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

Biochemical properties of sheep colostrum and its potential benefits for lamb survival: a review

Colostrum is the initial secretion of the mammary glands following parturition, which offers main food, protection, and biological active substances for the new born. The most threatening episode of neonate's life is the initial two weeks after birth. This period is associated with high neonatal mortality and morbidity. These worthwhile losses lead to a poor prolificacy rate, low profitability, and ultimately poor performance in animal production. Hence, both diseases and mortality cause valuable losses in terms of production and economic losses. The survival of neonate is correlated with their immune status and passive immune transfer (PIT). Colostrum provides the primary source of nutrition and immunity (PIT) that protects neonates against infections. It must be given as soon as possible after birth since its immunoglobulins are absorbed within the first 16-27 hours after birth, ideally within 2-4 hours. As a result, immunoglobulin (PIT) is the most important component of distressing infectious immunity, and a passable concentration of immunoglobulin in the blood of newborn lambs is linked to their health and survival rate. In this review, we summarized the importance of colostrum in early life and its association with neonatal lamb's survival, profitability and productivity of sheep farming.

Evaluation of physiological and morphological parameters for early prediction of prenatal litter size in goats

The aim of the present study was to evaluate the physiological and morphological parameters of pregnant does for early prediction of prenatal litter size. In total, 33 does were screened using ultrasonography and further categorized into three groups based on does bearing twins (n = 12), a single fetus (n = 12), or non-pregnant does (n = 9). The rectal temperature °F (RT) and respiration rate (RR) as physiological parameters, while abdominal girth in cm (AG) and udder circumference in cm (UC) as morphological parameters were recorded at different gestation times, i.e. 118, 125, 132 and 140 days. In addition to this, age (years) and weight at service (kg) were also used. The statistical analyses included analysis of variance (ANOVA) and linear discriminant analysis (LDA). The results indicated that groups had significant (P < 0.05) differences among morphological parameters at each gestation time, with higher AG and UC in does bearing twins followed by a single fetus and non-pregnant does. However, both physiological parameters were non-significantly (P > 0.05) associated with litter size groups. It was also revealed that the studied parameters showed increasing trends over gestation time in single and twin fetus categories, but they were on par among non-pregnant does. The results of the LDA revealed that estimated function based on age, weight at service, RR, RT, AG and UC had greater (ranging from 75.00 to 91.70%) accuracy, sensitivity and specificity at different gestation times. It was concluded that using an estimated function, future pregnant does may be identified in advance for single or twin litter size, with greater accuracy.

Endocrine changes during the peripartal period related to colostrogenesis in mammalian species

This review discusses endocrine and functional changes during the transition from late gestation to lactation that are related to the production of colostrum in different mammalian species. Species covered in this article include ungulate species (cattle, sheep, goats, pigs, horses), rodents (rat, mouse), rabbits, and carnivores (cats, dogs), as well as humans. An immediate availability of high quality colostrum for the newborn after birth is crucial in species where a transfer of immunoglobulins (Ig) does not or only partially occur via the placenta during pregnancy. Declining activity of gestagens, in most species progesterone (P4), is crucial at the end of pregnancy to allow for the characteristic endocrine changes to initiate parturition and lactation, but the endocrine regulation of colostrogenesis is negligible. Both, the functional pathways and the timing of gestagen withdrawal differ considerably among mammalian species. In species with a sustaining corpus luteum throughout the entire pregnancy (cattle, goat, pig, cat, dog, rabbit, mouse, and rat), a prostaglandin F2α (PGF2α)-induced luteolysis shortly before parturition is assumed to be the key event to initiate parturition as well as lactogenesis. In species where the gestagen production is taken over by the placenta during the course of gestation (e.g., sheep, horse, and human), the reduction of gestagen activity is more complex, as PGF2α does not affect placental gestagen production. In sheep the steroid hormone synthesis is directed away from P4 towards estradiol-17β (E2) to achieve a low gestagen activity at high E2 concentrations. In humans the uterus becomes insensitive to P4, as parturition occurs despite still high P4 concentrations. However, lactogenesis is not completed as long as P4 concentration is high. Early colostrum and thus Ig intake for immune protection is not needed for the human newborn which allows a delayed onset of copious milk secretion for days until the placenta expulsion causes the P4 drop. Like humans, horses do not need low gestagen concentrations for successful parturition. However, newborn foals need immediate immune protection through Ig intake with colostrum. This requires the start of lactogenesis before parturition which is not fully clarified. The knowledge of the endocrine changes and related pathways to control the key events integrating the processes of colostrogenesis, parturition, and start of lactation are incomplete in many species.

Effect of experimental stress and cortisol release induced by ACTH administration on expression of key genes related to milk synthesis and apoptosis during mammary involution of Saanen goats

This research paper addresses the hypothesis that stress, induced by ACTH administration and cortisol release increases somatic cell count (SCC) in mammary secretion, and improves the effectiveness of dry off in goats. We report indicators of milk synthesis and mammary gland involution during dry off. Thirty Saanen goats were subjected to abrupt dry off and treatments: (1) ACTH administration (ACTH) or (2) placebo (Control) on days 1, 3, 6, 9, 12, 15, 30, and 60 of dry off. The expression of target genes in mammary tissue that are related to milk synthesis and cell survival such as insulin-like growth factor 1 receptor (IGF1R), phosphatidylinositol-3-kinase (PIK3CA), protein kinase B (AKT1) and mechanistic target of rapamycin (MTOR), casein (CSN2), lactalbumin (LALBA) and lactoferrin (LF) were evaluated, and plasma cortisol concentration, SCC, leucocyte count, and microbiological analyses in milk and mammary secretions were assessed. ACTH significantly downregulated the expression of IGF1R and upregulated the expression of PIK3CA in mammary tissue, increased lactoferrin concentration and SCC, and changed immune cell levels in mammary secretions compared to Control. Furthermore, ACTH administration increased the percentage of dry goats compared to the Control (73 vs. 46%, respectively). We conclude that the effect of stress via ACTH administration and cortisol release accelerated mammary involution during the early dry-off period.

