Calcitriol-Dependent and -Independent Regulation of Intestinal Calcium Absorption, Osteoblast Function, and Skeletal Mineralization during Lactation and Recovery in Mice

Recovery from lactation-induced bone loss appears to be calcitriol-independent, since mice lacking 1-alpha-hydroxylase or vitamin D receptor (VDR) exhibit full skeletal recovery. However, in those studies mice consumed a calcium-, phosphorus-, and lactose-enriched "rescue" diet. Here we assessed whether postweaning skeletal recovery of Vdr null mice required that rescue diet. Wild type (WT) and Vdr null mice were raised on the rescue diet and switched to a normal (1% calcium) diet at Day 21 of lactation until 28 days after weaning. Unmated mice received the same regimen. In WT mice, cortical thickness was significantly reduced by 25% at 21 days of lactation and was completely restored by 28 days after weaning. Three-point bending tests similarly showed a significant reduction during lactation and full recovery of ultimate load and energy absorbed. Although Vdr null mice exhibited a similar lactational reduction in cortical thickness and mechanical strength, neither was even partially restored after weaning. Unmated mice showed no significant changes. In micro-computed tomography scans, diaphyses of Vdr null femora at 28 days after weaning were highly porous and exhibited abundant low-density bone extending into the marrow space from the endocortical surface. To quantify, we segregated bone into low-, mid-, and high-density components. In WT diaphyses, high-density bone was lost during lactation and restored after weaning. Vdr null mice also lost high-density bone during lactation but did not replace it; instead, they demonstrated a threefold increase in low-density bone mass. Histology revealed that intracortical and endocortical surfaces of Vdr null bones after weaning contained very thick (up to 20 micron) osteoid seams, covered with multiple layers of osteoblasts and precursors. We conclude that during the postweaning period, osteoblasts are potently stimulated to produce osteoid despite lacking VDRs, and that either calcitriol or a calcium-enriched diet are needed for this immature bone to become mineralized. © 2022 American Society for Bone and Mineral Research (ASBMR).

Incidence of Fracture Hospitalization and Surgery in Women Increases Steadily During the Puerperal and Lactation Period: A Retrospective Register-Based Cohort Study in Finland From 1999 to 2018

This retrospective cohort study assesses the incidences of major fractures and surgery in women during the puerperium and the lactation period in Finland between January 1, 1999, and December 31, 2018. Using nationwide data from the Finnish Care Register for Health Care and the Finnish Medical Birth Register, all women aged between 15 and 49 years with a fracture hospitalization within 12 months of delivery between 1999 and 2018 were included. During the study period, a total of 3140 fractures after delivery and 152,800 fractures of the female normal population of similar age were hospitalized. The incidence rate after delivery increased from 219/100,000 person-years during the first 4 months to 310 fractures/100,000 person-years during the latter 8 months of the first year after delivery. Altogether, 29% (n = 904/3140) of these fractures were treated operatively. The most common fractures were ankle and distal radius fractures, which made up one-third of all fractures. The incidence of pelvic fracture hospitalization was 15/100,000 person-years at 4 months after delivery, with an operation rate of 22%. Over half of all fractures occurred between 6 and 12 months after delivery (mean 6.6 months). The incidence of fracture hospitalization after delivery increased steadily during the puerperium and the lactation periods but remained lower than in the general population (age-adjusted incidence 554/100,000 person-years) with an incidence rate ratio of 0.51. However, a higher proportion of pelvic fractures were observed in the first months after delivery. Surgical rates were in line with the general population. Fractures of the wrist and ankle made up most of the fractures. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Changes in Bone Mineral Density During and After Lactation in Ugandan Women With HIV on Tenofovir-Based Antiretroviral Therapy

