postpartum; 42.1 ± 7.0 kg milk yield; mean ± SD) were blocked by DMI and randomly assigned to 1 of 3 treatment diets for an 8-d evaluation. Dietary concentration of CNO affected DMI, with the greatest depression at 4.0% CNO. Overall, dietary CNO depressed DMI and NDF digestibility of a high-starch diet compared with AFB. Feeding CNO to lactating cows equal to or greater than 2.5% decreased lactational performance or DMI.

Mapping the zinc-transporting system in mammary cells: molecular analysis reveals a phenotype-dependent zinc-transporting network during lactation

The mammary epithelial cell transitions from a non-secreting to a terminally differentiated, secreting cell during lactation. Zinc (Zn) is a key modulator of phenotypic transition as it regulates over 300 biological functions including transcription, translation, energy transformation, intracellular signaling, and apoptosis. In addition, Zn must be redirected from normal cellular functions into the secretory compartment, as many components of the secretory system are Zn-dependent and an extraordinary amount of Zn is secreted (1-3 mg Zn/day) into milk. Herein, we utilized a "systems biology" approach of genomic and proteomic profiling to explore mechanisms through which Zn is reallocated during phenotype transition in the lactating mammary gland from mice and cultured mammary cells. Nine Zn transporters play key roles in Zn redistribution within the network during lactation. Protein abundance of six Zip (Zip3, Zip5, Zip7, Zip8, Zip10, Zip11) and three ZnT (ZnT2, ZnT4, ZnT9) proteins was expanded >2-fold during lactation, which was not necessarily reflected by changes in mRNA expression. Our data suggest that Zip5, Zip8, and Zip10 may be key to Zn acquisition from maternal circulation, while multiple Zip proteins reuptake Zn from milk. Confocal microscopy of cultured mammary cells identified the Golgi apparatus (modulated in part by ZnT5, Zip7, and Zip11) and the late endosomal compartment (modulated in part by ZnT2 and Zip3) as key intracellular compartments through which Zn is reallocated during lactation. These results provide an important framework for understanding the "Zn-transporting network" through which mammary gland Zn pools are redistributed and secreted into milk.

The health benefits of calcium citrate malate: a review of the supporting science

There has been considerable investigation into the health benefits of calcium citrate malate (CCM) since it was first patented in the late 1980s. This chapter is a comprehensive summary of the supporting science and available evidence on the bioavailability and health benefits of consuming CCM. It highlights the important roles that CCM can play during various life stages. CCM has been shown to facilitate calcium retention and bone accrual in children and adolescents. In adults, it effectively promotes the consolidation and maintenance of bone mass. In conjunction with vitamin D, CCM also decreases bone fracture risk in the elderly, slows the rate of bone loss in old age, and is of benefit to the health and well-being of postmenopausal women. CCM is exceptional in that it confers many unique benefits that go beyond bone health. Unlike other calcium sources that necessitate supplementation be in conjunction with a meal to ensure an appreciable benefit is derived, CCM can be consumed with or without food and delivers a significant nutritional benefit to individuals of all ages. The chemistry of CCM makes it a particularly beneficial calcium source for individuals with hypochlorydia or achlorydia, which generally includes the elderly and those on medications that decrease gastric acid secretion. CCM is also recognized as a calcium source that does not increase the risk of kidney stones, and in fact it protects against stone-forming potential. The versatile nature of CCM makes it a convenient and practical calcium salt for use in moist foods and beverages. The major factor that may preclude selection of CCM as a preferred calcium source is the higher cost compared to other sources of calcium commonly used for fortification (e.g., calcium carbonate and tricalcium phosphate). However, formation of CCM directly within beverages or other fluid foods and/or preparations, and the addition of a concentrated CCM solution or slurry, are relatively cost-effective methods by which CCM can be incorporated into finished food and beverage products.

