Regulation of serum calcitriol by serum ionized calcium in rats during pregnancy and lactation

Renal calcium and magnesium handling during pregnancy: modeling and analysis

Pregnancy is associated with elevated demand of most nutrients, with many trace elements and minerals critical for the development of fetus. In particular, calcium (Ca2+) and magnesium (Mg2+) are essential for cellular function, and their deficiency can lead to impaired fetal growth. A key contributor to the homeostasis of these ions is the kidney, which in a pregnant rat undergoes major changes in morphology, hemodynamics, and molecular structure. The goal of this study is to unravel the functional implications of these pregnancy-induced changes in renal handling of Ca2+ and Mg2+, two cations that are essential in a healthy pregnancy. To achieve that goal, we developed computational models of electrolyte and water transport along the nephrons of a rat in mid and late pregnancy. Model simulations reveal a substantial increase in the reabsorption of Mg2+ along the proximal tubules and thick ascending limbs. In contrast, the reabsorption of Ca2+ is increased in the proximal tubules but decreased in the thick ascending limbs, due to the lower transepithelial concentration gradient of Ca2+ along the latter. Despite the enhanced transport capacity, the marked increase in glomerular filtration rate results in elevated urinary excretions of Ca2+ and Mg2+ in pregnancy. Furthermore, we conducted simulations of hypocalcemia and hypomagnesemia. We found that hypocalcemia lowers Ca2+ excretion substantially more than Mg2+ excretion, with this effect being more pronounced in virgin rats than in pregnant ones. Conversely, hypomagnesemia reduces the excretion of Mg2+ and Ca2+ to more similar degrees. These differences can be explained by the greater sensitivity of the calcium-sensing receptor (CaSR) to Ca2+ compared with Mg2+.NEW & NOTEWORTHY A growing fetus' demands of minerals, notably calcium and magnesium, necessitate adaptations in pregnancy. In particular, the kidney undergoes major changes in morphology, hemodynamics, and molecular structure. This computational modeling study provides insights into how these pregnancy-induced renal adaptation impact calcium and magnesium transport along different nephron segments. Model simulations indicate that, despite the enhanced transport capacity, the marked increase in glomerular filtration rate results in elevated urinary excretions of calcium and magnesium in pregnancy.

Effects of progesterone and interferon tau on ovine endometrial phosphate, calcium, and vitamin D signaling

Given recent reports of expression of postnatal mineral transport regulators at the maternal-conceptus interface during the peri-implantation period, this study tested the hypothesis that progesterone (P4) and/or interferon tau (IFNT) regulate phosphate, calcium, and vitamin D signaling in the ovine endometrium. Mature Rambouillet ewes (n = 24) were surgically fitted with intrauterine catheters on Day 7 of the estrous cycle. Ewes received daily intramuscular injections of 50 mg P4 in corn oil vehicle and/or 75 mg progesterone receptor antagonist (RU486) in corn oil from Days 8-15, and twice daily intrauterine injections of either control proteins (CX) or IFNT (25 μg/uterine horn/day) from Days 11-15 resulting in four treatment groups: P4 + CX; P4 + IFNT; RU486 + P4 + CX; and RU486 + P4 + IFNT. On Day 16, ewes were hysterectomized. RU486 + P4 + CX treated ewes had lower concentrations of 25 (OH) D in plasma than P4 + CX treated ewes (P < 0.05). Endometria from ewes treated with IFNT had greater expression of FGF23 (P < 0.01), S100A9 (P < 0.05), and S100A12 (P = 0.05) mRNAs, and lower expression of ADAM10 mRNA (P < 0.01) compared to ewes treated with CX proteins. Expression of FGF23 mRNA was greater in endometria of ewes that received RU486 + P4 + IFNT compared to ewes that received RU486 + P4 + CX (hormone x protein Interaction, P < 0.05). Expression of S100G mRNA was greater in endometria of ewes that received P4 + IFNT compared to ewes that received RU486 + P4 + IFNT (P < 0.05; hormone x protein Interaction, P < 0.01). These data implicate P4 and IFNT in the regulation of phosphate, calcium, and vitamin D signaling during the peri-implantation period of pregnancy and provide a platform for continued mechanistic investigations. Summary Sentence: Progesterone and interferon tau regulate phosphate, calcium, and vitamin D signaling during the ovine peri-implantation period.