Antiretroviral therapy (ART) in people living with human immunodeficiency virus (HIV) is associated with bone loss, but data are limited in lactation, when physiological bone mineral mobilization is occurring. This research charted changes in areal bone mineral density (aBMD) during and after lactation in Ugandan women with HIV (WWH) initiated onto ART in pregnancy, compared to women without HIV (REF). One-hundred WWH on tenofovir-based ART and 100 REF were enrolled in pregnancy. Lumbar spine (LS), total hip (TH), and whole-body-less-head (WBLH) aBMD were measured by dual-energy X-ray absorptiometry (DXA) at 2, 14, and 26 weeks of lactation, and at 3 months postlactation. The primary outcome was the difference between groups in mean percent change in LS aBMD between 2 and 14 weeks. Statistical analysis was performed in hierarchical repeated measures ANOVA models that corrected for multiple testing. Median age was 23.4 (IQR, 21.0 to 26.8) years. WWH had lower body weight. aBMD decreased in both groups during lactation, but WWH had greater decreases at TH (2-to-26 weeks: WWH [n = 63] -5.9% [95% CI, -6.4 to -5.4] versus REF [n = 64] -4.3% [95% CI, -4.8 to -3.8]; group*time point interaction p = .008). Decreases in LS aBMD were similar in WWH and REF (2-to-26 weeks: -2.0% [95% CI, -2.5 to -1.5]), although there was a tendency toward a smaller decrease in WWH between 2 and 14 weeks (WWH [n = 77] -1.8% [95% CI, -2.2 to -1.4] versus REF [n = 69] -2.9% [95% CI, -3.3 to -2.5]; group*time point interaction p = .08). Postlactation, LS aBMD was higher relative to week 2 in both groups. TH and WBLH aBMD did not return to week 2 values in WWH but did in REF (TH postlactation versus week 2: WWH [n = 61] -3.1% [95% CI, -3.6 to -2.6]; REF [n = 29] +0.1% [95% CI, -0.9 to +1.1]). These data show accentuated bone loss during lactation and only partial skeletal recovery by 3 months postlactation in Ugandan WWH on tenofovir-based ART. Studies are ongoing to understand longer-term consequences for bone health. © 2020 The Authors. Journal of Bone and Mineral Research published by American Society for Bone and Mineral Research.

Osteoporosis Treatments Affect Bone Matrix Maturation in a Rat Model of Induced Cortical Remodeling

To test how osteoporosis drugs affect bone matrix maturation during cortical bone remodeling, 72 pregnant rats were switched from a 0.4% to a 0.01% calcium diet at parturition for a 23‐day lactation period. At weaning, eight dams were sacrificed to establish baseline values, while the remaining dams were returned to 0.4% calcium and treated with vehicle (saline), sodium fluoride (NaF), zoledronic acid (ZA), or sclerostin antibody (Scl‐Ab) for either 7 or 28 days (eight animals per group per time point). Femora were examined by μCT, dynamic histomorphometry, Fourier transform infrared imaging, and three‐point bending of notched specimens. Cortical porosity decreased in all groups from baseline to day 28. Intracortical mineralizing surface (MS/BS) and mineral apposition rate (MAR), as well as the mineral‐to‐matrix ratio were unaffected by treatment, but intracortical crystallinity was increased in the ZA group at day 10 compared with vehicle. Cortical area increased in all groups over 28 days mainly because of an addition of bone at the endocortical surface. Endocortical MS/BS did not vary among the groups, but endocortical MAR was suppressed in the NaF group at day 2 and elevated in the Scl‐Ab group at day 4 compared with vehicle. Endocortical mineral‐to‐matrix ratio was increased at days 5 and 10 following NaF treatment and endocortical crystallinity was increased at day 5 following ZA treatment compared with vehicle. Fracture toughness did not differ among the groups. Thus, the treatments affected matrix maturation more strongly at the endocortical then intracortical envelope. In this model of induced remodeling, the bone formation phase is synchronized at multiple sites, facilitating study of the effects of drugs or other bone‐targeting agents on matrix maturation independent of their effects on the initiation of remodeling. © 2020 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Perceptions of Pregnancy and Lactation from the Pregnancy and Lactation Autoimmune Network Registry

OBJECTIVE

The Pregnancy and Lactation Autoimmune Network (PLAN) registry was established to evaluate the concerns of women with autoimmune or inflammatory rheumatic diseases (AIRD) pertaining to pregnancy and lactation.

METHODS

The registry was started as a survey of patients with AIRD at a single rheumatology specialty center in November 2016 and included questions regarding fertility, pregnancy, miscarriages, and lactation before and after diagnosis.

