The effects of prolactin and oxytocin on milk secretion and on the permeability of the mammary epithelium in the rabbit

Using domperidone to induce and sustain hyperprolactinemia in late-pregnant gilts

Prolactin controls mammary development as well as the lactogenic and galactopoietic processes in sows and increasing prolactin during gestation can augment milk yield. The dopamine receptor antagonist domperidone can increase circulating prolactin concentrations in pigs, but the ideal dose to achieve sustained hyperprolactinemia remains unknown. An experiment was performed to develop a protocol for using domperidone in studies of rapid and sustained hyperprolactinemia in late-pregnant gilts. On day 90 of gestation, gilts were divided into 4 groups: (1) intramuscular (IM) injections of canola oil (3 mL, controls [CTL], n = 9), (2) IM injections with 0.1 mg/kg BW of domperidone (low [LO], n = 8), (3) IM injections with 0.5 mg/kg BW of domperidone (medium [ME], n = 11), and (4) IM injections with 1.0 mg/kg BW of domperidone (high [HI], n = 11). Injections were given daily at 8:05 from days 90 to 109 of gestation. Treated gilts also received domperidone per os (0.5 mg/kg BW) at 8:00 and 20:00 on days 89, 90, and 91 of gestation. Three jugular blood samples were collected from all gilts at 6-h intervals on days 89, 90, and 91 of gestation, then twice daily on days 92, 93, and 94. Thereafter, samples were obtained at 8:00 every other day until day 114 of gestation. Blood was sampled serially from 9 CTL and 11 HI gilts on days 89 and 94 of gestation. On day 89 of gestation, prolactin concentrations for LO, ME, and HI gilts increased within 6 h of domperidone per os (P < 0.001). From days 89 until 93 of gestation, the area under the curve (AUC) for LO, ME, and HI gilts was greater than that for CTL gilts (P < 0.001), whereas from days 89 until 114, ME and HI gilts had greater AUC than CTL and LO gilts (P < 0.05). Results demonstrate that the combination of per os treatment with IM injections of 0.5 mg/kg of domperidone in an oil emulsion leads to the rapid and sustained release of prolactin over 24 d in late-pregnant gilts. Higher doses of domperidone failed to further increase circulating prolactin levels. These findings provide a useful strategy to induce sustained hyperprolactinemia in late-pregnant gilts.

Oxytocin injections in the postpartal period affect mammary tight junctions in sows

The potential impacts of injecting oxytocin (OXY) to sows in the early postpartum period on the quality of mammary tight junctions, milk composition, and immune status of sows and piglets were studied. Postparturient sows received i.m. injections of either saline (control [CTL]; = 10) or 75 IU of OXY ( = 10). Injections were given twice daily (0800 and 1630 h) starting on d 2 of lactation (i.e., between 12 and 20 h after birth of the last piglet), totaling 4 injections. Milk samples were obtained before the first injection (d 2 morning [AM]), before the second injection (d 2 afternoon [PM]), and on d 4 PM and d 5 PM. Blood samples were obtained from sows before milking on d 2 AM, d 2 PM, and d 5 PM. On d 5 of lactation, a blood sample was obtained from 3 piglets per litter. Circulating concentrations of prolactin, IGF-I, lactose, and IgA in sows did not differ between treatments at any time ( > 0.10), but OXY sows had less IgG than CTL sows ( < 0.01) on d 2 PM before the second OXY injection. There were differences in milk composition on d 2 PM, with OXY sows having more IGF-I ( < 0.01), solids ( < 0.05), protein ( < 0.01), energy ( < 0.05), and IgA ( < 0.01) and a greater Na:K ratio ( < 0.01) than CTL sows. These differences were not seen in the next 2 milk samples, except for protein and IgA that still tended ( < 0.10) to be greater in OXY vs. CTL sows on d 4 PM (for protein) and on d 5 PM (for IgA) after the last injection. Milk lactose content was lower in OXY vs. CTL sows on d 5 PM ( < 0.01). Values for immunoglobulin immunocrit, IgG, IgA, and IGF-I in piglet blood did not differ between treatments ( > 0.10). Injecting OXY to sows in the early postpartum period increased leakiness of the mammary tight junctions, improved composition of early milk, and may potentially affect immune status of neonatal piglets.

Mammary epithelium disruption and mammary epithelial cell exfoliation during milking in dairy cows

The presence of mammary epithelial cells (MEC) in the milk of ruminants indicates that some MEC are shed from the mammary epithelium; however, the mechanisms that regulate the MEC exfoliation process are not known. Through the release of oxytocin, prolactin, and cortisol and through oxytocin-induced mechanical forces on the mammary epithelium, milking could participate in regulating the MEC exfoliation process. The aims of the present study were to determine the rate of MEC exfoliation throughout milking and to investigate its relationship to mammary epithelium integrity and milking-induced hormone release. Milk samples from 9 Holstein dairy cows producing 40.6 ± 1.36 kg of milk/d were collected at the beginning (after 1 and 2 min), in the middle, and at the end of milking. Milk MEC were purified using an immunomagnetic method. Blood samples were collected before, during, and after milking, and the oxytocin, prolactin, and cortisol concentrations in the samples were measured. Tight junction opening was assessed by plasma lactose concentration and the Na⁺:K⁺ ratio in milk. The somatic cell count in milk varied during the course of milking; it decreased at the beginning of milking and then increased, reaching the highest values at the end of milking. Exfoliated MEC were present in all milk samples collected. The presence of MEC in the milk sample collected during min 1 of milking, likely corresponding to the cisternal milk fraction, suggests that MEC were exfoliated between milkings. The observed increase in the Na⁺:K⁺ ratio in milk and in the plasma concentration of lactose indicated that disruption of mammary epithelium integrity occurred during milking. The MEC exfoliation rate at milking was not correlated with the variables describing milking-induced prolactin release but was negatively correlated with cortisol release, suggesting that cortisol may play a role in limiting exfoliation. In conclusion, milking induced a disruption of the mammary epithelial barrier. Mammary epithelial cells may be continuously exfoliated between milkings or exfoliated during milking as a consequence of the oxytocin-induced mechanical forces and the disruption of mammary epithelium integrity.