Effect of methanolic crude extract of Aframomum melegueta (A.m) seeds on selected lactogenic hormones of Albino rats

The search for alternative control of excessive milk secretion associated with exclusive breast feeding necessitated this study. This study was designed to investigate the effect of methanolic crude extract of Aframomum melegueta (A.m) seed by measuring serum concentrations of selected lactogenic (prolactin, estradiol and progesterone) hormones in lactating and non-lactating Albino rats. A total of 18 non-lactating and lactating rats each were assigned into three (3) treatments of 6 rats each. Treatment 1 and 2 (controls) received 0.11 mg/kg/day of reference drug (bromocriptine) and 100 mg/kg/day of Dimethylsulfoxide (Dmso) 1 ml:9 ml of normal saline which is referred to as normal control in this study. Treatment 3 received 100 mg/kg/day of A.m methanolic crude extract. Serum samples were collected and analyzed for prolactin, estradiol and progesterone by ELISA techniques. The result obtained showed a 21.13% and 25.12% significant (P<0.05) reduction in serum concentration of prolactin for both non-lactating (26.15±0.15 ng/ml) and lactating (14.90±1.16 ng/ml) rats relative to (33.23±1.82 and 19.90±1.16 ng/ml) normal control rats but a significant (P<0.05) increase (26.15±0.15 ng/ml; 14.90±1.16 ng/ml) in serum levels of the same hormone relative to (12.56±0.89 ng/ml; 10.56±0.29 ng/ml) reference control drug (bromocriptine) in both groups. Serum estradiol levels were significantly (P<0.05) reduced by methanol extract relative to normal control rats in both groups. There was however, no significant (P>0.05) changes in serum levels of estradiol relative to reference control drugs in both rats. Serum levels of progesterone were not significantly altered in both groups relative to normal control rats. The above findings confer on the extract antilactogenic capability and hence a good alternative to bromocriptine.

Reciprocity in the developmental regulation of aquaporins 1, 3 and 5 during pregnancy and lactation in the rat

Milk secretion involves significant flux of water, driven largely by synthesis of lactose within the Golgi apparatus. It has not been determined whether this flux is simply a passive consequence of the osmotic potential between cytosol and Golgi, or whether it involves regulated flow. Aquaporins (AQPs) are membrane water channels that regulate water flux. AQP1, AQP3 and AQP5 have previously been detected in mammary tissue, but evidence of developmental regulation (altered expression according to the developmental and physiological state of the mammary gland) is lacking and their cellular/subcellular location is not well understood. In this paper we present evidence of developmental regulation of all three of these AQPs. Further, there was evidence of reciprocity since expression of the rather abundant AQP3 and less abundant AQP1 increased significantly from pregnancy into lactation, whereas expression of the least abundant AQP5 decreased. It would be tempting to suggest that AQP3 and AQP1 are involved in the secretion of water into milk. Paradoxically, however, it was AQP5 that demonstrated most evidence of expression located at the apical (secretory) membrane. The possibility is discussed that AQP5 is synthesized during pregnancy as a stable protein that functions to regulate water secretion during lactation. AQP3 was identified primarily at the basal and lateral membranes of the secretory cells, suggesting a possible involvement in regulated uptake of water and glycerol. AQP1 was identified primarily at the capillary and secretory cell cytoplasmic level and may again be more concerned with uptake and hence milk synthesis, rather than secretion. The fact that expression was developmentally regulated supports, but does not prove, a regulatory involvement of AQPs in water flux through the milk secretory cell.

The role of tight junctions in mammary gland function

Tight junctions (TJ) are cellular structures that facilitate cell-cell communication and are important in maintaining the three-dimensional structure of epithelia. It is only during the last two decades that the molecular make-up of TJ is becoming unravelled, with two major transmembrane-spanning structural protein families, called occludin and claudins, being the true constituents of the TJ. These TJ proteins are linked via specific scaffolding proteins to the cell's cytoskeleton. In the mammary gland TJ between adjacent secretory epithelial cells are formed during lactogenesis and are instrumental in establishing and maintaining milk synthesis and secretion, whereas TJ integrity is compromised during mammary involution and also as result of mastitis and periods of mammary inflamation (including mastitis). They prevent the paracellular transport of ions and small molecules between the blood and milk compartments. Formation of intact TJ at the start of lactation is important for the establishment of the lactation. Conversely, loss of TJ integrity has been linked to reduced milk secretion and mammary function and increased paracellular transport of blood components into the milk and vice versa. In addition to acting as a paracellular barrier, the TJ is increasingly linked to playing an active role in intracellular signalling. This review focusses on the role of TJ in mammary function of the normal, non-malignant mammary gland, predominantly in ruminants, the major dairy producing species.

Characterization of malignant breast tissue of breast cancer patients and the normal breast tissue of healthy lactating women volunteers using diffusion MRI and in vivo 1H MR spectroscopy

PURPOSE

To investigate the potential of diffusion weighted imaging (DWI) and in vivo proton MR spectroscopy (MRS) in the differentiation of breast tissue of healthy lactating women volunteers and breast cancer patients.

MATERIALS AND METHODS

DWI and MRS were carried out at 1.5 Tesla on 12 breast cancer patients and 12 normal lactating women volunteers. Apparent diffusion coefficient (ADC) and total choline (tCho) concentration were determined.

RESULTS

tCho was observed in all breast cancer patients and in 10/12 lactating women. Additionally a peak at 3.8 ppm corresponding to lactose was seen in 10/12 of lactating women. Concentration of tCho was similar in malignant breast tissue of patients (3.51 ± 1.72 mmol/kg) and in normal breast tissue of lactating women (3.52 ± 1.70 mmol/kg). However, ADC was significantly higher in the normal breast tissue of lactating women (1.62 ± 0.22 × 10(-3) mm(2)/s) compared with the malignant breast tissue of patients (1.01 ± 0.10 × 10(-3) mm(2)/s).

CONCLUSION

Observation of lactose peak with higher ADC in the breast tissue of healthy lactating women volunteers may aid in differentiation of changes that occur in breast tissue due to normal physiological conditions like lactation compared with malignant transformation.

