1,25-Dihydroxyvitamin D3 and osteocalcin in maternal and fetal guinea pigs

Vitamin D Binding Protein: A Historic Overview

Vitamin D and all its metabolites are bound to a specific vitamin D binding protein, DBP. This protein was originally first discovered by its worldwide polymorphism and called Group-specific Component (GC). We now know that DBP and GC are the same protein and appeared early in the evolution of vertebrates. DBP is genetically the oldest member of the albuminoid family (including albumin, α-fetoprotein and afamin, all involved in transport of fatty acids or hormones). DBP has a single binding site for all vitamin D metabolites and has a high affinity for 25OHD and 1,25(OH)2D, thereby creating a large pool of circulating 25OHD, which prevents rapid vitamin D deficiency. DBP of higher vertebrates (not amphibians or reptiles) binds with very high affinity actin, thereby preventing the formation of polymeric actin fibrils in the circulation after tissue damage. Megalin is a cargo receptor and is together with cubilin needed to reabsorb DBP or the DBP-25OHD complex, thereby preventing the urinary loss of these proteins and 25OHD. The total concentrations of 25OHD and 1,25(OH)2D in DBP null mice or humans are extremely low but calcium and bone homeostasis remain normal. This is the strongest argument for claiming that the "free hormone hypothesis" also applies to the vitamin D hormone, 1,25(OH)2D. DBP also transports fatty acids, and can play a role in the immune system. DBP is genetically very polymorphic with three frequent alleles (DBP/GC 1f, 1s, and 2) but in total more than 120 different variants but its health consequences, if any, are not understood. A standardization of DBP assays is essential to further explore the role of DBP in physiology and diseases.

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.

High vitamin D status before conception, but not during pregnancy, is inversely associated with maternal gestational diabetes mellitus in guinea pigs

BACKGROUND

Whether there is a dose-dependent effect of maternal dietary cholecalciferol during pregnancy on maternal glucose tolerance is unknown. In addition, circulating osteocalcin is increased by 1,25-dihydroxyvitamin D [1,25(OH)2D] and may improve glucose homeostasis.

OBJECTIVE

This study was designed to test whether dietary cholecalciferol during pregnancy dose-dependently affects maternal glucose tolerance and maternal and neonatal glucose concentrations in relation to plasma osteocalcin and body composition.

METHODS

Female guinea pigs (n = 45; 4 mo old) were randomly assigned to 5 doses of cholecalciferol (0, 0.25, 0.5, 1, or 2 IU/g diet) fed from mating to delivery. Plasma vitamin D metabolites, minerals, and osteocalcin, and blood glucose were measured before mating, at midgestation (day 42), and at day 2 postpartum in sows and in 2-d-old pups. At day 50 of pregnancy (early third trimester), a 3-h oral-glucose-tolerance test (OGTT) (2 g/kg) was conducted. Body composition was measured before mating and at day 2 postpartum in sows and in pups.

RESULTS

A positive dose-response to dietary cholecalciferol was observed for change in maternal plasma 25-hydroxyvitamin D [25(OH)D] through pregnancy (P < 0.0001), with 1,25(OH)2D increasing by 198% in the 1-IU/g group by midgestation vs. a reduction of 43.6% in the 0-IU/g group (P = 0.05). Twenty-four (54.5%) sows had gestational diabetes mellitus (GDM) on the basis of nonfed glucose and 39 (88.6%) had GDM on the basis of 2-h OGTT glucose concentrations. There were no group differences in maternal OGTT or changes in glucose, minerals, osteocalcin concentrations, and body composition. Pre-mating 25(OH)D was inversely related to 3-h area under the curve for blood glucose from the OGTT (r = -0.31, P = 0.05). In guinea pig pups, although both 25(OH)D (P < 0.0001) and 1,25(OH)2D (P < 0.0001) followed a dose-response to maternal diet, glucose, osteocalcin, minerals, and body composition were not altered.

CONCLUSIONS

Dietary vitamin D intake during pregnancy in guinea pigs does not affect the already high rate of GDM, whereas higher prepregnancy vitamin D status appears to be protective.