RESULTS

The study included 154 subjects from the PLAN registry. More than half (52%) of respondents indicated that their diagnosis negatively changed their views on pregnancy and nearly a third (30%) decided not to have children after AIRD diagnosis. Most (66%) women were concerned that medication use during the childbearing process would affect the baby. One-third (34%) indicated their views on breastfeeding negatively changed as a result of their disease diagnosis. The rates and duration of breastfeeding did not differ significantly for babies born before or after the mothers' diagnosis (p = 0.50 and p = 0.21, respectively). Eighteen women in our study avoided breastfeeding or stopped breastfeeding earlier than planned to start a medication (including etanercept, adalimumab, hydroxychloroquine, and certolizumab) they believed to be contraindicated during lactation. The PLAN registry included 19 women who breastfed 22 babies while being exposed to a disease-modifying antirheumatic drug or biologic. None of these 19 women reported a delay in their children's developmental milestones or higher infection rates.

CONCLUSION

This study highlights an unmet need in patients with AIRD of childbearing potential for data and education regarding pregnancy and lactation.

Absence of Calcitriol Causes Increased Lactational Bone Loss and Lower Milk Calcium but Does Not Impair Post-lactation Bone Recovery in Cyp27b1 Null Mice

We hypothesized that adaptation to calcium supply demands of pregnancy and lactation do not require calcitriol. Adult Cyp27b1 null mice lack calcitriol and have hypocalcemia, hypophosphatemia, and rickets. We studied wild-type (WT) and null sister pairs raised on a calcium-, phosphorus-, and lactose-enriched "rescue" diet that prevents hypocalcemia and rickets. Bone mineral content (BMC) increased >30% in pregnant nulls, declined 30% during lactation, and increased 30% by 4 weeks post-weaning. WT showed less marked changes. Micro-CT revealed loss of trabecular bone and recovery in both genotypes. In lactating nulls, femoral cortical thickness declined >30%, whereas endocortical perimeter increased; both recovered to baseline after weaning; there were no such changes in WT. Histomorphometry revealed a profound increase in osteoid surface and thickness in lactating nulls, which recovered after weaning. By three-point bend test, nulls had a >50% decline in ultimate load to failure that recovered after weaning. Although nulls showed bone loss during lactation, their milk calcium content was 30% lower compared with WT. Serum parathyroid hormone (PTH) was markedly elevated in nulls at baseline, reduced substantially in pregnancy, but increased again during lactation and remained high post-weaning. In summary, pregnant Cyp27b1 nulls gained BMC with reduced secondary hyperparathyroidism, implying increased intestinal calcium delivery. Lactating nulls lost more bone mass and strength than WT, accompanied by increased osteoid, reduced milk calcium, and worsened secondary hyperparathyroidism. This implies suboptimal intestinal calcium absorption. Post-weaning, bone mass and strength recovered to baseline, whereas BMC exceeded baseline by 40%. In conclusion, calcitriol-independent mechanisms regulate intestinal calcium absorption and trabecular bone metabolism during pregnancy and post-weaning but not during lactation; calcitriol may protect cortical bone during lactation. © 2017 American Society for Bone and Mineral Research.

Adaptations in the Microarchitecture and Load Distribution of Maternal Cortical and Trabecular Bone in Response to Multiple Reproductive Cycles in Rats

Pregnancy, lactation, and weaning result in dramatic changes in maternal calcium metabolism. In particular, the increased calcium demand during lactation causes a substantial degree of maternal bone loss. This reproductive bone loss has been suggested to be largely reversible, as multiple clinical studies have found that parity and lactation history have no adverse effect on postmenopausal fracture risk. However, the precise effects of pregnancy, lactation, and post-weaning recovery on maternal bone structure are not well understood. Our study aimed to address this question by longitudinally tracking changes in trabecular and cortical bone microarchitecture at the proximal tibia in rats throughout three cycles of pregnancy, lactation, and post-weaning using in vivo μCT. We found that the trabecular thickness underwent a reversible deterioration during pregnancy and lactation, which was fully recovered after weaning, whereas other parameters of trabecular microarchitecture (including trabecular number, spacing, connectivity density, and structure model index) underwent a more permanent deterioration, which recovered minimally. Thus, pregnancy and lactation resulted in both transient and long-lasting alterations in trabecular microstructure. In the meantime, multiple reproductive cycles appeared to improve the robustness of cortical bone (resulting in an elevated cortical area and polar moment of inertia), as well as increase the proportion of the total load carried by the cortical bone at the proximal tibia. Taken together, changes in the cortical and trabecular compartments suggest that whereas rat tibial trabecular bone appears to be highly involved in maintaining calcium homeostasis during female reproduction, cortical bone adapts to increase its load-bearing capacity, allowing the overall mechanical function of the tibia to be maintained. © 2017 American Society for Bone and Mineral Research.