Inhibition of prolactin with bromocriptine for 28days increases blood-brain barrier permeability in the rat

The blood-brain barrier (BBB) is necessary for the proper function of the brain. Its maintenance is regulated by endogenous factors. Recent evidences suggest prolactin (PRL) regulates the BBB properties in vitro, nevertheless no evidence of these effects have been reported in vivo. The aim of this study was to evaluate the role of PRL in the maintenance of the BBB in the rat. Male Wistar rats were treated with Bromocriptine (Bromo) to inhibit PRL production for 28days in the absence or presence of lipopolysaccharide (LPS). BBB permeability was evaluated through the Evans Blue dye and fluorescein-dextran extravasation as well as through edema formation. The expression of claudin-5, occludin, glial fibrillary acidic protein (GFAP) and the PRL receptor (PRLR) was evaluated through western blot. Bromo reduced the physiological levels of PRL at 28days. At the same time, Bromo increased BBB permeability and edema formation associated with a decrement in claudin-5 and occludin and potentiated the increase in BBB permeability induced by LPS. However, no neuroinflammation was detected, since the expression of GFAP was unchanged, as well as the expression of the PRLR. These data provide the first evidence that inhibition of PRL with Bromo affects the maintenance of the BBB through modulating the expression of tight junction proteins in vivo.

Prolactin and teleost ionocytes: new insights into cellular and molecular targets of prolactin in vertebrate epithelia

The peptide hormone prolactin is a functionally versatile hormone produced by the vertebrate pituitary. Comparative studies over the last six decades have revealed that a conserved function for prolactin across vertebrates is the regulation of ion and water transport in a variety of tissues including those responsible for whole-organism ion homeostasis. In teleost fishes, prolactin was identified as the "freshwater-adapting hormone", promoting ion-conserving and water-secreting processes by acting on the gill, kidney, gut and urinary bladder. In mammals, prolactin is known to regulate renal, intestinal, mammary and amniotic epithelia, with dysfunction linked to hypogonadism, infertility, and metabolic disorders. Until recently, our understanding of the cellular mechanisms of prolactin action in fishes has been hampered by a paucity of molecular tools to define and study ionocytes, specialized cells that control active ion transport across branchial and epidermal epithelia. Here we review work in teleost models indicating that prolactin regulates ion balance through action on ion transporters, tight-junction proteins, and water channels in ionocytes, and discuss recent advances in our understanding of ionocyte function in the genetically and embryonically accessible zebrafish (Danio rerio). Given the high degree of evolutionary conservation in endocrine and osmoregulatory systems, these studies in teleost models are contributing novel mechanistic insight into how prolactin participates in the development, function, and dysfunction of osmoregulatory systems across the vertebrate lineage.

Effects of feed restriction and prolactin-release inhibition at drying off on metabolism and mammary gland involution in cows

A cow's risk of acquiring a new intramammary infection during the dry period increases with milk production at drying off and decreases as mammary gland involution progresses. A method commonly used to reduce milk production is a drastic reduction in feed supply in the days that precede drying off. Milk production can also be reduced by inhibiting the lactogenic signal driven by prolactin (PRL). This study aimed to compare the effects of these 2drying-off procedures on metabolism, immunity, and mammary gland involution in cows. A total of 24Holstein cows in late lactation were assigned to 1 of 3treatments based on milk yield, somatic cell count, and parity. The cows were fed a lactation diet until drying off (control; n=8), only dry hay during the last 5d before drying off (DH; n=8), or the same lactation diet as the control cows but with twice-daily i.m. injections of 4mg of quinagolide, a specific inhibitor of PRL release, from 5d before drying off until 13d after (QN; n=8). Quinagolide induced a decrease in PRL concentration in blood and in milk and mammary secretions on all the injection days. Interestingly, PRL was also depressed in the blood and milk of the hay-fed cows before drying off. Both the QN and DH treatments induced a decrease in milk production, which averaged 17.9 and 10.1kg/d for the QN and DH cows, respectively, at drying off in comparison with 24.8kg/d for the control cows. Both BSA concentration and Na(+)-to-K(+) ratio increased faster in the mammary secretions of both the DH and QN cows than in those of the control cows, whereas citrate-to-lactoferrin ratio, another indicator of involution rate, decreased faster. The DH treatment decreased blood concentrations of glucose and most amino acids and increased blood concentrations of β-hydroxybutyrate and nonesterified fatty acids. Quinagolide increased blood glucose but did not affect the other metabolites. The serum harvested on d-1 from the hay-fed cows reduced peripheral blood mononuclear cell proliferation and IL-4 production, whereas the serum from the quinagolide-treated cows had no effect. In conclusion, this experiment shows that PRL-release inhibition could be a new alternative for reducing milk production before drying off and for hastening mammary gland involution without disturbing the metabolism of the cow.

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.

Ductal barriers in mammary epithelium

Tissue barriers play an integral role in the biology and pathobiology of mammary ductal epithelium. In normal breast physiology, tight and adherens junctions undergo dynamic changes in permeability in response to hormonal and other stimuli, while several of their proteins are directly involved in mammary tumorigenesis. This review describes first the structure of mammary ductal epithelial barriers and their role in normal mammary development, examining the cyclical changes in response to puberty, pregnancy, lactation and involution. It then examines the role of adherens and tight junctions and the participation of their constituent proteins in mammary tumorigenic functions such as migration, invasion and metastasis. Finally, it discusses the potential of these adhesion proteins as both prognostic biomarkers and potential therapeutic targets in breast cancer.

Effects of recombinant human prolactin on breast milk composition

OBJECTIVE

The objective of this study was to determine the impact of recombinant human prolactin (r-hPRL) on the nutritional and immunologic composition of breast milk.

METHODS

We conducted 2 trials of r-hPRL treatment. In the first study, mothers with documented prolactin deficiency were given r-hPRL every 12 hours in a 28-day, open-label trial. In the second study, mothers with lactation insufficiency that developed while they were pumping breast milk for their preterm infants were given r-hPRL daily in a 7-day, double-blind, placebo-controlled trial. Breast milk characteristics were compared before and during 7 days of treatment.

RESULTS

Among subjects treated with r-hPRL (N = 11), milk volumes (73 ± 36 to 146 ± 54 mL/day; P < .001) and milk lactose levels (155 ± 15 to 184 ± 8 mmol/L; P = .01) increased, whereas milk sodium levels decreased (12.1 ± 2.0 to 8.3 ± 0.5 mmol/L; P = .02). Milk calcium levels increased in subjects treated with r-hPRL twice daily (2.8 ± 0.6 to 5.0 ± 0.9 mmol/L; P = .03). Total neutral (1.5 ± 0.3 to 2.5 ± 0.4 g/L; P = .04) and acidic (33 ± 4 to 60 ± 6 mg/L; P = .02) oligosaccharide levels increased in r-hPRL-treated subjects, whereas total daily milk immunoglobulin A secretion was unchanged.