Serotonin: a local regulator in the mammary gland epithelium

Serotonin (5-hydroxytryptamine, 5-HT) is a very simple molecule that plays key roles in complex communication mechanisms within the animal body. In the mammary glands, serotonin biosynthesis and secretion are induced in response to dilation of the alveolar spaces. Since its discovery several years ago, mammary 5-HT has been demonstrated to perform two homeostatic functions. First, serotonin regulates lactation and initiates the transition into the earliest phases of involution. Second, serotonin is a local signal that induces parathyroid hormone-related peptide (PTHrP), which allows the mammary gland to drive the mobilization of calcium from the skeleton. These processes use different receptor types, 5-HT7 and 5-HT2, respectively. In this review, we provide synthetic perspectives on the fundamental processes of lactation homeostasis and the adaptation of calcium homeostasis for lactation. We analyze the role of the intrinsic serotonin system in the physiological regulation of the mammary glands. We also consider the importance of the mammary serotonin system in pathologies and therapies associated with lactation and breast cancer.

Protein composition and variation of caprine colostrum (Murciano-Granadina breed) by means of polyacrylamide-sds gel electrophoresis

The variation in the content of total protein, total casein, whey protein, α-casein, β-casein, k-casein, α-lactalbumin, β-lactoglobulin and ‘other’ whey proteins of the mammary secretion was studied in 44 goats of the Murciano-Granadina breed (Spain) from the 1st to the 4th day after parturition.

The concentration of all the protein variables showed a marked decrease from the 1st to the 4th day after parturition (P < 0·001). In whey proteins this decrease was more marked from the 1st (67·1 g/1) to the 2nd day after parturition (21·6 g/1), whereas the total caseins sharply decreased from the 2nd (62·6 g/1) to the 3rd day after parturition (39·0 g/1).

Stepwise discriminant analysis revealed that, except for the k-casein, all the proteins resulting from the electrophoretic fractionation had a discriminant power between the dates of sampling (P < 0·001). The multivariant analysis did not show statistical differences in the electrophoretic fractions corresponding to the samples on the 3rd and 4th days after parturition. According to such fractionation, the transition from colostrum to milk should take place on the 2nd day after parturition.

Initiation of human lactation: secretory differentiation and secretory activation

Theories for the origin of milk have been recorded since the time of Ancient Greeks. In those times it was believed that milk was derived from special vessels that connected the uterus to the breasts. The "chyle theory" on the origin of milk was another prominent theory which persisted well into the nineteenth century before the realisation that milk components were derived from blood and some milk constituents were actually synthesized within the breasts. The demonstration that milk ejection was the expulsion of milk that had already been secreted and that milk secretion was a separate continuous process, set the background for the development for the current understanding of milk synthesis and secretion. Today we know that there are two stages in the initiation of lactation- secretory differentiation and secretory activation. Secretory differentiation represents the stage of pregnancy when the mammary epithelial cells differentiate into lactocytes with the capacity to synthesize unique milk constituents such as lactose. This process requires the presence of a 'lactogenic hormone complex' of the reproductive hormones, estrogen, progesterone, prolactin and some metabolic hormones. Secretory activation on the other hand, is the initiation of copious milk secretion and is associated with major changes in the concentrations of many milk constituents. The withdrawal of progesterone triggers the onset of secretory activation but prolactin, insulin and cortisol must also be present. This review describes the works of pioneers that have led to our current understanding of the biochemical and endocrinological processes involved in the initiation of human lactation.

Evolution of Infant and Young Child Feeding: Implications for Contemporary Public Health

Evolutionary anthropological and ethnographic studies are used to develop a general conceptual framework for understanding prehistoric, historic, and contemporary variation in human lactation and complementary feeding patterns. Comparison of similarities and differences in human and nonhuman primate lactation biology suggests humans have evolved an unusually flexible strategy for feeding young. Several lines of indirect evidence are consistent with a hypothesis that complementary feeding evolved as a facultative strategy that provided a unique adaptation for resolving tradeoffs between maternal costs of lactation and risk of poor infant outcomes. This evolved flexibility may have been adaptive in the environments in which humans evolved, but it creates potential for mismatch between optimal and actual feeding practices in many contemporary populations.

Estrogen decrease in tight junctional resistance involves matrix-metalloproteinase-7-mediated remodeling of occludin

Estrogen modulates tight junctional resistance through estrogen receptor-alpha-mediated remodeling of occludin. The objective of the study was to understand the mechanisms involved. Experiments using human normal vaginal-cervical epithelial cells showed that human normal vaginal-cervical epithelial cells secrete constitutively matrix-metalloproteinase-7 (MMP-7) into the luminal solution and that MMP-7 is necessary and sufficient to produce estrogen decrease of tight junctional resistance and remodeling of occludin. Treatment with estrogen stimulated activation of the pro-MMP-7 intracellularly and augmented secretion of the activated MMP-7 form. Steady-state levels of MMP-7 mRNA and protein were not affected by estrogen. Estrogen modulated phosphorylation of the MMP-7, but the changes were most likely secondary to changes in cellular MMP-7 mass. Estrogen increased coimmunoreactivity of MMP-7 with the Golgi protein GPP130. Tunicamycin and brefeldin-A had no effect on cellular MMP-7 but monensin (inhibitor of Golgi traffic) blocked estrogen effects, suggesting estrogen site of action is at the Golgi system. Estrogen increased generalized secretory activity, including of luminal exocytosis of polycarbohydrates. However, estrogen increased coimmunoreactivity of MMP-7 with synaptosomal-associated protein of 25 kDa in apical membranes, suggesting soluble N-ethylmaleimide sensitive fusion factor attachment protein receptor-facilitated exocytosis of MMP-7. Treatment with the vesicular-ATPase inhibitor bafilomycin A(1) inhibited activation of MMP-7. These data suggest that estrogen up-regulates activation of the MMP-7 intracellularly, at the level of Golgi, and augments secretion of activated MMP-7 through soluble N-ethylmaleimide sensitive fusion factor attachment protein receptor-dependent exocytosis. On the other hand, estrogen acidification of the luminal solution would tend to alkalinize exocytotic vesicles and may lead to decreased activation of the MMP-7. These mechanisms acting in concert could be important for regulation and control of estrogen modulation of paracellular permeability in vivo.