Dietary vitamin D during pregnancy has dose-dependent effects on long bone density and architecture in guinea pig offspring but not the sows

BACKGROUND

The effects of vitamin D during pregnancy on maternal and neonatal bone health remain unclear.

OBJECTIVE

This study was designed to test whether dietary vitamin D dose-dependently affects maternal and neonatal bone health.

METHODS

Female guinea pigs (n = 45; 4 mo old) were randomly assigned at mating to receive 1 of 5 doses of vitamin D3 (cholecalciferol; 0, 0.25, 0.5, 1, or 2 IU/g diet) throughout pregnancy. Plasma vitamin D metabolites, mineral homeostasis, bone biomarkers, and bone mass were tested in sows throughout pregnancy and in 2-d-old pups. Microarchitecture and histology of excised bone were conducted postpartum.

RESULTS

By 3 wk of pregnancy, plasma 25-hydroxyvitamin D [25(OH)D] followed a positive dose-response, whereas 1,25-dihydroxyvitamin D [1,25(OH)2D] reached a plateau if vitamin D was ≥0.5 IU/g diet. Weight gain, areal bone mineral density (aBMD), volumetic bone mineral density (vBMD), and bone biomarkers did not differ among maternal groups. A positive dose-response was observed for mean ± SEM pup plasma concentrations of 25(OH)D (10.5 ± 1.50 to 113 ±11.6 nmol/L) and 1,25(OH)2D (123 ± 13.8 to 544 ± 53.3 pmol/L). Pup weight, plasma minerals, and osteocalcin were not different; plasma deoxypyridinoline was lower in the 1- and 0.25-IU/g groups than in all other groups. Pup femur aBMD was higher (9.2-13%; P = 0.04) in the 2-IU/g group than in all other groups except for the 0-IU/g group. Tibia and femur vBMD of pups responded to maternal diet in a U-shaped pattern. The femoral growth plate was 7.9% wider in the 0-IU/g group than in the 1-IU/g group.

CONCLUSIONS

Maternal vitamin D supplementation dose-dependently altered pup long bone architecture and mineral density in a manner similar to vitamin D deficient rickets whereas maternal bone was stable. These data reinforce that inadequate maternal vitamin D intake may compromise neonatal bone health and that exceeding recommendations is not advantageous.

Dietary supplementation with long chain polyunsaturated fatty acids in pregnant guinea pigs has sex-dependent effects on growth and bone outcomes in offspring

Long chain PUFA enhance bone mass in non-pregnant mammals. We examined the effects of arachidonic (AA; 20:4n-6) and docosahexaenoic (DHA; 22:6n-3) acid on bone mass of mothers and neonates. Guinea pig sows (n=15) were fed control, DHA or AA+DHA diets from mating to weaning. Measurements included: osteocalcin (OC), deoxypyridinoline (DPD), areal bone mineral density (aBMD) in sows and neonates; and volumetric density (vBMD) in neonates. Only vertebral aBMD and OC:DPD ratio declined during reproduction and only DHA reduced OC:DPD. Male pup weight was reduced by DHA and female weight elevated by AA+DHA. Whole body and femur aBMD were reduced by DHA and AA+DHA; whereas tibia vBMD was reduced by DHA in males. Female whole body, tibia and vertebrae aBMD plus tibia vBMD were elevated by AA+DHA; and DHA elevated whole body, tibia and vertebrae aBMD. Dietary AA+DHA and DHA elicit sex-dependent effects on neonatal bone, with minimal impact on mothers.

Postnatal vitamin D supplementation following maternal dietary vitamin D deficiency does not affect bone mass in weanling guinea pigs