Phosphate, Calcium, and Vitamin D: Key Regulators of Fetal and Placental Development in Mammals

Normal calcium and bone homeostasis in the adult is virtually fully explained by the interactions of several key regulatory hormones, including parathyroid hormone, 1,25 dihydroxy vitamin D3, fibroblast growth factor-23, calcitonin, and sex steroids (estradiol and testosterone). In utero, bone and mineral metabolism is regulated differently from the adult. During development, it is the placenta and not the fetal kidneys, intestines, or skeleton that is the primary source of minerals for the fetus. The placenta is able to meet the almost inexhaustible needs of the fetus for minerals by actively driving the transport of calcium and phosphorus from the maternal circulation to the growing fetus. These fundamentally important minerals are maintained in the fetal circulation at higher concentrations than those in maternal blood. Maintenance of these inordinately higher fetal levels is necessary for the developing skeleton to accrue sufficient minerals by term. Importantly, in livestock species, prenatal mineralization of the skeleton is crucial for the high levels of offspring activity soon after birth. Calcium is required for mineralization, as well as a plethora of other physiological functions. Placental calcium and phosphate transport are regulated by several mechanisms that are discussed in this review. It is clear that phosphate and calcium metabolism is intimately interrelated and, therefore, placental transport of these minerals cannot be considered in isolation.

Lacunar-canalicular bone remodeling: Impacts on bone quality and tools for assessment

Osteocytes can resorb as well as replace bone adjacent to the expansive lacunar-canalicular system (LCS). Suppressed LCS remodeling decreases bone fracture toughness, but it is unclear how altered LCS remodeling impacts bone quality. The first goal of this review is to assess how LCS remodeling impacts LCS morphology as well as the composition and mechanical properties of surrounding bone tissue. The second goal is to compare tools available for the assessment of bone quality at length-scales that are physiologically-relevant to LCS remodeling. We find that changes to LCS morphology occur in response to a variety of physiological conditions and diseases and can be classified in two general phenotypes. In the 'aging phenotype', seen in aging and in some disuse models, the LCS is truncated and osteocytes apoptosis is increased. In the 'osteocytic osteolysis' phenotype, which is adaptive in some physiological settings and possibly maladaptive in others, the LCS enlarges and osteocytes generally maintain viability. Bone composition and mechanical properties vary near the osteocyte and change with at least some conditions that alter LCS morphology. However, few studies have evaluated bone composition and mechanical properties close to the LCS and so the impacts of LCS remodeling phenotypes on bone tissue quality are still undetermined. We summarize the current understanding of how LCS remodeling impacts LCS morphology, tissue-scale bone composition and mechanical properties, and whole-bone material properties. Tools are compared for assessing tissue-scale bone properties, as well as the resolution, advantages, and limitations of these techniques.

Calcium restriction during lactation has minimal effects on post-weaning mineral metabolism and bone recovery

Dietary calcium (Ca) restriction during lactation in the rat, which induces intra-cortical and endocortical remodeling, has been proposed as a model to study bone matrix maturation in the adult skeleton. The purpose of this study was to assess the effects of dietary Ca restriction during lactation on post-weaning mineral metabolism and bone formation. Mated female Sprague-Dawley rats were randomized into groups receiving either 0.6% Ca (lactation/normal Ca) or 0.01% Ca (lactation/low Ca) diets during lactation. Virgin animals fed normal Ca were used as controls (virgin/normal Ca). At the time of weaning, animals on the low Ca diet were returned to normal Ca and cohorts of all three groups were sacrificed at days 0, 1, 2, 7, and 14 post-weaning. Lactation caused bone loss, particularly at the endocortical surface, but the amount was not affected by dietary Ca. Rats in the lactation/low Ca group had increased cortical porosity compared to the other groups, particularly within the size range of secondary osteons. Dietary Ca restriction during lactation did not affect post-weaning bone formation kinetics or serum Ca and phosphate levels. In both lactation groups, there was a transient increase in phosphate and fibroblast growth factor 23 (FGF23) post-weaning, which trended toward virgin/normal Ca levels over time. Thus, the additional challenge of low dietary Ca during lactation to induce intra-cortical remodeling in the rat has minimal effects on bone formation kinetics and mineral metabolism during the post-weaning period, providing further justification for this model to study matrix maturation in the adult skeleton.

Expression of prolactin receptors in the duodenum, kidneys and skeletal system during physiological and sulpiride-induced hyperprolactinaemia

INTRODUCTION AND AIM

Hyperprolactinaemia in pregnancy leads to mild and reversible changes in the maternal skeletal system, and medicamentous hyperprolactinemia causes more detrimental effects. We conducted an experimental study to evaluate differences between Prlr gene expression in the duodenum, vertebrae and kidneys during physiological and medicamentous hyperprolactinaemia, which could influence calcium homeostasis.