Regulation of Milk Lipid Formation and Secretion in the Mouse Mammary Gland

Cytosolic lipid droplets (CLDs), the immediate precursors of milk lipids in lactating animals, undergo cell-specific changes in their formation and intracellular distribution during mammary gland differentiation. Cell biological studies indicate that CLD formation in mammary epithelial cells is regulated in part by AKT-dependent increases in glucose uptake. Proteomic studies show that CLDs from lactating mammary epithelial cells possess a distinct protein composition enriched in molecules involved in their secretion and intracellular transport. CLD secretion is dependent on lactation and requires the purine catabolic enzyme xanthine oxidoreductase (XOR). Confocal immunofluorescence microscopy of XOR in lactating and nonlactating mammary glands and biochemical analysis of secreted CLDs link the secretion process to the formation of a stable tripartite complex between XOR, adipophilin (ADPH), and butyrophilin (Btn). Together these studies provide a molecular and cellular framework for understanding the process of milk lipid formation.

Functional regulation of xanthine oxidoreductase expression and localization in the mouse mammary gland: evidence of a role in lipid secretion

Xanthine oxidoreductase (XOR), a key enzyme of purine metabolism, has been implicated in the secretion of the milk fat droplet in lactating mammary epithelial cells, possibly through structural interactions with other milk fat globule proteins including butyrophilin (Btn) and adipophilin (ADPH). To help determine the mechanism by which XOR is regulated, we examined the expression and localization of XOR in the non-secretory states of late pregnancy and induced involution compared with the state of active secretion. XOR mRNA levels started to increase at mid-pregnancy, turned sharply upwards at the onset of lactation and decreased rapidly with forced involution, indicating transcriptional control of the enzyme level by differentiation and secretory function. During pregnancy and involution the enzyme was diffusely distributed in the cytoplasm, but moved rapidly to the apical membrane of the cells when secretion was activated, where it colocalized with both Btn and ADPH, similar to the situation in the milk fat globule itself. Size-exclusion chromatography of solubilized milk fat globule membrane proteins showed that XOR formed a sulphydryl-bond-dependent complex with Btn and ADPH in the milk fat globule membrane. XOR returned to a diffuse cytoplasmic localization shortly after induced involution, while Btn remained localized to the apical membrane, suggesting that localization of XOR is not dependent on the presence of Btn in the apical membrane. Our findings indicate that the expression and membrane association of XOR in the mammary gland are tightly regulated by secretory activity, and suggest that the apical membrane association of XOR regulates the coupling of lipid droplets to the apical plasma membrane during milk lipid secretion.

Initiation of lactation in women after preterm delivery

BACKGROUND

Lactogenesis II describes the onset of copious milk secretion, and the success of lactogenesis II has been determined in women by measuring the changes in the composition of mammary secretion in the immediate postpartum period.

AIM AND METHODS

Therefore, the aim of this study was to determine the success of lactogenesis II at day 5 postpartum in women expressing milk for their preterm infants (n = 22) by measuring the lactogenesis II markers (milk citrate, lactose, sodium and total protein) and comparing them with women breastfeeding full-term infants (n = 16).

RESULTS

There were no significant differences between the means (+/- SD) of the lactogenesis II markers for preterm (4.3 +/- 0.7 mM; 147 +/- 10 mM; 12 +/- 6 mM; 14.0 +/- 1.5 g/l, respectively) and term (3.4 +/- 1.4 mM; 126 +/- 17 mM; 30 +/- 13 mM; 15.3 +/- 2.5 g/l, respectively) women. However, variation about the mean was greater in preterm women (coefficient of variation for citrate, 40%; lactose, 14%; sodium, 42%; and total protein, 17%) compared with term women (17%, 7%, 33%, and 10%, respectively). All lactogenesis II markers were within 3 SD from the means for the term women and thus these women were considered to have successfully initiated their lactation. Only 18% of preterm women had all four lactogenesis II markers within 3 SD from the mean for term women. The remaining 82% of preterm women had at least one of the markers of lactogenesis II at pre-initiation concentrations (36% had 1 marker, 32% had 2 markers, and 14% had 3 markers). Furthermore, these women had significantly lower 24-hr milk production than those preterm women that had all four markers within 3 SD from the mean of the term women.

CONCLUSIONS

It was concluded that 82% of preterm women had a compromised initiation of lactation, and this was not uniform in all women.

Prediction of foaling using mammary secretion constituents

AIMS

To test published models for predicting time of foaling using data derived from prepartum mammary secretions and to develop a new model based on concentrations of mammary secretion constituents that accurately predicts time of foaling in Thoroughbred mares.

METHODS

Concentrations of sodium, potassium, calcium, citrate and lactose were measured in prepartum mammary secretions of 20 Thoroughbred mares in the 2 weeks before expected date of foaling. Models to predict time of foaling were fitted to data based on absolute concentration, change in concentration, and percentage change in concentration of mammary secretion constituents in relation to actual intervals to foaling.

RESULTS

Concentrations of potassium, calcium, citrate and lactose increased, and concentration of sodium decreased as foaling approached but variation between mares was large. Models to predict time of foaling based on percentage change in electrolyte concentrations were less accurate than those based on absolute concentration and change in concentration. When data from this study were fitted to two previously published models, the statistical sensitivity, specificity, and positive and negative predictive values of both models were lower than those originally reported.

CONCLUSIONS

The use of prepartum equine mammary secretion electrolyte concentrations for prediction of time of foaling is unreliable, due to large variation in both absolute concentrations and change in concentrations between mares. Models that use a combination of mammary secretion electrolytes and physical and behavioural factors may better predict foaling than those based on mammary secretions alone.