CONCLUSIONS

r-hPRL treatment increased milk volume and induced changes in milk composition similar to those that occur during normal lactogenesis. r-hPRL also increased antimicrobially active oligosaccharide concentrations. These effects were achieved for women with both prolactin deficiency and lactation insufficiency.

The effects of different intakes of crude protein on nitrogen utilization in the pregnant and lactating rabbit

Three experimental diets (L, M and H) containing 135, 175 and 210 g crude protein per kg dry matter respectively were given to crossbred does in two experiments. Protein quality was constant throughout.

In experiment 1 a total of 18 does (six per diet) were given the diets during two successive reproductive cycles. Losses of nitrogen in faeces and urine were estimated for four successive 7-day periods in each 28-day lactation period. Litters were culled at birth to give ‘high’ and ‘low’ lactational loads with 8 and 4 pups respectively. Total 28-day milk yields on diets L, M and H were 3·89, 4·82 and 5·27 kg respectively with eight pups sucking, and 302, 400 and 400kg respectively with four pups. These differences in milk yield were directly reflected in the weights of the pups when weaned at 28 days of age. There was no effect of diet on milk composition, pup birth weight or litter size. Does given diet L did not always consume their allotted ration and this may have affected the results.

The proportion of digested nitrogen utilized for milk protein synthesis differed significantly between diets and lactational loads, and was at its maximum (0·72) when diet L was given to does with eight pups. Does suckling eight pups retained 005 of the nitrogen they digested in their body tissues and those with four pups 0·13; there were no differences between diets in this respect.

In experiment 2 the nitrogen retention of 18 does (six per diet) was measured during pregnancy. Nitrogen retention increased as pregnancy progressed, and overall 0·20 of digested nitrogen was retained for conceptus growth and doe body tissue accretion.

Lactational performance of Giza White rabbits and its relation with pre-weaning litter traits

An experiment was carried out in Egypt using Giza White rabbits to investigate their lactational performance and to estimate the repeatabilities of milk yield and other litter traits. Data on 325 litters from 115 does fathered by 40 sires were used. Lactation traits involved milk yields during the first 7 days, 21 days and 35 days. Litter traits included litter size and weight at birth, number born alive, litter weight and gain at 21 days, litter size and weight at weaning and pre-weaning litter gain. Linear mixed models were used for analysing such data.

Phenotypic variation in milk yield was relatively high during the first 7 days, and decreased thereafter with advance of lactation stage. Year-season effects were significant for most litter traits, while month of kindling affected litter weight and gain at 21 days and milk yield during the first 7 days (P < 0·05 or P < 0·001). Litters kindled during winter had the highest milk yield compared with litters kindled in the other seasons. Milk production, litter size and weight and litter gain had curvilinear relationship with parity (P < 0·05 or P < 0·001). Sire of doe had no significant effects on litter traits studied. Repeatabilities of all traits were low and ranged from 0·001 to 0·134. Litter size and weight at birth were residually correlated with milk yield. Litter weight and gain at 21 days were significantly correlated with the lactational performance of the doe (P < 0·001).

Milk production of pseudopregnant multiparous does

The aim of the experiment was to evaluate the milk production of pseudopregnant does. In this experiment, data of multiparous, inseminated, pregnant (IP, n=15) does and two groups of presumably pseudopregnant multiparous does: inseminated, non-pregnant (INP, n=17) and induced to ovulation by GnRH (1.5 μg per animal) at the day of insemination (11 d after parturition) (non-inseminated, ovulating: NIO, n=15) were analysed. The progesterone level was measured at the 12th d after treatment to determine if females were pseudopregnant. All IP and NIO does were pregnant and pseudopregnant, respectively. Within the INP group, 10 and 7 does were diagnosed as positive (INPO) or negative (INPNO) for pseudopregnancy. Two of the INPNO does perished during lactation. The average milk production of groups IP, INP and NIO was 212, 92 and 72 g/d, respectively (P&lt;0.001). The proportion of rabbit does reaching daily milk yields of &lt;10, 10-50, 50-100, 100-160, &gt;160 g in the various groups were: IP=0, 0, 0, 0 and 100%, NIO=20, 13, 27, 40 and 0%, INP=15, 15, 15, 55 and 0%, respectively. The daily milk yield of the 5 INPNO does was 2, 6, 27, 84 and 139 g, respectively. These results demonstrated that multiparous empty does, pseudopregnant or non-pseudopregnant, can produce milk, but in lower quantities than multiparous does after kindling.

X-ray fluorescence microscopy reveals accumulation and secretion of discrete intracellular zinc pools in the lactating mouse mammary gland

Background

The mammary gland is responsible for the transfer of a tremendous amount of zinc (∼1–3 mg zinc/day) from maternal circulation into milk during lactation to support the growth and development of the offspring. When this process is compromised, severe zinc deficiency compromises neuronal development and immune function and increases infant morbidity and/or mortality. It remains unclear as to how the lactating mammary gland dynamically integrates zinc import from maternal circulation with the enormous amount of zinc that is secreted into milk.

Methodology/Principal Findings

Herein we utilized X-ray fluorescence microscopy (XFM) which allowed for the visualization and quantification of the process of zinc transfer through the mammary gland of the lactating mouse. Our data illustrate that a large amount of zinc first accumulates in the mammary gland during lactation. Interestingly, this zinc is not cytosolic, but accumulated in large, discrete sub-cellular compartments. These zinc pools were then redistributed to small intracellular vesicles destined for secretion in a prolactin-responsive manner. Confocal microscopy identified mitochondria and the Golgi apparatus as the sub-cellular compartments which accumulate zinc; however, zinc pools in the Golgi apparatus, but not mitochondria are redistributed to vesicles destined for secretion during lactation.

Conclusions/Significance

Our data directly implicate the Golgi apparatus in providing a large, mobilizable zinc storage pool to assist in providing for the tremendous amount of zinc that is secreted into milk. Interestingly, our study also provides compelling evidence that mitochondrial zinc pools expand in the mammary gland during lactation which we speculate may play a role in regulating mammary gland function.

The declining phase of lactation: peripheral or central, programmed or pathological?