Although vitamin D deficiency is common at birth, the consequences to growth and bone mass by weaning are unclear. This study was designed to determine whether maternal dietary vitamin D deficiency in pregnancy has a negative impact on the bone mass of full-term neonates and if postnatal supplementation could restore bone mass. Forty guinea pigs were randomized to receive a control (C) or deficient (D) diet (0.03 microg vs. 0.00 microg cholecalciferol/g) during pregnancy. Offspring were randomized at birth to receive 0.25 microg of cholecalciferol supplement (S) or a placebo (P) orally per day for 28 d. Measurements at birth and d 28 included whole body and regional bone mass and serum osteocalcin and deoxypyridinoline, plus biomechanical testing and peripheral quantitative computed tomography of excised tibias and femurs. Main and interactive effects were tested using mixed model ANOVA and post hoc Bonferroni's tests. At birth and d 28, offspring of the D sows had lower serum vitamin D and osteocalcin concentration, lower body weight, length, whole body and total tibia bone mineral content, and lower biomechanical integrity of tibia compared with those of the C sows, regardless of supplementation. Although postnatal vitamin D supplementation improved vitamin D status at d 28 in D offspring, values remained significantly lower than C groups. This study suggests that efforts should be made to optimize maternal vitamin D status in pregnancy, along with maintenance of vitamin D status in infancy, rather than relying on postnatal supplementation to normalize vitamin D status and bone mass.

Dietary calcium and phosphate restriction in guinea-pigs during pregnancy: fetal mineralization induces maternal hypocalcaemia despite increased 1α,25-dihydroxycholecalciferol concentrations

Guinea-pig fetuses at term are mineralized to a degree comparable with human fetuses, which makes the guinea-pig an attractive animal model to study maternal–fetal interactions with regard to Ca and phosphate (P) homeostasis. We studied non-pregnant and pregnant (day 57) vitamin D-replete guinea-pigs, fed either a normal guinea-pig chow with 9·6 g Ca/kg and 4·9 g P/kg or a study diet with 2 g Ca/kg and 1 g P/kg (low-Ca–P diet) for 7–8 weeks. Both pregnancy and the low-Ca–P diet decreased plasma concentrations of 25-hydroxycholecalciferol (25(OH)D3), but increased total and free 1α,25-dihydroxycholecalciferol (1,25(OH)2D3), strongly suggesting an additive stimulation of 1α-hydroxylase activity. Maternal and fetal 25(OH)D3and 1,25(OH)2D3levels were highly correlated (r0·82 and 0·92 respectively, P<0·001). Dual-energy absorption X-ray absorptiometry (DXA) showed that both pregnancy and the low-Ca–P diet decreased bone mineral density (BMD) of the maternal femur, particularly at the distal metaphysis. Despite higher 1,25(OH)2D3concentrations and lower BMD, pregnant animals on the low-Ca–P diet were hypocalcaemic; blood Ca2+levels were inversely correlated with the number of fetuses in this group (r-0·93, P<0·001). Fetal growth as well as mineralization (assessed by whole-body and femoral DXA, bone histomorphometry and plasma–bone osteocalcin measurements) were unaltered in the low-Ca–P group. In conclusion, fetal mineralization proceeds normally but induces maternal hypocalcaemia in guinea-pigs with dietary restriction of Ca and P.

Scurvy results in decreased collagen synthesis and bone density in the guinea pig animal model

The effect of severe ascorbic acid deficiency on bone remodeling and collagen synthesis was evaluated in a 21 day experiment, using the scorbutic guinea pig model. Animals (n = 6-7/group) were assigned to one of three groups: scorbutic, pair-fed ascorbic acid-replete, or ad libitum ascorbic acid-replete groups. After 2 weeks, scorbutic animals started voluntarily decreasing food intake and losing weight. By day 19-21, at which time bone and tissue samples were collected and analyzed, scorbutic animals decreased food intake to 46% of usual and lost 9% body weight. Serum 25OHD3, 1,25(OH)2D3, calcium, and albumin were significantly lower (p < 0.05) in the scorbutic animals than in the other groups. Bone mineral density and bone mineral content of the proximal and central femur were significantly lower in the scorbutic group than in the other groups (p < 0.05). Morphometric analysis of tibia indicated significantly lower bone volume, fewer and thinner trabeculae, and a thinner growth plate in the scorbutic group, compared to the pair-fed and ad libitum groups (p < 0.05). Osteoclast surface was about 60% higher in the scorbutic group than in the pair-fed and ad libitum control groups (0.05 < p < 0.10). Mechanical strength of the femur and lumbar vertebral body tended to be lower when bone mass was altered in the same group. Collagen synthesis of articular cartilage and tendons was lower in the scorbutic group than in the pair-fed or ad libitum groups (p < 0.05). In conclusion, scurvy but not food restriction, per se, results in alterations in bone mass and tissue collagen synthesis.