Lactogenesis. The transition from pregnancy to lactation

The most important factors in initiation of the cascade of changes in the mammary epithelium that constitute lactogenesis stage II seem to be a prepared mammary epithelium, progesterone withdrawal, maintained plasma prolactin (in most species), and removal of milk from the breast within an undefined interval after birth. Although the molecular mechanisms by which prolactin regulates milk protein synthesis are the subject of intense and productive studies, the specific mechanisms by which progesterone and milk removal interact with the mammary epithelial cell at parturition have not been studied, perhaps because no in vitro model system exists that mimics lactogenesis stage II, or because of the complexity of the changes that must be coordinated during this process, or because of a lack of general understanding of the complex progression of changes in the function of the breast as it goes from the quiescent state of pregnancy to the active secretory state of lactation. With new technologies designed to investigate the biology of complex systems arising from the growing knowledge of the genome of human and animal species and the growing availability of animal and tissue culture models for these processes, physicians can expect a rapid increase in the molecular understanding of lactogenesis in the near future. These fundamental studies must be coupled with good prospective clinical studies if physicians are to obtain a useful, comprehensive understanding of lactogenesis in women.

Biosynthesis of lactosamine in bovine mammary gland

Lactosamine (beta-D-Galp-(1-->4)-D-GlcN) was isolated from bovine milk sampled after intravenous infusion of glucosamine through the jugular vein of a lactating cow. The chemical structure was established by 2D NMR spectroscopy and electrospray ionisation mass spectrometry (ESIMS). Selected ion monitoring liquid chromatography-mass spectrometry (SIMLC-MS) of the perbenzoylated carbohydrate fraction showed the presence of the novel disaccharide in the milk sample after infusion, but not in the control bovine milk sample. The results showed the uptake of glucosamine in bovine mammary gland, and also indicated that a part of glucosamine was metabolised to the product lactosamine.

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.

Biosynthesis of beta1,4- and beta1,beta1-galactopyranosyl xylopyranosides in the mammary gland of lactating cow

Lactose is a principal carbohydrate in nearly all species of mammalian milk. In order to examine the acceptor substrate specificity of lactose synthase in vivo, D-xylose as an acceptor substrate was injected into the jugular vein of a Holstein cow during lactation, then a milk sample obtained by milking. beta1, beta1-galactopyranosyl xylopyranoside, a nonreducing disaccharide, was separated from the bovine milk sample after elimination of reducing sugars, identified by fast-atom bombardment (FAB)-MS and 1H-NMR analysis. A mixture of beta1,beta1- and beta1, 4-galactopyranosyl xylopyranoside fractions was also obtained by thin layer chromatography from the milk sample and elucidated by electrospray ionization (ESI)-MS and 1H NMR analysis. Comparison of the integrated intensity of the products shows that the beta1,beta1 and beta1,4 isomers are present in a ratio of 1.0 : 1.4, suggesting that D-xylose, transported from capillary blood across the plasma membrane of the mammary gland, was recognized by lactose synthase in its normal and reverse orientation owing to high symmetry of its structure. While the beta1,4-isomer is known as a fragment of the linkage region between the protein and the polysaccharide chain of proteoglycans, the beta1,beta1-isomer has not been identified in vivo. Here, we demonstrate that galactosylation of D-xylose transported from the capillary blood can occur by lactose synthase catalysis in the mammary gland while the usual galactosylation of D-glucose proceeds. In addition, these results suggest that the possibility of endogenous occurrence of the beta,beta-trehalose type disaccharide in the mammary gland of lactating mammals may not be ruled out.

Assessment of mammary gland metabolism in the sow. II. Cellular metabolites in the mammary secretion and plasma during lactogenesis II

The concentrations of lactose, glucose, glucose 6-phosphate, glucose 1-phosphate, UDPglucose, UDPgalactose, UDP, UMP, inorganic phosphate, ADP and AMP (metabolites involved in the lactose synthesis pathway), and cAMP, galactose and fructose were measured in the mammary secretion from sucked (n = 9) and unsucked (n = 4) mammary glands of nine sows during the first 5 d post partum. The concentrations of lactose, glucose, galactose and fructose were also measured in plasma during this time. The progressive increase in the concentration of lactose, and changes in the concentrations of cellular metabolites in the mammary secretion from sucked glands were consistent with an increase in the metabolic activity of those glands during lactogenesis II. In contrast, unsucked glands showed a progressive decrease in the concentration of lactose, while the concentrations of cellular metabolites in the milk generally remained unchanged. These results indicated that there was no increase in the metabolic activity of unsucked glands (no increase in lactose synthesis or utilization of glucose and ATP) and that the rate of lactose synthesis prior to milk removal was limited by the availability of glucose and/or UDPgalactose. Therefore, the removal of colostrum from the mammary gland was necessary for an increase in the rate of lactose synthesis (and probably de novo fatty acid synthesis) and implies that autocrine mechanisms are operating to control the rate of milk synthesis during lactogenesis in the sow. The low concentration of glucose in colostrum compared with that in plasma is discussed in view of the paracellular pathway.

Milk whey proteins in plasma of sows: variation with physiological state

The whey proteins alpha-lactalbumin and beta-lactoglobulin have been investigated as potential markers of mammary development in sows by measuring their concentrations in plasma. The whey proteins were isolated from porcine milk by gel filtration, ion-exchange and hydrophobic interaction chromatography, characterized by several criteria and used to raise antibodies. Specific radioimmunoassays were set up for porcine alpha-lactalbumin and beta-lactoglobulin and validated for use in porcine blood and milk. Plasma levels of the whey proteins were measured in sows that were pregnant, suckling litters post partum, weaned abruptly at birth or were pregnant but mastectomized. Both whey proteins showed similar patterns in plasma post partum, falling from a maximum 1 d after parturition to values < 0.02% those in milk by day 4-5 post partum in suckling sows and showing a transient peak associated with early involution before declining to very low concentrations in non-suckling sows. alpha-Lactalbumin was first detected in the last week prepartum, rising markedly in the 3 d before parturition, correlated with rising prolactin (r = 0.986) and falling progesterone (r = -0.998). beta-Lactoglobulin rose much earlier from 5 weeks prepartum, at the time when lobulo-alveolar mammary development is occurring, and correlated (r = 0.929) with oestradiol-17 beta. In mastectomized sows, concentrations of whey proteins in plasma were reduced by 90% or more when compared with intact animals, though following a similar pattern. This study shows that whey protein concentrations in plasma vary with physiological state and reflect aspects of the development of the mammary gland. The very different profiles for alpha-lactalbumin and beta-lactoglobulin prepartum indicate that they are differently controlled.