In most species the functional activity of the mammary gland during lactation follows a biphasic developmental pattern. This pattern starts with a rapid increase in milk output that occurs with secretory activation and continues with a more gradual increase until the point of peak lactation is reached. Following this gain-of-function phase, the ability of the gland to produce milk decreases. This decrease occurs even if the lactation is prolonged by the presence of continued suckling stimulus and complete milk removal. This review describes the current state of our knowledge concerning the factors that regulate milk synthesis capacity by the mammary gland during the lactation cycle. The review describes four potential alternatives as mechanisms governing the process, which we refer to as secretory diminution. These alternatives are not presented as mutually exclusive of each other or other possible mechanisms, but are proposed as potential contributing mechanisms.

Overexpression of des(1-3) insulin-like growth factor 1 in the mammary glands of transgenic mice delays the loss of milk production with prolonged lactation

During prolonged lactation, the mammary gland gradually loses the capacity to produce milk. In agricultural species, this decline can be slowed by administration of exogenous growth hormone (GH), which is believed to act through insulin-like growth factor 1 (IGF1). Our previous work demonstrated delayed natural mammary gland involution in des(1-3)IGF1-overexpressing transgenic mice (Tg[Wap-des{1-3}IGF1]8266 Jmr), hereafter referred to as WAP-DES mice. The present study tested the hypothesis that overexpressed des(1-3)IGF1 would delay the loss of milk production during prolonged lactation. Accordingly, we examined lactational performance in WAP-DES mice by artificially prolonging lactation with continual litter cross-fostering. Over time, lactational capacity and mammary development declined in both WAP-DES and control mice. However, the rate of decline was 40% slower in WAP-DES mice. Mammary cell apoptosis increased by 3-fold in both groups during prolonged lactation but was not different between genotypes. Plasma concentrations of murine IGF1 were decreased in WAP-DES mice, while those of the transgenic human IGF1 were elevated during prolonged lactation. Phosphorylation of the mammary IGF1 receptor was increased in the WAP-DES mice, but only during prolonged lactation. Plasma prolactin decreased with prolonged lactation in nontransgenic mice but remained high in WAP-DES mice. The WAP-DES mice maintained a higher body mass and a greater lean body mass during prolonged lactation. These data support the conclusion that overexpressed des(1-3)IGF1 enhanced milk synthesis and mammary development during prolonged lactation through localized and direct activation of the mammary gland IGF1 receptor and through systemic effects on prolactin secretion and possibly nutrient balance.

Prolactin effects on cultured pavement cell epithelia and pavement cell plus mitochondria-rich cell epithelia from freshwater rainbow trout gills

The physiological effects of ovine prolactin (oPRL) and recombinant rainbow trout prolactin (rbtPRL) on cultured gill epithelia derived from freshwater rainbow trout were assessed. Epithelia composed of either pavement cells only (single seeded inserts, SSI) or both pavement and mitochondria-rich cells (double seeded inserts, DSI) were cultured in media, supplemented with doses of oPRL ranging from 10 to 100 ng/ml. Under symmetrical culture conditions (L15 media apical/L15 media basolateral), oPRL had no effect on transepithelial resistance, paracellular permeability (assessed with PEG-4000), or Na(+) and Cl(-) transport across both preparations of cultured gill epithelia. Under asymmetrical conditions (freshwater apical/L15 media basolateral), SSI epithelia treated with oPRL (10 and 50 ng/ml), in comparison to comparably treated epithelia receiving no oPRL, exhibited a greater increase in the transepithelial resistance, particularly during the first 12h of freshwater exposure, no difference in paracellular permeability and Na(+)-K(+)-ATPase activity, and lowered net Na(+) flux rates (i.e., reduced basolateral to apical loss rates). These reflected reduced unidirectional efflux rates. The PRL effect appeared to be mainly a reduction in transcellular permeability. SSI epithelia treated with rbtPRL (10 ng/ml) exhibited similar patterns of response to those treated with oPRL. Na(+)-K(+)-ATPase activity increased in DSI epithelia treated with oPRL; however, oPRL did not stimulate ion uptake across either SSI or DSI epithelial preparations. The data demonstrated that, as the sole hormone supplement for cultured gill epithelia, PRL did not promote active ion uptake. However, the observed PRL-induced alterations in cultured gill epithelial physiology were consistent with the in vivo actions of PRL on the gills of freshwater teleost fish.

ENaC- and CFTR-dependent ion and fluid transport in mammary epithelia

Mammary epithelial 31EG4 cells (MEC) were grown as monolayers on filters to analyze the apical membrane mechanisms that help mediate ion and fluid transport across the epithelium. RT-PCR showed the presence of cystic fibrosis transmembrane conductance regulator (CFTR) and epithelial Na(+) channel (ENaC) message, and immunomicroscopy showed apical membrane staining for both proteins. CFTR was also localized to the apical membrane of native human mammary duct epithelium. In control conditions, mean values of transepithelial potential (apical-side negative) and resistance (R(T)) are -5.9 mV and 829 Omega x cm(2), respectively. The apical membrane potential (V(A)) is -40.7 mV, and the mean ratio of apical to basolateral membrane resistance (R(A)/R(B)) is 2.8. Apical amiloride hyperpolarized V(A) by 19.7 mV and tripled R(A)/R(B). A cAMP-elevating cocktail depolarized V(A) by 17.6 mV, decreased R(A)/R(B) by 60%, increased short-circuit current by 6 microA/cm(2), decreased R(T) by 155 Omega x cm(2), and largely eliminated responses to amiloride. Whole cell patch-clamp measurements demonstrated amiloride-inhibited Na(+) currents [linear current-voltage (I-V) relation] and forskolin-stimulated Cl(-) currents (linear I-V relation). A capacitance probe method showed that in the control state, MEC monolayers either absorbed or secreted fluid (2--4 microl x cm(-2) x h(-1)). Fluid secretion was stimulated either by activating CFTR (cAMP) or blocking ENaC (amiloride). These data plus equivalent circuit analysis showed that 1) fluid absorption across MEC is mediated by Na(+) transport via apical membrane ENaC, and fluid secretion is mediated, in part, by Cl(-) transport via apical CFTR; 2) in both cases, appropriate counterions move through tight junctions to maintain electroneutrality; and 3) interactions among CFTR, ENaC, and tight junctions allow MEC to either absorb or secrete fluid and, in situ, may help control luminal [Na(+)] and [Cl(-)].

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.