Variation of milk citrate with stage of lactation and de novo fatty acid synthesis in dairy cows

Citrate is a normal constituent of milk that affects milk-processing characteristics. It is an intermediate in the tricarboxylic acid cycle and plays an indirect role in fat synthesis by providing reducing equivalents in the form of NADPH. The objective of this study was to investigate variation in citrate with stage of lactation and de novo fatty acid synthesis, without confounding dietary effects. Twenty-four cows were fed the same diet, and milk citrate and fatty acids were determined over a 10-d period. Eight cows were in early lactation [13 +/- 1.8 d in milk (DIM; mean +/-standard error], 8 in midlactation (130 +/-4.6 DIM), and 8 in late lactation (283 +/-3.4 DIM). For cows in early, mid, and late lactation, milk yield was 34.4, 34.4, and 21.4 L/d [standard error of difference (SED) 1.78]; milk fat was 50.4, 40.3, and 41.4 g/L (3.68); milk citrate was 11.3, 9.7, and 10.1 mmol/L (0.64); the ratio of 4-14 C:18-20 C fatty acids was 0.9, 1.3, and 1.2 (0.07). Activity of the fatty acid synthase enzyme system (EC 2.3.1.85) was calculated as acetate used for chain elongation (ACE); ACE (mol/d) for cows in early, mid, and late lactation, was 7.3, 11.1, and 8.1 (SED 1.05). For individual cows, citrate (mmol/L) = 14.3 -0.44 xACE (r2 = 0.58). We propose that ACE provides a more accurate indication of synthase activity than do fatty acid ratios or yields. This study confirms the hypothesis that variation in milk citrate with stage of lactation is related to de novo synthesis of fatty acids and that the relationship is independent of diet and milk yield.

Calcium secretion into milk

Ionized calcium ([Ca(2+)]) is present in milk at concentrations around 3 mM, a concentration that drives the formation of complexes with citrate, phosphate, and casein, thereby generating compounds that carry the major portion of calcium in milk. In humans and cows, where it has been studied, changes in milk calcium appear to be regulated by the amount of citrate and casein in milk rather than changes in [Ca(2+)]. Most or all of the calcium in milk is likely derived through exocytosis of secretory vesicles derived from the Golgi compartment where a calcium ATPase mediates transport from the cytoplasm. The identity of the transporters is not yet certain but gene expression for the plasma membrane calcium ATPase, PMCA2bw, and the secretory pathway calcium ATPase, SPCA, is highly upregulated during lactation. Currently nothing appears to be known about the mechanisms that mediate transport of calcium across the basolateral membrane of the alveolar cell.

Diurnal variation and the effect of feed restriction on plasma and milk metabolites in TMR-fed dairy cows

The objective was to study the diurnal variation in metabolites in plasma and milk of dairy cows fed total mixed rations (TMR) with a low-energy (LE) or high-energy content (HE) expected to give a minor and a major diurnal variation, respectively. Further, the purpose was to quantify and compare the responses in plasma and milk parameters when cows changed from ad libitum to restrictive feeding. Eight multiparous, early-lactating Danish Holstein cows were used in a cross-over design with two consecutive 14-day periods. Blood and milk samples were collected hourly on day 11 of each period and on days 12-14 of each period, the cows were fed restrictively (65% of ad libitum dry-matter intake). The concentration of beta-hydroxybutyrate (BHB) in plasma was significantly higher in the evening for cows fed the HE TMR, than for cows fed the LE TMR. There was a significant diurnal variation in BHB in milk, with the highest concentrations between milkings and the lowest concentrations at milking. Non-esterified fatty acids (NEFA) in plasma showed significant diurnal variation that was caused by high concentrations in the morning. Plasma glucose did not show any diurnal variation. It has been argued that feeding a TMR removes diurnal changes related to feeding, which is contrary to earlier diurnal studies where concentrates have been fed twice daily. Feed restriction increased (P < 0.001) NEFA and BHB in plasma by 121 and 90%, respectively, while the glucose concentration decreased (P < 0.001) by 19%. Milk concentrations of BHB, citrate and fat increased (P < 0.001) by 163, 11 and 26%, respectively, because of feed restriction, while there were no changes in milk protein and lactose. The relatively high increase in BHB during feed restriction suggests that BHB is more advantageous as a milk indicator of metabolic status in dairy cows than citrate and fat.