Bromocriptine treatment of periparturient goats: long-term suppression of prolactin and lack of effect on lactation

British Saanen dairy goats (n = 10) were treated with bromocriptine or vehicle from day 147 of pregnancy to day 4 post partum, a treatment duration of 8.8 +/- 1.7 d (mean +/- SEM). The periparturient prolactin surge was abolished by this treatment, but there were no significant effects on plasma growth hormone or insulin concentrations. Lactogenesis was delayed in the bromocriptine-treated goats, milk yields being significantly depressed (P < 0.01) for the first week of lactation. Yields had recovered to control values by day 10 when prolactin concentrations were still significantly depressed. Mammary gland biopsies were taken on day 4 post partum from five animals in each group. Using this tissue, no significant differences could be shown in mammary morphology or DNA synthesis, but the RNA:DNA ratio was significantly reduced (P < 0.05). After week 1, there were no significant differences between bromocriptine-treated and control goats in milk yield, milk composition, udder volume, time of peak yield or persistence. The goats given short-term bromocriptine treatment at parturition showed prolonged effects on prolactin secretion, their seasonal prolactin rise being severely blunted (P < 0.001). A normal lactation is therefore not prevented in goats by a delay in lactogenesis, suppression of prolactin at parturition or the resulting prolonged depression of circulating prolactin. Goats in established lactation given bromocriptine for 8 d showed, by contrast, a rapid recovery of plasma prolactin concentrations within 5 d post treatment. Milk yield declined significantly (P < 0.03) compared with pretreatment values during and for 1 week after bromocriptine but then began to recover, with no significant change in vehicle-treated goats.

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-derived growth inhibitor protein and messenger ribonucleic acid concentrations in different physiological states of the gland

Expression of mammary-derived growth inhibitor in tissue from lactating and involuting bovine mammary glands was investigated. Seventeen lactating, pregnant (220 to 272 d in gestation) cows were divided in two groups of 8 and 9 cows each. Cows of the first group were slaughtered while in lactation. Cows of the second group (9 involuting cows) were slaughtered at 13 to 52 d following sudden cessation of milking. High concentrations of mammary-derived growth inhibitor (.63% of the total protein) were detected in mammary tissue of lactating cows. Mammary-derived growth inhibitor (less than .10% of the total protein) was dramatically reduced during most of the involution period (13 to 45 d following cessation of milking). Mammary-derived growth inhibitor was again detected (.28% of the total protein) during the last stage of the involution (46 to 53 d after cessation of milking), which coincided with colostrum formation. When steady state concentrations of mammary-derived growth inhibitor mRNA were examined, the results obtained mirrored those obtained at the protein concentration. These data suggest that regulation of mammary-derived growth inhibitor occurs via modulation of the steady state concentration of its mRNA. Furthermore, there is a strong correlation between mammary-derived growth inhibitor expression and lactation in dairy cows.

Changes in the lipid composition of the secretions of the bovine mammary gland during the dry period

To study initiation of milk fat synthesis, lipid composition of mammary secretions at -60, -40, and -10 d prepartum was studied in lactating and nonlactating Holstein cows. Eleven cows were dried off, and 13 cows were milked twice per day throughout the normal dry period. Total neutral lipid was similar in late lactation milk (-60 d) from lactating cows, 2.1 g/dl, and in milk from the dry group, 2.2 g/dl. Neutral lipids decreased to 1.3 and .9 g/dl in quarters from dry cows at -40 and -10 d prepartum. In secretions from dry quarters, triglycerides were 97% of total lipids at -60 d and decreased to 85 and 91% at -40 and -10 d, respectively. Conversely, FFA and monoglycerides increased during the dry period. Lipids associated with fat globule membrane components increased during the dry period. These increases were 10 times for cholesterol, 20 times for cholesteryl esters, and twice for phospholipids. In general, the content of fat globule core lipids (triglycerides) exhibited a pattern opposite that of membrane lipids (cholesterol, cholesteryl esters, phospholipids) during the prepartum period. Proportions of core lipids tended to decrease, whereas proportions of membrane lipids increased in prepartum mammary secretions. Lipid composition of prepartum secretions may be influenced by blood lipids, somatic cells, and alterations in mammary lipid synthesis.

Serum concentrations of the milk proteins alpha-lactalbumin and beta-lactoglobulin in pregnancy and lactation: correlations with milk and fat yields in dairy cattle

Serum concentrations of alpha-lactalbumin and beta-lactoglobulin in first pregnancy, parturition, lactation, involution, and second parturition in 37 Holstein cattle were determined and used as an index of mammary status and in predicting milk yield. During first pregnancy, serum alpha-lactalbumin increased in the last 3 mo and reached a peak at parturition (approximately 1100 ng/ml). Changes in alpha-lactalbumin could not be described by a simple exponential equation, whereas changes in serum beta-lactoglobulin were described by a single exponential from second trimester until 4 wk prepartum and reached a peak at parturition (approximately 460 ng/ml). By 2 wk after parturition, alpha-lactalbumin had dropped to approximately 140 ng/ml, and beta-lactoglobulin dropped to approximately 25 ng/ml. In late lactation, alpha-lactalbumin was approximately 70 ng/ml and beta-lactoglobulin approximately 20 ng/ml. Short-term elevations were found after cessation of milking in both alpha-lactalbumin and beta-lactoglobulin in serum. The concentrations of alpha-lactalbumin and beta-lactoglobulin at second parturition were similar to those at first parturition with no differences found between parity. Both alpha-lactalbumin and beta-lactoglobulin in serum were functionally associated with mammary growth and development. In heifers late in pregnancy, both serum concentrations of alpha-lactalbumin and beta-lactoglobulin were positively correlated with mature equivalent milk and fat yields in the subsequent lactation. Serum beta-lactoglobulin concentrations at 16 wk prepartum in heifers were highly correlated with the sum of first and second lactation milk (r = .60) and fat (r = .60) yields. The potential value of using serum beta-lactoglobulin as an index for prescreening of heifers for lactation potential is discussed.

