Circulating levels and function of 1,25-(OH)2D3 in lactation

Peri-parturient hypocalcemia in goats: Clinical, hematobiochemical profiles and ultrasonographic measurements of postpartum uterine involution

Background and Aim:

Hypocalcemia in goats occurs around the time of parturition and is caused by decreasing level of calcium less than 10 mg/dL. This investigation characterized the hematological and biochemical profiles of peri-parturient hypocalcemia in goats and study the effects of hypocalcemia on uterine involution during the postpartum period on day 0 and then weekly postpartum (day +7, +14, +21, +28, +35, till +42).

Materials and Methods:

Forty-five polyparous native breed does age 3-5 years and weighing 40–60 kg were assigned to control and hypocalcemia groups based on their health history, clinical and biochemical findings. The control group included 10 clinically healthy pregnant does, and the hypocalcemia group included 35 late pregnant does that suffered from anorexia, weakness, muscle tremors of the hind limbs, and an inability to stand. Clinical examination and blood sampling in both groups were performed approximately 14 days before the expected time of kidding (day −14), at kidding day (day 0), on day +21, and +42 postpartum. Sonographic measurements were used to monitor uterine involution on day 0 and then weekly once the animal was postpartum (day +7, +14, +21, +28, +35, till +42) in both groups.

Results:

Results of sonographic measurement revealed that the hypocalcemia group had a greater (p<0.05) size of the uterus as assessed transrectally and retarded uterine involution when compared with the control group. Laboratory findings revealed that does in both groups showed lower red blood cells, packed cell volume%, and hemoglobin (Hb) concentrations on day −14 before kidding and day 0 when compared with postpartum measurements. A significant increase (p<0.05) in total leukocyte counts, neutrophil, serum glucose, and cortisol levels with a significant decrease (p<0.05) in lymphocytes. Significant decreases (p<,0.05) in serum calcium, phosphorous, vitamin D, and total antioxidant capacity levels with significant (p<,0.05) increases in serum malondialdehyde were recorded on day 0 and day −14 in the hypocalcemia group when compared with the control group.

Conclusion:

This investigation provides evidence that hypocalcemia in does causes both metabolic and oxidative stress during peri-parturient periods while also retarding uterine involution during postpartum periods.

Vitamin D-binding protein influences total circulating levels of 1,25-dihydroxyvitamin D3 but does not directly modulate the bioactive levels of the hormone in vivo

Mice deficient in the expression of vitamin D-binding protein (DBP) are normocalcemic despite undetectable levels of circulating 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)]. We used this in vivo mouse model together with cells in culture to explore the impact of DBP on the biological activity of 1,25(OH)(2)D(3). Modest changes in the basal expression of genes involved in 1,25(OH)(2)D(3) metabolism and calcium homeostasis were observed in vivo; however, these changes seemed unlikely to explain the normal calcium balance seen in DBP-null mice. Further investigation revealed that despite the reduced blood levels of 1,25(OH)(2)D(3) in these mice, tissue concentrations were equivalent to those measured in wild-type counterparts. Thus, the presence of DBP has limited impact on the extracellular pool of 1,25(OH)(2)D(3) that is biologically active and that accumulates within target tissues. In cell culture, in contrast, the biological activity of 1,25(OH)(2)D(3) is significantly impacted by DBP. Here, although DBP deficiency had no effect on the activation profile itself, the absence of DBP strongly reduced the concentration of exogenous 1,25(OH)(2)D(3) necessary for transactivation. Surprisingly, analogous studies in wild-type and DBP-null mice, wherein we explored the activity of exogenous 1,25(OH)(2)D(3), produced strikingly different results as compared with those in vitro. Here, the carrier protein had virtually no impact on the distribution, uptake, activation profile, or biological potency of the hormone. Collectively, these experiments suggest that whereas DBP is important to total circulating 1,25(OH)(2)D(3) and sequesters extracellular levels of this hormone both in vivo and in vitro, the binding protein does not influence the hormone's biologically active pool.

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.

Calcium and vitamin D metabolism during lactation

Calcium transfer to the fetus in late pregnancy and the subsequent transfer of calcium to milk represent the greatest challenges to calcium homeostasis in adult animals. The adaptation of the maternal calcium homeostatic mechanisms is the result of a complex interplay between calciotropic hormones and the tissues, intestine, bone, and kidney, responsible for providing the large amounts of calcium needed to support fetal skeletal growth and lactation. In this review, we will discuss general calcium homeostasis followed by a review of the specific adaptations required by the human, rat, and cow to meet fetal and lactational demands for calcium. Finally, we will review what is known about the regulation of calcium transfer from the plasma to the milk.

Calciotropic hormones during reproduction

This review summarizes the reported effects of the menstrual cycle, pregnancy and lactation on serum concentration of the calciotropic hormones PTH and 1,25(OH)2D. A midcycle rise in PTH and 1,25(OH)2D has been observed, but in the majority of studies there was no change in PTH and 1,25(OH)2D concentrations throughout the menstrual cycle. Both total and free 1,25(OH)2D levels are increased during pregnancy. The renal 1,25(OH)2D production is stimulated, and there is some evidence of 1,25(OH)2D production by decidua/placenta and fetal kidney in vitro; the decidual/placental production should not be overestimated in vivo. The increased renal 1 alpha-hydroxylase activity is possibly mediated by estrogens and PTH, although the effect of pregnancy on PTH remains uncertain. Increased serum 1,25(OH)2D concentrations probably result in a rise of intestinal calcium absorption during pregnancy. There is a postdelivery drop in PTH and 1,25(OH)2D levels, but they are increased when lactation is prolonged, or in mothers nursing twins. The l alpha-hydroxylase activity during lactation may be stimulated by PTH, but also by prolactin.

Estrogens reduce bone loss in the ovariectomized, lactating rat model

Estrogen treatment of ovariectomized, lactating rats improved retention of bone mineral mass by 15-25% compared to ovariectomized, lactating rats receiving vehicle only. On the second day postpartum all lactating rats were ovariectomized and were placed along with age-matched non-mated controls on a whole-wheat flour-based diet with 0.1% calcium and 0.4% phosphorus. On day 6 postpartum estrogen treatment was begun with either implantation of a slow-release 17 beta-estradiol pellet or with the first of daily subcutaneous injections in sesame oil (vehicle). Increasing doses of estrogen resulted in decreased food consumption and decreased litter weight gain, both well-known effects of estrogens. Ovariectomized, lactating rats implanted with a slow-release pellet containing 0.35 mg 17 beta-estradiol had mean serum estradiol levels of 113.5 pg/ml. At the end of 21 days of lactation, femurs of dams with placebo pellets showed loss of 54% of bone ash weight compared with the non-mated controls versus only 42% loss by rats receiving estradiol treatment. Rats were also injected with estradiol benzoate in a sesame oil vehicle at 3 dose levels of 1.6, 5, or 16 micrograms/day. Only the 5 and 16 micrograms/day doses significantly improved retention of bone mineral mass during lactation (+17% and +18%, respectively, vs vehicle-injected, lactating rats). Estrone administered by subcutaneous injection also improved retention of bone during lactation; however, injection of 50 micrograms/day of estrone was required to produce an equivalent bone retention compared to 5 micrograms/day of estradiol. Thus, treatment of ovariectomized, lactating rats with estrogens results in a significant reduction of the loss of bone mineral mass associated with lactation.