Transport of milk constituents by the mammary gland

This review deals with the cellular mechanisms that transport milk constituents or the precursors of milk constituents into, out of, and across the mammary secretory cell. The various milk constituents are secreted by different intracellular routes, and these are outlined, including the paracellular pathway between interstitial fluid and milk that is present in some physiological states and in some species throughout lactation. Also considered are the in vivo and in vitro methods used to study mammary transport and secretory mechanisms. The main part of the review addresses the mechanisms responsible for uptake across the basolateral cell membrane and, in some cases, for transport into the Golgi apparatus and for movement across the apical membrane of sodium, potassium, chloride, water, phosphate, calcium, citrate, iodide, choline, carnitine, glucose, amino acids and peptides, and fatty acids. Recent work on the control of these processes, by volume-sensitive mechanisms for example, is emphasized. The review points out where future work is needed to gain an overall view of milk secretion, for example, in marsupials where milk composition changes markedly during development of the young, and particularly on the intracellular coordination of the transport processes that result in the production of milk of relatively constant composition at a particular stage of lactation in both placental and marsupial mammals.

Maternal selenium status influences the concentration and binding pattern of zinc in human milk

To study the health effects of high dietary maternal selenium intake breast milk, blood and toe-nails were collected from 143 women (20-24 days of lactation) living in Venezuelan states of Yaracuy and Portuguesa. Depending on the regional selenium intake level three regions were defined within the total range of 90-980 microgram per day. The samples were analyzed by means of INAA for the determination of trace elements, including selenium and zinc. The significant inverse correlation between Se and Zn in breast milk found in former studies was confirmed. Investigation of the Zn-binding pattern in milk whey was carried out by an on-line combination HPLC (SEC) for protein separation and ICP-AES for element detection. Six Zn-binding compounds including citrate were detected. A highly significant negative correlation was found between the citrate, which is the main low-molecular Zn-binding compound, and the maternal daily selenium intake. We determined that the decrease in zinc concentration is due to a decrease in the citrate level, which depends on the selenium concentration in breast milk. The selenium concentration in breast milk is, in turn, proportional to the dietary intake. In addition, significant changes in the UV- and Zn-profiles were observed in the milk whey with the highest selenium content, indicating that above a certain maternal intake level substantial changes in the composition of mammary secretory cells occur. This effect can be of interest for estimation of the safe dietary intake level of selenium.

Assessment of mammary gland metabolism in the sow. I. Development of methods for the measurement of cellular metabolites in milk and colostrum

We have modified deproteinization methods and a number of spectrophotometric and bioluminescent methods in order to measure the concentrations of cellular metabolites in small volumes (< 0.3 ml) of sows' colostrum and milk. For the majority of the assays, recoveries ranged from 92 to 105%. The binding of ATP and UTP to a calcium phosphate-citrate-caseinate complex in milk, and the decrease in ATP (92%/h) and UTP (18.1%/h) concentrations during in vitro incubation of the whey fraction suggested that it was unlikely ATP and UTP (both < 1 microM) could exist free in sows' milk. The mean concentrations (range) of cellular metabolites in milk (6-11 d post partum) were: glucose, 669 microM (220-1367); glucose 6-phosphate, 63.0 microM (27.6-101.4); glucose 1-phosphate, 18.3 microM (13.1-24.8); UDPglucose, 296 microM (170-494); UDPgalactose, 635 microM (230-945); lactose, 162 mM (124-187); UDP, 105 microM (85-130); UMP, 1760 microM (1326-2587); inorganic phosphate, 13.5 mM (1.4-29.3); ATP, < 0.5 microM; ADP, 53.6 microM (10.5-171.25); AMP, 215 microM (61.6-491.6); cAMP, 22.3 microM (3.5-61.6); galactose, 198 microM (118-474) and fructose, 226 microM (172-283). Differences in the concentrations of glucose, glucose 6-phosphate, glucose 1-phosphate, UDPgalactose and cAMP between fore and hind milk samples indicated postsecretory changes in the concentrations of certain metabolites. Changes in the concentrations of metabolites during in vitro incubation of milk and of colostrum suggested that these postsecretory changes were probably due to the actions of enzymes present in mammary secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Concentration of citrate in the mammary secretion of sows during lactogenesis II and established lactation

The functional significance of citrate in the mammary secretion of six sows was investigated during the second stage of lactogenesis (lactogenesis II) and established lactation. The changes in the concentrations of progesterone and lactose in the maternal blood, and lactose, Na and K in the mammary secretion, suggested that lactogenesis II began during the final day of pregnancy. The concentration of citrate in the mammary secretion of the sows during lactogenesis II was high and varied from 5.4 (SEM 0.5) mM at day 0.5 post partum to 6.8 (SEM 0.4) mM at day 1.5 post partum. There was a decline of approximately 30% in the concentration of citrate in the milk of sows during the first week of lactation. These findings suggest that, in contrast to all other species studied previously, milk citrate is not a harbinger of lactogenesis II in the sow. However, the changes in the concentration of citrate in the mammary secretions of sows may reflect changes in the rate of de novo synthesis of fatty acids that take place in the mammary glands of sows during lactogenesis II and established lactation.