METHODS

Experimental animals (18 weeks old, Wistar female rats) were divided as follows: group P (nine rats that were 3 weeks pregnant), group M (ten rats that were intramuscularly administrated sulpiride (10 mg/kg) twice daily for 3 weeks), and the control group (C, ten age-matched nulliparous rats, 18-week-old). Laboratory investigations included measurements of serum ionized calcium, phosphorus, urinary calcium and phosphorus excretion, osteocalcin (OC), serum procollagen type 1 N-terminal propeptide (P1NP), vitamin D, parathyroid hormone (PTH) and prolactin (PRL). Relative quantification of gene expression for prolactin receptors in the duodenum, vertebrae and kidneys was determined using real-time PCR.

RESULTS

Expression of the Prlr gene was significantly higher in the duodenum (p < 0.001) and lower in vertebrae (p < 0.001) and kidneys (p < 0.01) in rats with physiological hyperprolactinaemia (PHP) than in the control group. Significantly lower Prlr expression in the duodenum was verified (p < 0.001), along with increased Prlr gene expression in vertebrae (p < 0.001) and kidneys (p < 0.01), in rats with medicamentous hyperprolactinaemia (MHP) than in the C group.

CONCLUSIONS

Downregulation of Prlr gene expression in the duodenum may explain the diminished intestinal calcium absorption in medicamentous hyperprolactinaemia. Prolactin takes calcium from the skeletal system following increased Prlr gene expression in the vertebrae to maintain calcium homeostasis, which increases the harmful effect on bone metabolism compared to that of physiological hyperprolactinaemia.

Serum 25-hydroxyvitamin D levels throughout pregnancy: a longitudinal study in healthy and preeclamptic pregnant women

BACKGROUND

Worldwide there is a high prevalence of 25-hydroxyvitamin D (25OHD) deficiency and has been associated with adverse outcomes during pregnancy.

OBJECTIVE

This is a nested, case-control study in a longitudinal cohort to compare the serum 25OHD levels and other biomarkers throughout pregnancy in a group of 20 preeclamptic women and 61 healthy pregnant women. An additional group of 29 healthy non-pregnant women were also studied during the two phases of the menstrual cycle.

RESULTS

Mean 25OHD levels in non-pregnant women were 31.9 ng/mL and 34.9 ng/mL during follicular and luteal phase, respectively (P < 0.01). Mean serum 25OHD levels in healthy pregnant women were 26.5, 30.1 and 31.9 ng/mL, at first, second and third trimester, respectively (P < 0.001). The first trimester levels of 25OHD were lower than those of healthy non-pregnant women (P < 0.001), showing a significant recovery at third trimester. In the group of healthy pregnant women, the 25OHD levels were 25.7 ng/mL and 27.2 ng/mL at 3 and 6 months postpartum, respectively; both values were lower than those observed in the non-pregnant women (P < 0.001). In preeclamptic women, 25OHD serum levels were similar to those of healthy pregnant women; nevertheless, they remained almost unchanged throughout pregnancy.

CONCLUSION

There were no significant differences between healthy and preeclamptic pregnant women in terms of 25OHD levels throughout the pregnancy. Serum 25OHD levels in non-pregnant women were higher during luteal phase compared with follicular phase. The 25OHD levels of non-pregnant women tended to be higher than those of pregnant women.

Maternal Mineral and Bone Metabolism During Pregnancy, Lactation, and Post-Weaning Recovery

During pregnancy and lactation, female physiology adapts to meet the added nutritional demands of fetuses and neonates. An average full-term fetus contains ∼30 g calcium, 20 g phosphorus, and 0.8 g magnesium. About 80% of mineral is accreted during the third trimester; calcium transfers at 300-350 mg/day during the final 6 wk. The neonate requires 200 mg calcium daily from milk during the first 6 mo, and 120 mg calcium from milk during the second 6 mo (additional calcium comes from solid foods). Calcium transfers can be more than double and triple these values, respectively, in women who nurse twins and triplets. About 25% of dietary calcium is normally absorbed in healthy adults. Average maternal calcium intakes in American and Canadian women are insufficient to meet the fetal and neonatal calcium requirements if normal efficiency of intestinal calcium absorption is relied upon. However, several adaptations are invoked to meet the fetal and neonatal demands for mineral without requiring increased intakes by the mother. During pregnancy the efficiency of intestinal calcium absorption doubles, whereas during lactation the maternal skeleton is resorbed to provide calcium for milk. This review addresses our current knowledge regarding maternal adaptations in mineral and skeletal homeostasis that occur during pregnancy, lactation, and post-weaning recovery. Also considered are the impacts that these adaptations have on biochemical and hormonal parameters of mineral homeostasis, the consequences for long-term skeletal health, and the presentation and management of disorders of mineral and bone metabolism.