The Skeleton Is a Storehouse of Mineral That Is Plundered During Lactation and (Fully?) Replenished Afterwards

During lactation, mammals resorb mineral from the maternal skeleton to provide calcium to milk. Rodents lose 25% to 35% of skeletal ash weight, ash calcium content, and bone mineral content as measured by dual-energy X-ray absorptiometry (DXA), and have compromised material properties of bone as assessed by crushing vertebrae and 3-point bend tests of femora or tibias. The strength, stiffness, and toughness of vertebrae, femora, and tibias are reduced by as much as 60%. The effects of lactation are not uniform throughout the skeleton, but instead resorption is much more marked in the trabecular-rich spine than in the appendicular skeleton or whole body. Women who breastfeed exclusively lose an average of 210 mg calcium in milk each day, whereas nursing of twins or triplets can double and triple the output of calcium. Clinical data are also consistent with skeletal calcium being released during lactation to provide much of the calcium needed for milk production. Lumbar spine bone mineral density (BMD), as assessed by DXA, declines by a mean of 5% to 10% among numerous studies during 3 to 6 months of exclusive lactation, whereas largely cortical sites (hip, forearm, whole body) show half that loss or no significant changes. Micro-CT of rodents and high-resolution peripheral quantitative computed tomography (HR-pQCT) imaging of women confirm that lactation causes microarchitectural deterioration of bone. These skeletal losses occur through two pathways: upregulated osteoclast-mediated bone resorption and osteocytic osteolysis, in which osteocytes remove mineral from their lacunae and pericanalicular spaces. After weaning, the skeleton is fully restored to its prior mineral content and strength in both animal models and humans, despite persistent microarchitectural changes observed in high-resolution imaging. Osteoblasts upregulate to lay down new osteoid, while osteocytes remineralize their surroundings. The factors that stimulate this post-weaning skeletal recovery remain unclear. In most studies, a history of lactation does not increase the risk, but may protect against, low BMD and fragility fractures. © 2017 American Society for Bone and Mineral Research.

Irreversible Deterioration of Cortical and Trabecular Microstructure Associated With Breastfeeding

Estrogen deficiency associated with menopause is accompanied by an increase in the rate of bone remodeling and the appearance of a remodeling imbalance; each of the greater number of remodeling transactions deposits less bone than was resorbed, resulting in microstructural deterioration. The newly deposited bone is also less completely mineralized than the older bone resorbed. We examined whether breastfeeding, an estrogen-deficient state, compromises bone microstructure and matrix mineral density. Distal tibial and distal radial microarchitecture were quantified using high-resolution peripheral quantitative computed tomography in 58 women before, during, and after breastfeeding and in 48 controls during follow-up of 1 to 5 years. Five months of exclusive breastfeeding increased cortical porosity by 0.6% (95% confidence interval [CI] 0.3-0.9), reduced matrix mineralization density by 0.26% (95% CI 0.12-0.41) (both p < 0.01), reduced trabecular number by 0.22 per mm (95% CI 0.15-0.28), and increased trabecular separation by 0.07 mm (95% CI 0.05-0.08) (all p < 0.001). Relative to prebreastfeeding, at a median of 2.6 years (range 1 to 4.8) after cessation of breastfeeding, cortical porosity remained 0.58 SD (95% CI 0.48-0.68) higher, matrix mineralization density remained 1.28 SD (95% CI 1.07-1.49) lower, and trabeculae were 1.33 SD (95% CI 1.15-1.50) fewer and 1.06 SD (95% CI 0.91-1.22) more greatly separated (all p < 0.001). All deficits were greater than in controls. The results were similar at distal radius. Bone microstructure may be irreversibly deteriorated after cessation of breastfeeding at appendicular sites. Studies are needed to establish whether this deterioration compromises bone strength and increases fracture risk later in life. © 2016 American Society for Bone and Mineral Research.