Mammary amino acid utilization in dairy cows fed fat and its relationship to milk protein depression

Changes in mammary AA utilization associated with dietary fat-induced milk protein depression were assessed in cows with disturbed AA status. Four first lactation cows fitted with rumen cannulas were assigned to a factorial arrangement of treatments: two diets and two casein infusion sites within a 4 x 4 Latin square design. Diets were formulated with yellow grease at 0 or 4% of DM. An 8% sodium caseinate solution was infused continuously at 5.04 kg/d into the rumen or abomasum during the last 5 d of each 21-d period. Arterial AA concentrations were reduced by dietary fat treatment and increased by abomasal sodium caseinate infusion. Mammary arteriovenous differences of essential AA tended to increase with both abomasal casein infusion and dietary fat treatments. With ruminal casein infusion treatments, yellow grease supplementation increased the percentage of extraction of blood essential AA into mammary tissue. Mammary blood flow rate dropped 7% on high fat treatments, preventing an increase in uptakes of critical AA to accompany the improved efficiency of milk synthesis, which was evident from a significantly reduced ratio of mammary blood flow to milk volume, resulting in depressed milk protein content.

Calcium, phosphate and citrate in human milk at initiation of lactation

The onset of copious milk secretion (lactogenesis II) in women occurs between 1 and 3 d after birth, and during this period the composition of breast milk changes. During the first 5 d of lactation we measured the concentrations of total, diffusible and ionized Ca (Catot, Cad, Ca2+), diffusible phosphate (Pid), diffusible citrate (Citd) and lactose in the breast milk. On day 1 after birth the concentrations (mean +/- SEM) were Catot, 5.71 +/- 0.30 mM; Cad, 2.66 +/- 0.19 mM; Ca2+, 2.90 +/- 0.18 mM; Pid, 0.26 +/- 0.16 mM; Citd, 0.25 +/- 0.03 mM and lactose, 76 +/- 11 mM. Between day 1 and day 4 the concentration of Catot increased 1.7-fold to 9.56 +/- 0.39 mM, Cad increased 1.8-fold to 4.75 +/- 0.26 mM, Ca2+ decreased by 20% to 2.33 +/- 0.13 mM, Pid increased 6.6-fold to 1.69 +/- 0.11 mM, Citd increased 20-fold to 5.06 +/- 0.21 mM, and lactose increased 2.3-fold to 173 +/- 4 mM. A high correlation has been found between [Cad] and [Citd] in the milk of both ruminant and non-ruminant species, which show a wide range in concentrations of [Cad] and [Citd], and the data fit a simple physicochemical model of ion equilibria in the aqueous phase of milk. The results of the present study confirm the relationship between [Cad] and [Citd] in human milk, even during lactogenesis II when the composition of the milk is changing very rapidly.

Characterization of the transport of tri- and dicarboxylates by pig intestinal brush-border membrane vesicles

1. Transport of citrate and fumarate across the pig intestinal brush-border membrane (BBM) was investigated using isolated BBM vesicles. 2. Citrate and fumarate uptake was stimulated by an inwardly directed Na+ gradient consistent with Na+/citrate (fumarate) co-transport. Cis-inhibition and trans-stimulation experiments strongly suggest the existence of a common transport site for tri- and dicarboxylates. 3. The protonated forms of citrate (citrate-1, citrate-2) seem to be much better transported than citrate-3, indicated by the strong stimulation of citrate uptake at an extravesicular pH of 5.6 compared to pH 7.8. 4. Uptake of tri- and dicarboxylates seems to be potential-sensitive since citrate and in particular fumarate transport was enhanced by an inside negative potential difference. 5. Kinetics of succinate transport revealed a single carrier-mediated component with apparent kinetic constants of 0.43 nmol/mg protein-3 s (Vmax) and 0.14 mmol/l (Km).