Adapting to the transition between gestation and lactation: differences between rat, human and dairy cow

Adequate blood calcium concentrations are vital for the normal function of mammals. Mechanisms for maintaining normal blood calcium function adequately most of the time; however, occasionally they fail and calcium homeostasis is compromised. Milk fever or periparturient hypocalcemia in dairy cattle is a well-documented example of a breakdown in the mechanisms of calcium homeostasis. This disease occurs at the time of parturition and is unique to adult dairy animals. The disease results from the inability of animals to cope with the sudden demand for calcium in support of colostrum formation. Animals developing the disease become hypocalcemic and require intravenous calcium to survive. The precise metabolic disorder(s) responsible for the onset of milk fever is still being debated. This report will highlight some of the current concepts related to the causes and prevention of milk fever in dairy cattle, as well as contrasting differences in calcium demands that exist between dairy cattle, humans and rats at the onset of lactation.

Bone mass changes in vivo during the entire reproductive cycle in rats feeding different dietary calcium and calcium/phosphorus ratio content

The purpose of this study was to quantify in vivo the impact of different dietary Ca contents on the maternal total skeleton and skeletal sub-areas in adult rats during pregnancy and lactation, using DXA. Twenty-four female Wistar rats (approximately 5 months old) were mated and divided into three groups (n = 8) and fed one of the following diets, varying only in Ca content (LCD: 0.14%, NCD: 0.6% or HCD: 1.2%). Pups were adjusted to 8-9 per dam. Maternal ionic calcium and in vivo bone mineral density (BMD) were measured at the beginning, after delivery and after weaning. Regardless of the diet, ionized calcium decreased from onset to weaning ( P < 0.05). At weaning, bone mass decreased 7.3% in NCD, 15% in LCD and 10.5% in HCD from initial values. Total skeleton, whole and proximal tibia and spine BMDs only decreased at delivery in the LCD group ( P < 0.05) but, irrespective of the diet, at weaning, they were lower compared to delivery and initial values ( P < 0.05). LCD group presented the lowest BMD in the proximal tibia and spine regions ( P < 0.05). At birth, pups did not present differences, however, at weaning, LCD pups reached the lowest body weight ( P < 0.05), NCD presented the highest body Ca content ( P < 0.05) and there were no differences between LCD and HCD. This in vivo study showed that regardless of the dietary calcium content, the maternal skeleton is slightly affected by pregnancy but severely affected by lactation. However, the degree of such response appears to depend not only on dietary Ca content but also on dietary Ca/P molar ratio.

Reduced fetal, placental, and amniotic fluid PTHrP in the growth-restricted spontaneously hypertensive rat

Evidence implicates pivotal roles for parathyroid hormone-related protein (PTHrP) in stimulating cell growth and differentiation, placental calcium transport, and placental vasodilatation. As spontaneously hypertensive rat (SHR) fetuses are growth restricted compared with those of its normotensive control, the Wistar Kyoto (WKY) rat, we examined intrauterine PTHrP and total and ionic calcium concentrations in these rats. Fetal plasma PTHrP concentrations, but not total calcium concentrations, were lower in the SHR compared with WKY (P < 0.05). SHR placental concentrations of PTHrP were lower than in WKY (P < 0.03) and failed to show the increase observed in WKY near term (P < 0.05). PTHrP concentrations in amniotic fluid from SHR were not raised near term and were lower compared with WKY (P < 0.0005). The increased ionic calcium concentrations in amniotic fluid in the WKY near term (P < 0.05) were not detected in the SHR. Thus SHR fetal plasma, placental, and amniotic fluid PTHrP concentrations were reduced and associated with fetal growth restriction. We suggest that PTHrP may play a role in the etiology of both growth restriction during pregnancy and hypertension later in life.