Lactation-Induced Changes in the Volume of Osteocyte Lacunar-Canalicular Space Alter Mechanical Properties in Cortical Bone Tissue

Osteocytes can remove and remodel small amounts of their surrounding bone matrix through osteocytic osteolysis, which results in increased volume occupied by lacunar and canalicular space (LCS). It is well established that cortical bone stiffness and strength are strongly and inversely correlated with vascular porosity, but whether changes in LCS volume caused by osteocytic osteolysis are large enough to affect bone mechanical properties is not known. In the current studies we tested the hypotheses that (1) lactation and postlactation recovery in mice alter the elastic modulus of bone tissue, and (2) such local changes in mechanical properties are related predominantly to alterations in lacunar and canalicular volume rather than bone matrix composition. Mechanical testing was performed using microindentation to measure modulus in regions containing solely osteocytes and no vascular porosity. Lactation caused a significant (∼13%) reduction in bone tissue-level elastic modulus (p < 0.001). After 1 week postweaning (recovery), bone modulus levels returned to control levels and did not change further after 4 weeks of recovery. LCS porosity tracked inversely with changes in cortical bone modulus. Lacunar and canalicular void space increased 7% and 15% with lactation, respectively (p < 0.05), then returned to control levels at 1 week after weaning. Neither bone mineralization (assessed by high-resolution backscattered scanning electron microscopy) nor mineral/matrix ratio or crystallinity (assessed by Raman microspectroscopy) changed with lactation. Thus, changes in bone mechanical properties induced by lactation and recovery appear to depend predominantly on changes in osteocyte LCS dimensions. Moreover, this study demonstrates that tissue-level cortical bone mechanical properties are rapidly and reversibly modulated by osteocytes in response to physiological challenge. These data point to a hitherto unappreciated role for osteocytes in modulating and maintaining local bone mechanical properties. © 2016 American Society for Bone and Mineral Research.

Upregulation of calcitriol during pregnancy and skeletal recovery after lactation do not require parathyroid hormone

Pregnancy invokes a doubling of intestinal calcium absorption whereas lactation programs skeletal resorption to provide calcium to milk. Postweaning bone formation restores the skeleton's bone mineral content (BMC), but the factors that regulate this are not established. We used Pth-null mice to test whether parathyroid hormone (PTH) is required for postweaning skeletal recovery. On a normal 1% calcium diet, wild-type (WT) and Pth-null mice each gained BMC during pregnancy, declined 15% to 18% below baseline during lactation, and restored the skeleton above baseline BMC within 14 days postweaning. A 2% calcium diet reduced the lactational decline in BMC without altering the gains achieved during pregnancy and postweaning. The hypocalcemia and hyperphosphatemia of Pth-null mice normalized during lactation and serum calcium remained normal during postweaning. Osteocalcin and propeptide of type 1 collagen (P1NP) each rose significantly after lactation to similar values in WT and Pth-null. Serum calcitriol increased fivefold during pregnancy in both genotypes whereas vitamin D binding protein levels were unchanged. Absence of PTH blocked a normal rise in fibroblast growth factor-23 (FGF23) during pregnancy despite high calcitriol. A 30-fold higher expression of Cyp27b1 in maternal kidneys versus placenta suggests that the pregnancy-related increase in calcitriol comes from the kidneys. Conversely, substantial placental expression of Cyp24a1 may contribute significantly to the metabolism of calcitriol. In conclusion, PTH is not required to upregulate renal expression of Cyp27b1 during pregnancy or to stimulate recovery from loss of BMC caused by lactation. A calcium-rich diet in rodents suppresses skeletal losses during lactation, unlike clinical trials that showed no effect of supplemental calcium on lactational decline in BMC.