Prolactin, alone or in combination with glucocorticoids, enhances tight junction formation and expression of the tight junction protein occludin in mammary cells

Tight junctions (TJ) between adjacent epithelial cells play an important role in maintaining mammary function in the differentiated mammary gland. Mouse mammary cell lines (HC11 and Comma-1D) were used to investigate the effect of the lactogenic hormones prolactin (PRL) and glucocorticoids on the formation of mammary TJ. TJ formation was assessed by an increase in transepithelial electrical resistance and a decrease in paracellular flux of radiolabeled inulin. Both PRL and the synthetic glucocorticoid dexamethasone (DEX) stimulated TJ formation. The biggest effect on TJ formation was observed when both hormones were used in combination, but only when cells were pretreated with DEX. The effects of PRL and DEX are mediated, at least in part, via expression of the transmembrane TJ protein occludin. In summary, these data are the first to show an effect of PRL on mammary TJ formation and the expression of TJ proteins, and confirm the TJ-stimulating effects of glucocorticoids that have been reported previously.

Tight junction regulation in the mammary gland

Tight junctions form a narrow, continuous seal that surrounds each endothelial and epithelial cell at the apical border, and act to regulate the movement of material through the paracellular pathway. In the mammary gland, the tight junctions of the alveolar epithelial cells are impermeable during lactation, and thus allow milk to be stored between nursing periods without leakage of milk components from the lumen. Nonetheless mammary epithelial tight junctions are dynamic and can be regulated by a number of stimuli. Tight junctions of the mammary gland from the pregnant animal are leaky, undergoing closure around parturition to become the impermeable tight junctions of the lactating animal. Milk stasis, high doses of oxytocin, and mastitis have been shown to increase tight junction permeability. In general changes in tight junction permeability in the mammary gland appear to be the results of a state change and not assembly and disassembly of tight junctions. Both local factors, such as intramammary pressure and TGF-beta, and systemic factors, such as prolactin, progesterone, and glucocorticoids, appear to play a role in the regulation of mammary tight junctions. Finally, the tight junction state appears to be closely linked to milk secretion. An increase in tight junction permeability is accompanied by decrease in the milk secretion rate, and conversely, a decrease in tight junction permeability is accompanied by an increase in the milk secretion rate.

Dietary thiamin supply during lactation influences thiamin status in lactating rats and their offspring and the thiamin level in milk

This study was conducted to examine the effect of dietary thiamin, ranging from deficient to excessive supplies, on thiamin status of lactating rats and their offspring, and the thiamin level in milk. Therefore, after parturition, rat dams were divided into eight groups of 10 each, and were fed diets with 0, 2, 4, 6, 7, 40, 350 and 3500 mg/kg thiamin over a total of 13 days during lactation. Milk for determining the thiamin concentration was obtained from day 6 and 13 of lactation. At day 14 of lactation rat dams and their offspring were used to ascertain the thiamin status including transketolase activity of blood, liver and brain, and thiamin concentration in body. Thiamin supplies ranging from deficient to excessive dietary concentrations influenced both the thiamin levels of the lactating dams and their offspring within 13 days. Lactating rat dams fed a thiamin-free diet and their offspring were classified as thiamin-deficient on the basis of growth retardation and a lower activity of transketolase in blood, liver and brain. Within these variables transketolase in blood has been shown to be most sensitive, and reached a plateau feeding 6 mg/kg thiamin. The concentration of thiamin in milk ranged between 0.1 and 19 mg/kg. The findings also show that dietary thiamin had the strongest effect on thiamin in milk obtained from day 6 and 13 of lactation, and a deficient or suboptimal supply with thiamin was therefore not compensated for an intensified transfer of reserved body thiamin into milk. Also thiamin levels in tissues and carcass, which did not show any clear-cut saturation characteristic, increased with increasing dietary thiamin, and this dose-dependence was more marked in blood and liver than in carcass.

Maternal protein reserves and their influence on lactational performance in rats. 3. The effects of dietary protein restriction and stage of lactation on milk composition

The effects of severe protein restriction following parturition on the changes in rat milk composition during lactation were investigated using multiparous female Sprague-Dawley rats caged individually following mating and offered a high-protein diet (H; 215 g crude protein (N x 6.25; CP)/kg dry matter (DM)) ad lib. until parturition. Following parturition, half the females continued to receive diet H, whilst the remainder were offered a diet low in protein (L; 90 g CP/kg DM) ad lib. On days 2, 4, 8 and 12 of lactation groups of females from both dietary treatments were used to provide a milk sample. Milk samples were analysed for their lactose (enzymically), protein (binding to Coomassie blue), lipid (gravimetrically) and mineral (spectrophotometrically) contents. The milk lactose concentration of group H increased with stage of lactation (r2 0.85, P < 0.001). Such an increase was prevented by diet L, and from day 8 of lactation the milk lactose of group L was lower (P < 0.05) than in group H. Group H milk protein concentration did not change during lactation and averaged 90.7 mg/g. Dietary protein restriction reduced the milk protein concentration of group L so that on days 2, 4 and 12 of lactation it was lower (P < 0.05) than that of group H. On day 8 of lactation the milk protein concentration of group L had increased (P < 0.05) and was comparable with that of group H. For group H, milk lipid averaged 166.8 mg/g and was generally unchanged during lactation. Diet L increased (P < 0.01) the milk lipid concentration (205.5 mg/g) compared with diet H and this was also significant on days 4 and 8 of lactation (P < 0.05). Group L milk lipid concentration also increased between days 4 and 8 of lactation (P < 0.05). Milk Na concentration declined during lactation in both dietary groups (P < 0.01) but was unaffected by dietary treatment. Both milk Ca and P concentrations increased (P < 0.01) during lactation in both dietary groups, whilst protein restriction also increased the Ca and P concentrations (P < 0.05). Milk K and Mg concentrations were unaffected by dietary treatment or stage of lactation. This significant alteration in the milk composition of severely protein-restricted dams, while possibly favouring the disposal of greater quantities of energy-yielding nutrients, suggests that equations developed for the estimation of milk production in rats cannot be used under such conditions.