Effect of ACTH and oxytocin treatment on lactoferrin and citrate in cows' milk

Six dairy cows were treated before milkings with either oxytocin (Pitocin, 20 i.u.) or ACTH (Synacthen, 150 i.u.), principally to determine their effect on the ratio of citrate: lactoferrin concentrations in the milk. With ACTH treatment, after 3 d milk yield and citrate concentration decreased significantly, lactoferrin and bovine serum albumin (BSA) concentrations increased significantly. Somatic cell counts (SCC) increased temporarily in the milk of three of the cows which previously had greater than 100 000 cells/ml. Lactoferrin yield remained fairly constant but citrate yield was significantly reduced. The citrate: lactoferrin molar ratio decreased from 1373 to 606. With oxytocin treatment, after 4 d milk yield first increased and then significantly decreased, citrate concentration decreased significantly while there were no significant changes in lactoferrin or BSA concentration or in the yield of any other milk constituents. The citrate: lactoferrin molar ratio decreased from 1621 to 1301. There were no significant changes in SCC either during treatment or 4 d after treatment but there was a significant rise at 16 d after treatment. It was concluded that in lactating cows both hormones affected citrate and lactoferrin concentrations in the direction that would improve the antibacterial properties of milk, but that this was accompanied by adverse effects on milk secretion. The extent of the change was not sufficient to be likely to produce inhibition of coliform bacteria.

Feed-back control of milk secretion in the goat by a chemical in milk

In order to investigate the nature of the inhibition of milk secretion during a long milking interval, goats were treated in three possible ways: (i) milked twice daily at 08.00 h and 16.00 h or (ii) milked thrice daily at 00.00 h, 08.00 h and 16.00 h, or (iii) milked thrice daily at 00.00 h, 08.00 h and 16.00 h, but at 00.00 h the milk removed was replaced with an equal volume of isosmotic sucrose solution. The latter treatment was carried out in order to subject the gland to a degree of physical distension equivalent to that on treatment (i). On either thrice-daily milking or thrice-daily milking with sucrose replacement, milk secretion rate over the 16.00-08.00 h period was significantly higher (by about 10% in both cases) than on twice-daily milking. Secretion rates of lactose, milk protein, citrate and calcium during the 00.00-08.00 h period were similar on either thrice-daily milking or thrice-daily milking with sucrose replacement; the secretion rate of fat was significantly higher on thrice-daily milking with sucrose replacement. Secretion rates of Na+, K+ and Cl- were significantly higher on thrice-daily milking with sucrose replacement. In the case of Na+, the increased Na+ secretion rate was sufficient to create a normal Na+ concentration in the milk/sucrose mixture removed at the next milking. In the cases of K+ and Cl-, their secretion rates were not sufficient to restore their concentrations to normal by the next milking.(ABSTRACT TRUNCATED AT 250 WORDS)

Vitamin D-deficient rats produce reduced quantities of a nutritionally adequate milk

Pups raised by lactating rats deficient in vitamin D do not grow normally, and we have shown previously that this growth failure results from a maternal rather than a neonatal defect. This result indicates that vitamin D-deficient rats produce reduced amounts of milk and/or the milk they do produce is nutritionally incomplete. To examine the first of these possibilities, 3H2O and 42K in separate experiments were given to lactating rats at 13 days postpartum, and the amount of isotope transferred to their pups during the next 24 h was determined. The amount of milk produced during this period was calculated from the measured concentrations of 3H2O and 42K in milk. The specific activity of 3H2O in maternal plasma was kept constant by providing 3H2O in the drinking water of the dams. Vitamin D-deficient rats were found to produce only 19.5 +/- 1.8% determined by the 3H2O method and 23.2 +/- 1.1% by the 42K method (means +/- SD) of the amount of milk produced by vitamin D-replete rats. The composition of milk from vitamin D-deficient rats was examined to determine its nutritional value. Vitamin D-deficient milk contains elevated levels of fat, and the skim fraction contains more protein, potassium, calcium, and inorganic phosphorus but less carbohydrate than normal milk. When vitamin D-deficient dams were given two pups to nurse rather than the eight usually provided, pup growth was equivalent to that of vitamin D-replete pups. Furthermore, femur dry weights and degree of mineralization were similar in vitamin D-deficient and -replete pups of equivalent body weight.(ABSTRACT TRUNCATED AT 250 WORDS)