A study of the component glycerides of cow and buffalo milk fats with reference to the possible mechanism of their production during lactation

The component glycerides of a cow and a buffalo milk fat have been studied quantitatively after resolving them by intensive crystallization from acetone or ether at temperatures between — 50 and 0°C into groups of glycerides which, although still complicated in fatty acid composition, contain much simplified mixtures of mixed glycerides as compared with the original fats. From the component acids of each separated glyceride group and of the trisaturated glycerides present therein, it has been possible to produce an approximately quantitative statement of the many individual mixed glycerides present in the original fats, and a more closely quantitative statement of the various glyceride categories (trisaturated, mono- or di-unsaturated, mono- or di-palmito, mono- or di-oleo, etc.) present. The bearing of the data so obtained on the question of the mechanism of the production of the characteristic short-chain acyl glycerides of ruminant milk fats is discussed. Evidence is collected which favours the view that the main source of milk fats is glycerides in the blood entering the lactating gland, the oleo- (or other unsaturated) groups in which are transformed in greater or less degree into acyl groups with shorter carbon chains (down to butyric, C 4 ). It is shown that the alternative hypothesis of direct synthesis in the mammary gland of short-chain glycerides from carbohydrate or acetate is inconsistent with the quantitative com­position of the component acids of milk fats of a range of non-ruminant as well as ruminant mammals. On the other hand, it is considered that the maximal production of short-chain acyl glycerides observed in milk fats of the ruminant animals is clearly connected with the presence of unusual amounts of acetic and associated lower fatty acids in their blood, and that this causes conditions (pH range or other factor) in the lactating gland which favour the action of the enzyme system responsible for the conversion of oleo- into short-chain glycerides.

EFFECT IN THE COW OF INTRARUMINAL INFUSIONS OF VOLATILE FATTY ACIDS AND OF LACTIC ACID ON THE SECRETION OF THE COMPONENT FATTY ACIDS OF THE MILK FAT AND ON THE COMPOSITION OF BLOOD

1. The effects in the cow of intraruminal infusions of acetic acid, propionic acid or butyric acid on the secretion of the component fatty acids of the milk fat, and of these acids and of lactic acid on the composition of the blood plasma of the jugular vein, have been studied. 2. The infusion of acetic acid or butyric acid increased the yield of the C(4)-C(16) acids of milk fat but decreased the yield of C(18) acids. The infusion of propionic acid decreased the yields of all major component acids except palmitic acid and possibly lauric acid. 3. The changes in the concentrations in blood plasma of glucose and of ketone bodies were consistent with the glucogenic effect of propionic acid and the ketogenic effects of butyric acid and acetic acid. The effects of lactic acid were not consistent from cow to cow. Only with the infusion of acetic acid was a significant increase in the concentration of total volatile fatty acids in blood plasma found. Infusions of butyric acid and of propionic acid tended to depress the concentration of citric acid in the blood plasma and infusion of acetic acid increased it. No consistent effects of the infused acids on the concentration in blood plasma of esterified cholesterol, free cholesterol, triglyceride or phospholipid were observed. 4. The possibility is discussed that the effects of the infused acids on milk-fat secretion are caused through an alteration of the concentrations of precursors of milk fat in mammary arterial blood.

QUANTITATIVE ASPECTS OF RELEASE OF OXYTOCIN BY SUCKLING IN UNANAESTHETIZED RABBITS

A method of estimating oxytocin output following physiological stimuli is described which is applicable to experiments in conscious, undisturbed and unrestrained animals. The method consists of recording the uterine responses in puerperal rabbits with a permanently inserted intrauterine balloon as an index of neurohypophysial activation.

Previous work has been confirmed on the essential role of oxytocin in milk ejection in rabbits. Only negligible amounts of milk are obtained by the young without any endogenous or exogenous oxytocin, but normal release of oxytocin can occur in spite of diminished or completely inhibited milk flow.

The amount of oxytocin released during suckling depends on the strength of the stimulus in a characteristic fashion, and the strength of stimuli again depends both on the number of young suckling simultaneously and on their physical maturity.