Milk secretion in the rat: progressive changes in milk composition during lactation and weaning and the effect of diet

1. Progressive changes in the composition of milk from rats has been studied from day 0 to 20 of lactation and for 3 days following separation of the dams and pups at day 20 post partum. 2. The changes in concentration of Na, K and lactose suggested that secretion both prepartum and following weaning occurred by a paracellular mechanism whereas a transcellular pathway existed during established lactation. 3. The concentration of total protein and casein increased gradually throughout lactation. In contrast, the concentration of serum albumin increased and transferrin decreased markedly during early lactation. The fat content of milk declined 3-fold within 5 days of birth but the concentration of Ca, Mg and inorganic P increased. The concentration of each of these milk constituents remained constant during established lactation. 4. Following weaning the pronounced decline in lactose, K and inorganic P was negatively correlated with an increase in all other milk constituents except fat. 5. Rats fed a low energy diet produced milk with a lower fat content but with an unaltered concentration of protein and carbohydrate. The growth rate of these litters was similar for the first 5 days of lactation when compared to litters from dams fed a high energy diet. The growth rate of litters thereafter and following weaning was greater for rats fed a high energy diet.

Prolactin and propranolol prevent the suckling-induced inhibition of lactation in rabbits

The normal once-a-day frequency of suckling in the rabbit was increased on day 31 (late lactation) by the addition of two extra sucklings (8 and 16 hr after) the daily suckling. In confirmation of previous data, two additional sucklings significantly decreased milk yield acutely on day 31 in comparison with the average 4-day milk yield before and after day 31. The decrease in milk secretion after the two additional sucklings was prevented by a single injection of 3 mg prolactin (given 24 hr before the two extra sucklings) and/or by injections of the beta-adrenergic-blocking drug, propranolol (100 micrograms/kg b. wt. given 30 min before each additional suckling). Since prolactin secretion is decreased in these species and the mammary gland is less responsive to the hormone during late lactation, the present results suggest that in addition to these factors, suckling-induced activation of sympathetic influences may contribute to the decline in milk production at this stage of lactation. Taken together, these results suggest that suckling may regulate lactation in the rabbit through antagonistic mechanisms at different stages of lactation.

Milk synthesis and secretion rates in cows with milk composition changed by oxytocin

Supraphysiological doses of oxytocin were used to investigate the association among milk composition, secretion rates, and cell electrolyte concentrations. Groups of 8 Holstein cows were injected i.v. with 1, 10, 100, or 1000 IU oxytocin after a normal milking. Milk yield over the following 12-h period declined 5.5, 8.1, 17.0, and 43.7% respectively. Milk Na increased in proportion to oxytocin dose. Yield and concentration of lactose and protein declined with increasing oxytocin doses, but milk fat yield remained unchanged. To determine whether the decreased milk lactose secretion was caused by a decline in synthesis or leakage into blood plasma and subsequent renal clearance, milk and urine were collected for 24-h periods before and after 100 IU oxytocin. Urinary lactose accounted for 2.7 +/- 3.19 and 12.9 +/- 8.9% of the total collected on the days before and after oxytocin. This increase (46.7 g/d) was only one-fifth of the 25% decline in total lactose recovery, suggesting that reduced mammary lactose synthesis is primarily responsible for lower milk yield. Measurement of Na and K in biopsied mammary tissue suggests that a high Na:K ratio in milk will increase that ratio in secretory cells, which may be part of the mechanism for lower milk yield under circumstances that increase permeability of mammary tight junctions.

Serum albumin secretion in rat milk

1. The concentration of serum albumin in rat milk, rat serum, the stomach contents of suckling rats and mammary homogenates has been measured. 2. Serum albumin in expressed milk ranges in concentration between 3.5 and 6 mg/ml and amounts to an average of 4-5 mg/100 mg total protein whereas, in the stomach contents of suckling rats, serum albumin was 3.2 mg/100 mg total protein. 3. Corresponding concentrations of a second soluble milk protein, alpha-lactalbumin, were 2.9 mg/100 mg total protein in the milk and 1.2 mg/100 mg total protein in the stomach contents. 4. Serum albumin is a major protein in mammary homogenates at 10 mg/100 mg total soluble protein and the calculated total mammary pool of albumin amounts to 110 mg/rat. 5. There was a significant negative correlation between the serum albumin concentration and the ratio of potassium to sodium ions in milk samples obtained from rats milked with varying oxytocin treatments. 6. No evidence for albumin mRNA could be detected when mammary total RNA was probed with a complementary DNA (cDNA) for rat albumin. 7. The data are consistent with albumin being a major whey protein in the rat, it being synthesized at an extramammary site and transferred to the milk space by a paracellular mechanism from an extravascular mammary pool rather than directly from the serum.

Sodium and potassium distribution in the lactating mouse mammary gland in vivo

The ratio of intracellular potassium to sodium in the mammary cells of the lactating mouse was compared with the ratio of potassium to sodium in the mouse milk to determine whether the sodium and potassium concentrations in milk are governed by a Donnan equilibrium as postulated earlier (Peaker 1977a, 1978). An efflux technique was used to determine the average intracellular sodium of 23.0 +/- 1.2 mumol/g tissue (+/- S.E. of mean). The intracellular potassium, determined by calculating the amount of potassium contained in both the interstitial and milk spaces and subtracting these values from the total tissue potassium, was 62 +/- 1 mumol/g tissue. The mean intracellular potassium to sodium ratio, calculated from individual efflux experiments, was 2.7 +/- 0.2. The total, interstitial, and milk water spaces were measured by tissue drying, sodium efflux, and lactose efflux, respectively. The average values (+/- S.E. of mean) obtained were 0.700 +/- 0.004 ml/g tissue, 0.150 +/- 0.016 ml/g tissue and 0.064 +/- 0.004 ml/g tissue. Based on these values the intracellular water space was 0.49 +/- 0.02 ml/g tissue. Intracellular concentrations of sodium and potassium calculated from the intracellular amounts of sodium and potassium and intracellular water space were 47 +/- 3 mM and 129 +/- 5 mM, respectively. The concentrations of potassium and sodium in mouse milk were 47 +/- 1 mM and 26 +/- 1 mM. The mean potassium to sodium ratio (+/- S.E. of mean) calculated from individual milk samples was 1.8 +/- 0.1. The milk ratio of potassium to sodium is significantly different (P less than 0.001) from the intracellular water ratio of potassium to sodium. This finding, in a tight epithelium such as the lactating mouse mammary gland, suggests that both sodium and potassium cannot be distributed passively across the apical membrane and an active transport process must exist for one or both of these ions in this membrane.