Propionate for fatty acid synthesis by the mammary gland of the lactating goat

Isolated mammary glands of lactating goats were perfused with heparinized and oxygenated blood for 8 to 15 h. Adequate quantities of glucose, acetate, and amino acid were added to the perfusate. After addition of propionate to the perfusion blood, concentrations of odd-numbered and of monomethyl-substituted fatty acids other than those with iso and anteiso configuration increased in the milk fat. These acids seem to be synthesized de novo in the mammary gland. The increase of C17:0 concentration was weak and problematic. We suggest that propionate is acting as a precursor for monomethyl-substituted fatty acids by way of methylmalonyl-CoA. The activating effect of propionate administration upon milk fatty acid production was largest for odd-numbered followed by monomethyl-substituted fatty acids. No increase of iso acids was observed in milk fat in the propionate-infused glands whereas the increase of anteiso acids was extremely small. This agrees with the conception that iso and anteiso fatty acids are synthesized by rumen bacteria.

Swelling of Golgi vesicles in mammary secretory cells and its relation to the yield and quantitative composition of milk

A quantitative description is given of some cellular secretory mechanisms in the mammary epithelium, that can be used in the interpretation of changes in milk yield and composition. The volume of the aqueous phase is considered to depend on a flux of vesicles derived from the Golgi apparatus and on the degree of vesicle swelling prior to exocytosis. Milk proteins secreted by the Golgi route are diluted during vesicle swelling and secreted fat globules are diluted to their final, milk, concentration in the alveolar lumen. Swelling of Golgi vesicles is considered to occur through an osmotic flow of water induced by the intravesicular synthesis of lactose and by the net transport of ions coupled to lactose synthesis. A hypothetical model is given of ion transport mechanisms across the Golgi vesicle membrane to illustrate how the coupling of lactose synthesis to salt transport might occur.

Citrate accumulation by a Golgi apparatus-rich fraction from lactating bovine mammary gland

1. Golgi apparatus-rich fractions from lactating bovine mammary gland rapidly accumulated citrate from incubation medium. Characteristics of this process suggested that a citrate transport system may be present in Golgi apparatus membranes. 2. Endoplasmic reticulum fractions accumulated citrate at nearly the same rate as Golgi apparatus; secretory vesicle fractions displayed lower ability to accumulate citrate. Intact epithelial cells (acini) from lactating mammary gland did not accumulate citrate. 3. Citrate accumulation by Golgi apparatus was pH and temperature sensitive but was not altered by metabolic inhibitors. 4. These observations suggest a role for Golgi apparatus in packaging intracellular citrate for secretion into milk.

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.

Milk yield and secretion composition following intramammary infusion of colchicine

Twenty-five lactating dairy cows were in a split block design to investigate effects of intramammary colchicine infusion on milk yield and composition. Right udder halves of 15 cows were infused with colchicine while left udder halves received no infusion. Ten cows received no infusion in either right or left udder halves. At 24 h postintramammary colchicine infusion, milk yield decreased 38.5% in treated halves and 9.3% in control halves. Udder halves of uninfused cows had an 8.7% reduction. Milk from treated halves compared to untreated halves had elevated somatic cells, serum albumin, and pH. Citrate and the molar ratio of citrate to lactoferrin were lower. Lactoferrin and immunoglobulin G did not differ between infused and uninfused halves; however, they increased in treated halves postinfusion as compared to preinfusion. Serum albumin and citrate increased, and the index of selective accumulation of immunoglobulin G decreased in left halves. Control cows did not differ.

Composition, stability and electrolyte permeability of Golgi membranes from lactating-rat mammary gland

1. Golgi membrane vesicles, isolated from lactating-rat mammary gland and greatly enriched in galactosyltransferase (EC 2.4.1.22), contained over 40 separate bands of protein, including some periodic acid)(Schiff-staining material and free thiol groups, when analysed by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. 2. The membrane lipids were enriched in phosphatidylcholine, phosphatidylethanolamine and unesterified cholesterol. 3. Membrane fluidity, as monitored by the fluorescence polarization of 1,6-diphenylhexa-1,3,5-triene, increased linearly over 5-37 degrees C. 4. The vesicle membranes were impermeable to lactose over a wide pH range, but admitted electrolytes of molecular weight below about 300. 5. These properties are discussed with respect to other cellular membranes and the secretion of milk products.