In the early puerperium (up to 3–4 days post-partum) each suckling young evokes a response comparable to that of 0.5–1.0 mU of oxytocin injected intravenously, whether suckling alone or together with its litter-mates. Only a few grams of milk are removed by each young at that time. After this period, the suckling of 1 or 2 young separately still elicits the release of only a small amount of oxytocin (1–2 mU), but during suckling of a whole litter from 50 to 100 mU are liberated, which corresponds to 10–15 mU for each young. The full milk yield can only be obtained by each young while suckling simultaneously with the whole litter.

The amount of oxytocin released is independent of the duration of suckling, of the quantity of milk available, and of the milk flow. Once lactation has been established, the oxytocin output during suckling on an »once-daily« regime remains fairly constant at least for a large part of the lactation period. It is possible that at the end of the lactating period, the amount secreted during suckling is increased to about 250 mU or even more.

THREE TYPES OF MAMMARY TUMOUR INDUCED IN RATS BY FEEDING WITH DMBA

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CONTRACTION OF LACTATING RAT MAMMARY GLAND IN RESPONSE TO DIRECT MECHANICAL STIMULATION

Experiments were performed to determine whether the mammary gland myoepithelium of the lactating rat responds to direct mechanical stimulation. Rat pups on postpartum day 14 obtained 8.6 and 12.2% of the total extractable milk in the mammary glands in 31–41 min from anesthetized spinal cord-sectioned and sham-operated mothers, respectively. The mammary glands of both groups responded with a rise in intramammary pressure following mechanical stimulation of the skin overlying the cannulated mammary gland. The responses occurred after a latent period of 1–2 sec, attained an amplitude of 1–13 cm H2O, and lasted 8–18 sec. Summation of the responses did not occur. Temporary fatigue of the response could be induced with repetitive stimuli. Stimulation of mammary glands contralateral to or adjacent to the cannulated gland never produced a response in the cannulated gland. The exposed mammary gland responded to lightly applied mechanical stimulation with alveolar contractions confined to the area of stimulus contact. Each rat responded to intravenous oxytocin (25 mU) with a 10–29 cm H2O intramammary pressure rise of 4–31 min duration.

EFFECT OF PREGNANCY AND LACTATION ON GROWTH OF MAMMARY TUMOURS INDUCED BY 7,12-DIMETHLBENZ(A) ANTHRACENE (DMBA)

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UPTAKE OF FREE PLASMA AMINO ACIDS BY THE LACTATING COW'S UDDER AND AMINO ACID COMPOSITION OF UDDER LYMPH

1. Total alpha-amino N and the amounts of 24 ninhydrin-positive substances were determined in several samples of plasma and lymph from the cow's udder. The arteriovenous differences of these substances across the mammary glands were measured in several experiments performed on lactating cows and in one experiment on a ;dry' cow. Udder lymph obtained from live lactating cows by a lymph fistula and taken after killing lactating cows was analysed. 2. The concentrations of the individual free amino acids in udder lymph obtained from the live cow were similar to those found in cow's plasma. The concentrations of many amino acids in udder lymph taken immediately after death were two- to four-fold higher than those of the corresponding amino acids in udder lymph obtained from the live cow. 3. Most amino acids of the blood showed a considerable decrease in concentration by passage across the lactating mammary gland. Ornithine, a non-casein amino acid, showed arteriovenous differences of up to 60% of the arterial plasma concentration. No substantial amino acid uptake by the udder could be demonstrated in the experiment on the non-lactating cow. 4. The arteriovenous differences obtained for arginine, glutamine, isoleucine, leucine, lysine, valine, threonine and histidine were probably large enough to provide all the respective amino acid residues in milk protein. 5. The uptake of aspartic acid, asparagine, glutamic acid, serine and proline by the lactating cow's udder was not sufficient to account for all these respective amino acid residues found in milk protein.

IMMUNOASSAY FOR HUMAN CHORIONIC "GROWTH HORMONE-PROLACTIN" IN SERUM AND URINE

Chorionic "growth hormone-prolactin" in serum and urine from pregnant women, in serum from umbilical cord, and in amniotic fluid was assayed by a sensitive immunological method dependent on the combination of rabbit antiserum to the growth hormone-prolactin with the iodine-131-labeled hormone. The hormone is detectable in serum and urine early in gestation, and with advancing pregnancy its concentration in serum continues to increase to a maximum during the last trimester. It was not found in serum within 8 hours after delivery.