Lactose in plasma during lactogenesis, established lactation and weaning in sows

The concentration of lactose in plasma was determined in different sows at all phases of their reproductive cycle and related to the compositional changes in mammary secretion during lactogenesis, established lactation and weaning. Lactose was present in low concentrations (3-4 microM) in the blood of virgin sows and pregnant sows up to 107 days of gestation. From day 4 pre-partum to day 1 pre-partum circulating lactose rose gradually to 34.5 +/- 7.7 microM (mean +/- S.E. of mean). Maximal concentrations of 262 +/- 168.4 microM were reached 6 h after parturition. The concentration of lactose in plasma was correlated with the amount of lactose in mammary secretion (r = 0.88, P less than 0.01) at the beginning of farrowing. During established lactation the concentrations of lactose, Na and K in milk, and of lactose in plasma (72-86 microM), were constant. The concentration of lactose in plasma did not vary significantly during periods of suckling, or after stimulation of milk ejection by oxytocin. However, the amount of lactose in plasma rose significantly (P less than 0.02) after the administration of oxytocin if milk ejection was not accompanied by suckling. The mean plasma concentration of lactose began to rise 36 h after weaning to a peak value of 241.8 +/- 53.6 microM at 48 h; thereafter it declined to 10.2 +/- 2.0 microM by 6 days. This study has shown that lactose concentrations in the plasma vary according to the secretory activity of the mammary gland. Its plasma concentration provides an earlier temporal measure of lactogenesis in individual sows than is obtained either from observation or analysis of mammary secretion.

Prolactin effects on ion transport across cultured mouse mammary epithelium

Prolactin is a known osmoregulatory hormone in lower vertebrates, and recent evidence indicates that this hormone modulates ionic concentrations in milk. In an ultrastructurally and biochemically differentiated primary cell culture system in which mouse mammary epithelium is maintained on floating collagen gels, prolactin causes an increase in short-circuit current (Isc) of monolayers of cells derived from midpregnant (24.6 to 48.0 microA . cm-2) and lactating (10.4 to 16.1 microA . cm-2) glands. Transepithelial potential differences (basal side ground) average about -12 mV and are similar to those seen in vivo. Prelactating mammary epithelial cell cultures have transepithelial resistances ranging from 374 omega . cm2 (prolactin present) to 507 omega . cm2 (prolactin absent), and lactating cell cultures have resistances averaging almost 1,000 omega . cm2. Prolactin effects require at most one day of culture maintenance in prolactin-containing medium, and the effects are not due to known contamination of prolactin preparations with arginine vasopressin or growth hormone. Medium concentrations of prolactin as low as 1 ng/ml can elicit these effects. In prelactating cell cultures not treated with prolactin, the Isc is equal to the rate of sodium absorption. Prolactin increases sodium absorption fourfold but increases Isc only twofold. Clearly, prolactin induces other active transport; neither potassium nor chloride movements can account for this additional transport. Resistance values, current-voltage plots, and permeability coefficients indicate that in vitro mammary epithelium is a moderately "tight" tissue. Comparisons with intact glands indicate that in vitro mammary epithelium closely resembles its in vivo counterpart. Floating collagen gel cultures appear suitable for elucidating transport properties in cellularly heterogeneous and structurally complex mammalian tissues.

The secretion of calcium and phosphorus into milk

1. The time course of appearance of radioactivity in milk was studied following close-arterial infusion of labelled phosphate, Ca or leucine into the mammary artery of lactating goats. Maximum activities were reached at 1.5 hr in all milk fractions including inorganic soluble phosphate, inorganic colloidal phosphate, casein P, soluble Ca, protein-associated Ca and casein. 2. At 0.5 hr, labelling of the soluble and colloidal phosphate fractions was significantly higher than that of the casein P. 3. Recovery of 32P or 47Ca 3 or more hours after infusion into the cistern of the mammary glands was 98% or greater, indicating that the mammary epithelium is virtually impermeable to [32P]phosphate and 47Ca in the milk to blood direction. 4. Ca and P failed to enter milk in excess of the normal secretion rate when the milk was diluted with isosmotic sucrose given by intraductal injection. 5. These data suggest that milk Ca and phosphate in their various forms are secreted, like protein and lactose, by exocytosis of Golgi vesicles. Unless a paracellular pathway is present, as in oxytocin-treated animals, the milk concentrations are maintained by virtue of the impermeability of the mammary epithelium to these substances.

Mouse mammary epithelial cells on floating collagen gels: transepithelial ion transport and effects of prolactin

Epithelial cells dissociated from midpregnant BALB/c mouse mammary glands were cultured for as long as 20 days as confluent monolayers on floating collagen gels. Detached gels bearing monolayers were placed in lucite Ussing chàmbers for measurement of transepithelial potential difference (PD), short-circuit current (I(sc)), resistance (R), and unidirectional fluxes of Na(+) and Cl(-) during short-circuit current conditions (PD = 0). With Hanks' solution bathing both sides of cultures maintained with insulin and cortisol, PD = -12.8 mV (serosal side ground), I(sc) = 24.6 muA/cm(2), and R = 507 omega.cm(2). Net absorption of Na(+) equaled I(sc), and there was no net Cl(-) transport. PD and I(sc) were reduced 50% by mucosal addition of 10 muM amiloride and to zero by metabolic inhibition with nitrogen gas or by serosal addition of 0.1 mM ouabain. In similar cultures supplemented with prolactin, PD and I(sc) increased to -15.8 mV and 48.0 muA/cm(2), respectively, and R decreased to 374 omega.cm(2). Inhibitor effects were similar to those seen in prolactin-free cultures. Prolactin exposure resulted in a 3-fold increase in net absorption of Na(+). Na(+) absorption was not equivalent to I(sc), and there was little Cl(-) absorption; therefore, prolactin induced active transport of other, as yet unidentified, ions. These effects of prolactin require at least 3 days to occur and cannot be attributed to the known contamination with neurohypophysial hormones. The prolactin-induced increase in Na(+) absorption parallels its Na(+)-retaining ability in lower vertebrates and could be part of the mechanism that keeps milk Na(+) concentration low in intact glands.

Early effects of prolactin on lactating rabbit mammary gland. Ultrastructural changes and stimulation of casein secretion

Effects of prolactin on the secretion of milk proteins have been investigated by incubating mammary tissue fragments from lactating rabbits. Within 15 min of adding the hormone to the incubation medium, cell morphology is modified: the relative volume occupied by the Golgi region is greatly increased. When prolactin is added immediately after a pulse labelling of proteins (3 min with 3H-L-leucine), the amount of labelled caseins secreted during one hour is significantly increased. This increase proceeds neither from an acceleration of intracellular transit of caseins (as shown by electron microscopic autoradiography) nor by an enhancement of amino acid uptake (as measured by incorporation of non-metabolizable amino acids) nor by an increase of overall protein synthesis, during the first hour. The action of prolactin on the morphology of such subcellular organelles as the Golgi apparatus and its influence on casein secretion are discussed.

