Placenta calcium

Hydroxy vitamin D pools ia the blood and is transported on DBF to the kidney, where further hydroxylation takes place at C-1 or C-24 ia response to calcium levels. l-Hydroxylation occurs primarily ia the kidney mitochondria and is cataly2ed by a mixed-function monooxygenase with a specific cytochrome P-450 (52,179,180). 1 a- and 24-Hydroxylation of 25-hydroxycholecalciferol has also been shown to take place ia the placenta of pregnant mammals and ia bone cells, as well as ia the epidermis. Low phosphate levels also stimulate 1,25-dihydtoxycholecalciferol production, which ia turn stimulates intestinal calcium as well as phosphoms absorption. It also mobilizes these minerals from bone and decreases their kidney excretion. Together with PTH, calcitriol also stimulates renal reabsorption of the calcium and phosphoms by the proximal tubules (51,141,181—183). 

Epithelial calcium channel 1 (ECaCl), synonym TRJPV5, is a member ofthe TRP family of ion channels, implicated in vitamin D-dependent transcellular Ca2+ transport in epithelial cells ofthe kidney, placenta and the intestine. 

Distribution - Calcium is rapidly incorporated into skeletal tissue. Normal serum calcium concentrations range from 9 to 10.4 mg/dL (4.5 to 5.2 mEq/L), but only ionized calcium is active. Calcium crosses the placenta and reaches higher concentrations in fetal blood than maternal blood. Calcium also is distributed in milk. 

The circulating levels of l,25(OH)2D3 rise progressively towards the end of pregnancy, probably in response to the increased mineral demands. At the same time, an extra-renal synthesis of l,25(OH)2D3 takes place in the fetoplacental unit [4,47,48]. This synthesis is thought to occur in the decidual cells rather than in the placenta itself, although some controversy still remains upon the exact location of this process. Placental tissues have also been shown to contain specific receptors for l,25(OH)2D3 [49] as well as the faculty to synthesize the small calbindin (9 kDa) [50], However, the transplacental calcium transport is independent of the overall maternal vitamin D status [51]. The hypothesis of a differential regulation of calcium transport within the feto/placental unit may be in relation with the in situ synthesis of l,25(OH)2D3 or the fetal synthesis of this hormone. 

Activated Partial Thromboplastin Time (APTT). To 0.1 ml of citrated plasma 0.1 ml of human placenta lipid extract (Pathrombin, Behring Werke, Marburg) is added and the mixture is incubated for 2 min at 37 °C. The coagulation process is initiated by the addition of 0.1 ml 25 mM calcium chloride when the coagulometer is started and the time to clot formation is determined. The APTT measures effects on the endogenous pathway of coagulation. 

Calbindin-D9k has been identified in both the placenta and yolk sac of rats and mice, and increases in the later stages of gestation when there is considerable fetal uptake of calcium for mineralization of the skeleton. In both birds and mammals, calcitriol is also required for ovulation (Halloran, 1989). 

Transfer of uranium across the placenta was investigated in an animal study, but no information is available for humans. In the animal study, only 0.01-0.03% of an intravenous dose of uranium to rat dams crossed the placenta (Sikov and Mahlum 1968) thus if an inhalation, oral, or dermal exposure was sufficient to raise the blood uranium level, a very limited amount of uranium might cross the placenta. No studies were located regarding uranium in breast milk. Based on the chemical properties of uranium, it seems unlikely that there would be preferential distribution from the blood to this high-fat compartment. It is not known if uranium has any effect on the active transport of calcium into breast milk. Most of the adult body burden of uranium is stored in bone (ICRP 1979, 1995, 1996). It is not known if maternal bone stores of uranium (like those of calcium and lead) are mobilized during pregnancy and lactation. 

Berlin M, Blanks R, Catton M, Kazantzis G, Mottet N, Samiullah Y. Birth weight of children and calcium accumulation in placentas of female nickel-cadmium (long-life) battery workers. In Cadmium in the human environment toxicity and carcinogenicity. Nordberg G, Herber R, Alessio L (editors). International Agency for Research on Cancer (lARC) Scientific Publications, Vol 118,... 

Cadherins are transmembrane glycoproteins that are prime mediators of cell-cell adhesion via calcium-dependent interactions. Different members of the family are found in different locations (e.g., E-cadherin is found in epithelial tissue, N-cadherin is found in muscle and adult neural tissues, P-cadherin is found in the placenta). 

During pregnancy and lactation, some of the calcium stored in the bones of the mother is released and transferred to the fetus through the placenta or to an infant through breast milk. If the mother s bones contain strontium from previous exposure, a similar release and transfer may occur. If a mother is newly exposed to strontium, some will be transferred to the fetus, although the percentage of transfer is likely to be low. 

Stulc J, Stulcova B, Svihovec J. 1990. Transport of calcium across the dually perfused placenta of the rat. J Physiol 420 295-311. 

Twardock AR, Austin MK. 1970. Calcium transfer in perfused guinea pig placenta. Am J Physiol 219(2) 540-545. 

Twardock AR, Downey HF, Kirk ES, et al. 1969. Comparative transfer of calcium and strontium and of potassium and cesium in the guinea pig placenta. AEC Symp Ser 17 97-104. 

Vertebrates require much calcium in their food in the USA the recommended daily allowance (RDA) for adult humans is 800 mg, and most other countries have comparable recommendations. During gestation in mammals, calcium must be transported across the placenta into the fetus, in particular during those phases of pregnancy when bone formation is most rapid. Interestingly, there appear to be some parallels between intestinal and placental transport that will be discussed further below. The role of calcium in biominerals is a vast subject that we can treat only superficially in this chapter. 

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