Transfer of selenium from blood to milk in goats and noninterference of copper with selenium metabolism

The mechanism of selenium secretion by the mammary gland and effects of dietary copper and selenium metabolism on selenium in milk were investigated. Radioactive sodium selective selenite was injected into the jugular vein of lactating goats fed concentrates containing 15 or 115 ppm copper. Blood and milk samples were collected hourly for 8 h and daily for 1 wk. Whole blood, plasma, whole milk, and casein, whey, and cream fractions of milk were counted for selenium-75. No significant differences due to dietary copper were seen. The selenium-75 was primarily associated with the casein in milk. The association of selenium with the whey fraction was greater for early periods than later and varied between animals. The specific activity based on protein content was greater in casein than in whey. Peak selenium-75 in whole milk occurred 2 h after the peak in plasma. On day 7 following selenium-75 dosing, kidney had the highest specific activity, and liver was slightly higher than mammary tissue. All subcellular organelle fractions of liver, kidney, and mammary tissue by homogenization and differential centrifugation contained significant selenium-75. Selenium normally is secreted from the mammary secretory cell in combination with protein through the secretory vesicles, metabolism of selenite prior to milk secretion is important, and copper does not interfere with normal metabolism of selenium.

Energy-dependent calcium sequestration activity in a Golgi apparatus fraction derived from lactating rat mammary glands

A fraction isolated from lactating rat mammary glands was shown by marker enzyme assays to be rich in Golgi apparatus vesicles. This Golgi apparatus-rich fraction was shown to accumulate calcium in the presence of ATP but not in its absence. Other nucleoside triphosphates were only partially effective in promoting calcium transport. Mg2+ was required for the uptake which was also temperature and pH dependent. The uptake was sustained by the use of oxalate and phosphate as intravesicular trapping agents. In the presence of 10 mM oxalate the apparent Km for calcium uptake was 0.24 muM ionized calcium. The V was 4.45 nmol calcium/min per mg protein. Preloaded calcium could be rapidly released by the addition of the ionophore A23187 indicating an intravesicular location for the sequestered ion. Addition of ethylene glycol bis(beta-aminoethylether)-N,N'-tetraacetic acid resulted in a slower release of preaccumulated calcium, indicating the existence of one or more efflux routes by which calcium leaves the vesicles in the presence of MgATP. Ruthenium red partially inhibited the uptake but lanthanum and particularly the sulphydryl inhibitor p-hydroxy-mercuribenzoate were much more effective. The properties of the calcium-sequestering system in the Golgi apparatus-rich fraction were similar to those reported for other non-muscular tissues and lend support to the hypothesis that calcium is secreted into milk via the Golgi apparatus of the mammary gland secretory gland.

Effect of atropine on plasma amino acid levels and milk secretion of cows

Subcutaneous injections of 30 mg atropine into lactating cows induced a 20--40% decrease of free amino acid (AA) levels in arterial plasma. Minimum levels were observed after 30--50 min. The decline persisted for more than 6 h. The greatest fall in concentration was noted for tyrosine, methionine, lysine, arginine, phenylalanine and threonine. Arterial glucose levels remained unaffected. The effect of atropine on milk secretion was studied in 2 cows which were milked every hour with the aid of oxytocin. Maximal effects were observed after 3--4 h. They included reduction in concentration of casein and alpha-lactalbumin (alpha-la) and a decline in production of milk (20%), casein (35%), alpha-la (45%) and lactose (18%). Uptake by the lactating udder over a period of about 1 h after injection of atropine was studied in 2 cows. Mammary blood flow and glucose uptake remained unaffected. There was a positive correlation between arteriovenous differences of essential AA and arterial plasma concentrations. The uptake of essential AA decreased by approximately 50%. There was no evidence that atropine has a direct inhibiting effect on the udder. It is suggested that the decrease of alpha-la synthesis might induce an inhibition of lactose synthesis and milk production.