1Alpha-hydroxyvitamin D3 prevents the decrease of bone mineral density in lactating beagles

We assessed the change of bone mineral density (BMD) in lactating beagles with dual energy X-ray absorptiometry (DXA) and the preventive effect of 1alpha-hydroxyvitamin D3 (1alpha(OH)D3) on the BMD. Beagles, two to five years old, were used for detecting the time course change of BMD. Since the coefficient of variation (CV(%)) on detecting lumber vertebral (L2-L4) and tibial BMD by DXA was about 0.5%, DXA was useful to detect the change of BMD in beagles. There was a marked decrease in vertebral BMD during lactational period in the control group. The BMD levels after weaning were found to reverse to the initial level at mating. The same tendency was observed in tibial BMD as vertebral BMD, though the BMD changes were not marked. Beagles were administered at a dose of 0.1 microg/kg of 1alpha(OH)D3 three times in a week, and it was found to suppress the decrease in vertebral BMD during the breast feeding period. Also, the administration of 1alpha(OH)D3 promoted the prevention of decreased BMD during lactation both in vertebrae and tibiae. Significant effects of 1alpha(OH)D3 administration on tibial BMD were not observed. No adverse effects, such as hypercalcemia and hypercalciuria, were observed during the experimental period. Therefore, DXA was useful for detecting the changes of BMD in lactating beagles and the change of BMD was marked in lumber vertebrae, which are rich in trabecular bone. The preventive effect of 1alpha(OH)D3 on the decrease of BMD during the lactation period was observed in beagles.

Bone mass changes during pregnancy and lactation in the rat

We examined bone mass changes in the total, axial, and appendicular skeleton as well as in the different subareas of femur and tibia in rats fed on a normal calcium diet. A total of 16 virgin Wistar rats, approximately 5 months of age (270+/-30 g), were assigned to two groups of eight rats each. One group was mated and, for each pregnant rat, a nonpregnant control rat was studied simultaneously. Weaning was performed when the pups reached 38+/-3 g body weight. At the beginning (t = 0), on the first day postpartum (t = 22 days), and at weaning (t = 45 days), total skeleton bone mineral content (BMC), area, and bone mineral density (BMD) were determined by dual-energy X-ray absorptiometry (DXA) in vivo under anesthesia. Body weight increased significantly during pregnancy (p < 0.05) and decreased at weaning, whereas control rats showed a slow, gradual increment without reaching a significant difference. During pregnancy, BMC and area of the total skeleton increased significantly in pregnant rats, but the changes in BMD were not different compared with the control group. A completely different pattern was observed between groups during the 23 days of lactation. While the skeleton continued to grow in the control group (BMC and area increased), the total skeleton of lactating rats showed no change in area (size), small decreases in BMC, and a significant decrease in BMD (p < 0.05). Consequently, although BMC and BMD of both groups were similar at the time of delivery, BMC was 12.0% lower and BMD 4.9% lower at the end of lactation in the lactating rats compared with the control group. The contribution of the maternal skeleton to the lactation period was not similar; that is, the areas with the highest trabecular component showed the greater average differences in BMD at the time of weaning (proximal tibia -19.9%, distal femur -12.6%, spine -10.9%) (p < 0.05), compared with relatively minor, nonsignificant losses in areas where cortical bone predominates (distal tibia -5%, middle tibia -5.2%). Our experimental results demonstrated the usefulness of DXA in vivo to visualize changes in BMD during the reproductive cycle of the rat. Moreover, the data confirm that normal pregnancy in the rat appears to exert little influence on bone, whereas lactation induces significant bone loss, mainly in the areas of predominant trabecular bone.

Altered regulation of parathyroid hormone secretion by calcium in pregnant and lactating rats

We have previously shown that the serum parathyroid hormone (PTH) concentration in lactating (L) rats is not suppressed by high serum Ca2+ to the same extent as in nonmated (NM) rats. To investigate further Ca2+ regulation of PTH secretion, parathyroid cells from NM rats and rats in late pregnancy and at peak lactation were dispersed and incubated for 2 h in medium containing 0.52-2.05 mM Ca2+. Medium PTH was assayed with a homologous immunoradiometric assay (IRMA). At the two highest Ca2+ levels (1.81 and 2.05 mM), medium PTH was significantly higher (p = 0.031) for cells from L rats than for cells from NM rats. In contrast, significantly less (p < 0.001) PTH was secreted for the L group versus the NM group at medium Ca2+ values of 1.27 and 1.46 mM. Estimated set points for L and NM groups were 1.17 mM and 1.35 mM, respectively, corresponding closely to the prevailing serum Ca2+ for these two groups. Consistent with the present in vitro data, high serum PTH (> 40 pg/ml) in L rats occurred only at serum Ca2+ values below 1.27 mM. Elevated serum PTH at lower serum Ca2+ levels was also seen in pregnant rats. Dispersed parathyroid cells from 20- to 21-day pregnant rats secreted significantly more PTH (p = 0.028) than cells from NM rats at all Ca2+ levels tested (1.1-1.6 mM). In conclusion, the relationship between extracellular Ca2+ and PTH secretion is altered in rats during late pregnancy and at peak lactation, perhaps as part of the adaptation to the demands for calcium for pre- and postnatal growth.