Response to selection for milk yield and metabolic challenges in primiparous dairy cows

The effect of selection for milk yield on lactation yield, net energy balance, and on plasma growth hormone, insulin, prolactin, nonesterified fatty acids and glucose was studied in two groups of primiparous Holstein cows of differing genetic merit. Net energy balance was calculated and serial blood samples were collected for a 7 hr period at 0, 45, 90 and 180 days postpartum. Growth hormone releasing factor (.2 microgram/kg BW) was administered after 2.5 hr at 0, 45 and 180 days postpartum, while epinephrine (.7 microgram/kg BW) was administered at 90 days postpartum. Milk yield was greater, net energy balance was decreased and plasma growth hormone was greater in genetically superior selection group cows compared to control cows. Growth hormone showed similar increases in both genetic groups in response to growth hormone releasing factor, while prolactin, insulin and glucose were not altered. Epinephrine stimulated an increase in plasma nonesterified fatty acids, glucose and insulin, but responses did not differ between genetic groups. Results indicate differences exist in production efficiency, net energy balance and plasma growth hormone concentration among dairy cattle as a result of selection for milk yield and suggest that selection pressure may act to alter homeorhetic control of nutrient metabolism.

Approaches to the manipulation of mammary involution

Mammary involution is a gradual process that occurs following cessation of milking. Regression of mammary secretory tissue accompanies dramatic changes in secretion composition during the transition from lactation to involution. Conversely, rapid differentiation of secretory tissue and copious accumulation of colostrum occur as parturition approaches. The duration of the nonlactating period, mammary gland health, and secretory cell response to hormones influence subsequent lactational performance in most species. Manipulation of the bovine mammary gland in an attempt to hasten involution has been studied. The primary objective of these studies was to determine if hastened involution would decrease new intramammary infections during the early nonlactating period. Results of these studies have also led to a more fundamental understanding of events that occur during physiological transition of the mammary gland. Adequate regression, proliferation, and differentiation of mammary secretory epithelium during the nonlactating period of ruminants appear to be essential for maximal milk production during lactation. Factors that interfere with these mechanisms can adversely affect mammary function during the impending lactation. A greater understanding of these processes may provide new approaches for increasing milk production in dairy cattle.

Udder health in the periparturient period

The periparturient period is associated with rapid differentiation of secretory parenchyma, intense mammary growth, copious synthesis and secretion, and marked accumulation of colostrum and milk. Udder health during this time is an important factor associated with the production of maximum quantities of high quality milk. Intramammary infections that occur during the dry period can adversely affect udder health, resulting in decreased milk production, altered milk composition, and impaired mammary function. Bovine mammary glands are markedly susceptible to new infections during the periparturient period, especially prior to parturition. Many infections that occur at this time are associated with clinical mastitis during early lactation. Methods of controlling mastitis in the dry period have focused primarily on the use of antibiotics. However, antibiotic therapy at drying off does not appear to prevent new infections in the periparturient period. This is most likely due to lack of persistence of antibiotics. Furthermore, antibiotics used currently are less effective against environmental pathogens, in particular coliform bacteria, which can cause a high proportion of intramammary infections during the periparturient period. Methods of controlling bovine mastitis during the periparturient period is an important area that requires additional research. Procedures need to be developed that are effective against a variety of bacteria, including environmental mastitis pathogens, if additional control is to be achieved.

ACTH initiation of mammary secretion in pregnant goats is influenced by the stage of gestation and pre partum milking

Pregnant goats were treated with ACTH by intramuscular injection (1 mg/d for 2 d) at various stages of pregnancy to investigate its effect on initiation of mammary secretion. ACTH stimulated lactose secretion and increased udder volume at or after 84 d of gestation, when compared with untreated controls. Treatment with ACTH between d 109 and 127 of gestation increased circulating plasma glucose concentration, mammary blood flow and mammary uptake of glucose which returned to pre injection levels 6 d after treatment. Arterial concentrations of progesterone were not affected by ACTH but mammary progesterone uptake increased 2-fold. Pre partum milking alone stimulated mammary secretion in eight of 13 goats. Subsequent treatment with ACTH resulted in no further change in the composition of the secretion but increase in yield was maintained. ACTH stimulated mammary secretion in those goats which did not respond to pre partum milking. It was concluded that ACTH initiated mammary secretion in pregnant goats by mechanisms which interact with the stimulation of milking but which are independent of circulating progesterone levels.

Alpha-lactalbumin in bovine serum: relationships with udder development and function

Concentrations of alpha-lactalbumin in blood from cattle in various physiological states were measured as an index of udder development and function. Included were primiparous heifers during gestation and the peripartum period, nonlactating, nonpregnant cows hormonally induced into lactation, and cows milked two or three times daily in early, middle, or late lactation. Concentrations of alpha-lactalbumin in serum increased in two phases during gestation. Initial values (7.3 ng/ml, up to 120 d prepartum) rose, then leveled at 29.9 ng/ml on d 120 to 30 prepartum. Concentrations subsequently increased, averaging 133.2 ng/ml over the 30 d prior to parturition. During the peripartum period, alpha-lactalbumin rose from 221.2 ng/ml on d 4 prepartum, peaked at calving (918.8 ng/ml), then declined, stabilizing at approximately 500 ng/ml (1.5 to 3 d postpartum). Concentrations of alpha-lactalbumin in cows induced to lactate were low on d 1 to 9 of hormone treatment (15.7 ng/ml), rose to a maximum on d 17 (803.4 ng/ml), then fell to a plateau (185.8 ng/ml) on d 21 to 25. alpha-Lactalbumin concentrations were higher in early (101.9 ng/ml) than in middle or late lactation (81.4 and 79.2 ng/ml, respectively). Concentrations were also greater in twice versus thrice milked cows (101.9 vs. 73.0 ng/ml). Changes in alpha-lactalbumin concentrations in serum are associated with developmental and functional status of the udder. The measurement provides a noninvasive method to assess mammary gland activity.