Changes in the composition of the mammary secretion of women after abrupt termination of breast feeding

1. The composition of mammary secretion has been followed in seven women before and after abrupt termination of breast feeding. The period of full lactation was 39 days for 1 woman and a mean of 332 days (range 251--443 days) for the six others. Small samples of mammary secretion (0.5--5.00 ml.) were collected by manual expression from three women at monthly intervals throughout 12 months of lactation and from seven women at frequent intervals for 42 days of involution. 2. During full lactation (12 months, three women) the mean values (+/- S.E. of mean) for lactose, total protein, alpha-lactalbumin, Na, K and Cl were 7.03 +/- 0.13 g/100 ml., 1.68 +/- 0.08 g/100 ml., 163 +/- 6.39 mg/100 ml., 8.5 +/- 0.90 mM, 13.4 +/- 0.34 mM and 11.93 +/- 0.53 mM, respectively. 3. After termination of breast feeding, the concentrations of lactose and K decreased while Na, Cl, fat and total protein increased progressively over 42 days. The increase in the protein content was contributed to by increases in the concentration of lactoferrin, IgA, IgG, IgM, albumin, alpha-lactalbumin and casein. There was no significant difference in the concentration of the milk constituents between the right and left breast throughout full lactation and after its termination. 4. These observations indicated that the secretory capability of the mammary gland of women changed dramatically after complete cessation of breast feeding but that the involuting gland remained partially functional for a long period.

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

1. In conscious lactating goats a significant correlation was found between blood-milk potential difference (p.d.) and milk [lactose] such that in goats with a lower milk [lactose], milk was more negative with respect to blood.2. When mannose was substituted for glucose in the substrate mixture of isolated perfused goat mammary glands, milk yield and milk [lactose] fell while milk [Na] and [K] increased; in parallel experiments the blood-milk p.d. changed such that milk became more negative with respect to blood. These changes were reversed following the addition of glucose.3. When milk was made hypertonic by the addition of hyperosmotic sucrose or lactose solutions, water entered milk osmotically and milk became electrically less negative or even positive with respect to blood in goats, cows and guinea-pigs.4. No effect on p.d. was apparent following the addition of isosmotic sucrose to milk in goats.5. When milk was held in the teat of goats by a pneumatic cuff around the base of the teat, no effect on p.d. was apparent when hyperosmotic sucrose was introduced into this teat pouch.6. It is suggested that waterflow-induced potentials (the streaming potential and the transport number effect) can be induced across the mammary epithelium.7. In goats exogenous oxytocin lowered milk [lactose] and blood-milk p.d. became less negative with respect to blood.8. In non-lactating and mastitic glands of goats the blood-milk p.d. was within 0.5-2.5 mV of zero.9. The effects of oxytocin, and the low p.d. in non-lactating and mastitic glands, are compatible with the view that in such circumstances there is a paracellular pathway across the mammary epithelium which partially short-circuits the two sides.10. It is suggested that, with water being drawn osmotically into milk to dilute newly formed lactose, waterflow-induced potentials may be responsible for establishing the normal p.d. across the apical membrane of the secretary cell, thereby keeping milk [K] and [Na] lower than in intracellular fluid.

The secretion of citrate into milk

1. The time course of changes in specific activities of citrate, lactose and fatty acids in milk during frequent milking, following the I.V. administration of labelled glucose, acetate and chylomicrons in goats has been studied. Peak specific activities of lactose and citrate in milk were reached at 2-3 hr, while peak specific activites of fatty acids were reached at 5-7 hr. 2. Following short I.A. infusions of 24Na, 36Cl, and 42K, peak specific activities in milk were reached in 1 hr or less. 3. The mammary epithelium of lactating goats was found to be virtually impermeable to labelled citrate in both directions. 4. Labelled citrate had an apparent volume of distribution in lactating guinea-pigs mammary slices in vitro similar to that of extracellular space markers. 5. Treatment of goats with large doses of oxytocin markedly increased the permeability of the secretory epithelium to labelled citrate. 6. In the goat mammary gland, citrate, protein and calcium failed to enter milk which had been diluted with isosmotic lactose by intraductal injection, whereas Na, K and Cl did enter, thus tending to restore the concentrations of these ions to normal. 7. It is suggested that citrate, which is formed within the sucretory cell, enters milk not by passage across the apical cell membrane but, in common with lactose and milk protein, by exocytosis of Golgi vesicles. It appears that citrate is held at high concentrations in milk by virtue of the impermeability of the mammary epithelium to the forms in which it occurs in milk.

Milk secretion in the rabbit: changes during lactation and the mechanism of ion transport

Changes in the yield and composition of milk and in the permeability of the mammary epithelium to labelled disaccharides and ions have been studied during lactation in rabbits of the Dutch breed. 2. Milk yield increased to reach a peak on day 20 of lactation and then declined, but by 30-32 days the yield was still relatively high. Milk [protein] and [fat] increased in late lactation; [Na] and [Cl] decreased from early to established lactation (11-14 days) and then increased, whereas milk [K] and [lactose] showed an inverse pattern to that displayed by[Na] and [Cl]. 3. At all stages of lactation [Na] and [Cl] were both inversely related to milk [lactose] while [K] showed a positive correlation. 4. Labelled lactose and sucrose were found to cross the mammary epithelium at all stages but in increased amounts during late lactation. Sucrose entry from blood into milk was positively correlated with milk [Na], and inversely correlated with [K] and [lactose]. 5. The entry of (24)Na and (36)Cl into milk from blood paralleled the changes in milk [Na] and [Cl]. 6. Intracellular ionic composition determined in vitro was similar for [Na] and [K] in both established (11-14 days) and late (25-28 days) lactation, but [Cl] was higher in late lactation. 7. Intracellular potentials recorded in vivo were -31 mV (mean) and -36 mV in established and late lactation respectively. Transepithelial p.d. was close to zero at both stages. 8. It is suggested that ions and lactose and other small molecules can cross the mammary epithelium by a paracellular, as well as by a transcellular route, throughout lactation and that the paracellular pathway is increased in late lactation. 9. The site of the proposed paracellular pathway and the implications of such a pathway's presence on ion transport are discussed, and a scheme is suggested to account for the ionic composition of milk in this species.