Radioimmunoassay for measurement of bovine alpha-lactalbumin in serum, milk and tissue culture media

A sensitive, specific radioimmunoassay for bovine alpha-lactalbumin (alpha-la) suitable for measurement of serum, tissue culture media and milk was developed. alpha-La was not detected in serum from prepubertal male or female cattle, but was detected as early as 60 d of gestation in nulliparous Holstein heifers, the level being greatly increased during the last one third of gestation. Serum from cross bred beef heifers contained less alpha-la and it was not detected until late gestation. Concentrations of alpha-la in serum samples from pregnant multiparous Holstein cows decreased at drying-off and subsequently increased just before parturition. Secretion of alpha-la by mammary tissue explants from steroid-primed prepubertal Holstein heifers was induced by the addition of bovine prolactin, ovine prolactin or human growth hormone to tissue culture media.

Variation among species in the endocrine control of mammary growth and function: the roles of prolactin, growth hormone, and placental lactogen

Prolactin, growth hormone, and placental lactogen form a family of structurally related hormones, which may have evolved from a common ancestral peptide. Prolactin and growth hormone are present in all mammals, but the biological activity associated with placental lactogen has been detected in only some groups. Attempts to detect placental lactogen using bioassay and radioreceptor assay are reported and have been unsuccessful in an insectivore (the shrew), a bat, an edentate (the armadillo), a lagomorph (the rabbit), several carnivores (dog, cat, ferret), perissodactyls (horse, zebra, rhino), and, within the artiodactyls, pigs. Placental lactogenic activity has been detected in primates (chimpanzee, orangutan), rodents (voles, Pinon mouse, guinea-pig, mara), and in numerous artiodactyls (llama, giraffe, several species of deer, antelope, gnu, gazelle, musk ox, cape buffalo, Barbary sheep, several sheep of the genus Ovis, goat, and cow). These results confirm and extend the work of others and are discussed in relation to the evolution of these hormones. In synergism with steroid and thyroid hormones, protein hormones of the prolactin and growth hormone family play a crucial role in stimulating the development of the mammary gland, the differentiation and function of mammary cells to secrete milk, and in the systemic adjustments in maternal metabolism in pregnancy and lactation. Studies in vitro have shown that mammary tissues from several species synthesize milk components in response to insulin plus adrenal corticoid plus prolactin. However, there are also species differences in minimal hormonal requirements for lactogenesis. In vivo, for example, rabbits will initiate or sustain lactation in response to prolactin alone, whereas sheep and goats require prolactin plus growth hormone plus adrenal corticoid plus thyroid hormone. Measurement of hormone concentrations in the plasma of pregnant animals shows considerable differences among species in the pattern of secretion of lactogenic hormones to bring about mammary development. A surge of prolactin secretion occurs at parturition but may not be essential in the initiation of lactation. The timing of progesterone withdrawal correlates well with lactogenesis in eutherian mammals, but species differ in the mechanisms at parturition which bring this about. Marsupials show a quite different pattern of suckling-induced lactation. In maintaining lactation the greatest contrast is between ruminants, in which growth hormone is of particular importance, and other mammals, in which reduction of prolactin secretion with bromocriptine rapidly suppresses milk synthesis and secretion.(ABSTRACT TRUNCATED AT 400 WORDS)

Preliminary studies of mammary secretions in the mare to assess foetal readiness for birth

The status of the mare and foetus in relation to readiness for birth was assessed by measurement of the electrolytes sodium, potassium and calcium in mammary secretions pre-partum. Sixteen Thoroughbred mares were allowed to foal spontaneously and the ionic status of their mammary secretions was measured over three to five weeks pre-partum. From these measurements, a scoring system was developed where an ionic score of 35 points or more suggested that the mare was within 24 h of foaling. On the basis of this ionic score, 10 pony mares were induced with either oxytocin or fluprostenol and assessment of foal maturity was made by physical, behavioural and physiological criteria. Eight pony mares, induced when the ionic score was 35 points or more, delivered full term foals; two mares were induced when their scores were 30 and 20 points and delivered a full term and slightly immature foal respectively. These results suggest that foetal maturity may be related to electrolyte concentrations in mammary secretions and that an ionic score of 35 points or more may indicate that induction would be successful in terms of maturity of the newborn foal.

Caprine mammary differentiation and initiation of lactation following prepartum colchicine infusion

Milk from both colchicine-infused and uninfused udder halves had similar levels of somatic cells, serum albumin, pH, citrate, and lactose throughout the experimental period. Milk citrate and lactose concentrations gradually increased in both colchicine-infused and uninfused udder halves during early lactation while levels of somatic cells and serum albumin decreased as lactation progressed. No differences in parenchymal development or cytological differentiation were observed between colchicine-treated and untreated mammary tissue obtained prepartum, at parturition, or 7 days postpartum. Colchicine-infused udder halves produced about 9% less milk than uninfused controls during the first 30 days of lactation.

Cyclic nucleotides in goats' milk: changes with physiological state

Concentrations of cAMP and cGMP were measured in mammary secretions of goats. [cAMP] and [cGMP] increased during late pregnancy. At the onset of copious milk secretion, [cGMP] decreased while [cAMP] increased further. After the cessation of milking in late lactation, [cGMP] increased and [cAMP] tended to decrease. During withdrawal of food for 48 hr, the rate of milk secretion fell, [cGMP] increased and [cAMP] decreased; the changes were reversed on re-feeding. During temporary inhibition of milk secretion by intramammary treatment with colchicine, [cGMP] increased while [cAMP] decreased. It is concluded that cyclic nucleotide concentrations in milk change with physiological state.

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.

Alterations in sex-steroid hormone receptors during mammary gland differentiation in the rat

Sprague-Dawley and Wistar strains were the best tissue sources for the estrogen receptors of the mammary gland of 7 rat strains examined. Tissue DNA content reached a maximum in late lactation (15-19 days). Cytosolic protein concentrations reached a maximum in late lactation (15-19 days). Both tissue DNA and cytosolic protein concentrations decreased upon weaning. The dissociation constant of the cytosolic estrogen receptor showed little change in pregnancy and lactation. The cytosolic estrogen receptor concentration was maximal at 19 days lactation. The cytosolic protesterone receptor was undetectable in late pregnancy (14-20 days) and in mid and late lactation (10-19 days).