Calcium homeostatic system

Vitamin D functions in the process of calcium mobilization from previously formed bone making it available to the extracellular fluid upon demand by the calcium homeostatic system as described under the regulation of vitamin D metabolism. From the experiments described in the metabolism section it is clear that 1, 25-(0H)2D3, rather than 25-OH-D3 or vitamin D3 functions in the mobilization of calcium from bone under physiologic conditions. The mechanism whereby 1,25-(0H)2D3 initiates mobilization of calcium from bone is not at all understood. In contrast to the response of intestinal calcium transport to 1, 25-(0H)2D3 this process is blocked by the prior administration of actinomycin D suggesting that in fact a transcriptive event is involved in this activation As previously pointed out, bone possesses a specific receptor for 1,25bone cells has not yet been determined however. In vivo the 1,25-(OH)2D3 activation of bone calcium mobilization requires the presence of parathyroid hormone but the nature of... 

The calcium homeostatic system, originally attributed exclusively to the parathyroid hormone, now includes the renal hormone la,25-di-hydroxyvitamin D3. Calcium is the fifth most abundant element in vertebrate systems and is the most important structural component of the body 45, 72). Calcium is also required for the following purposes ... 

Disruption of Ca Homeostasis and Neurological Effects. Neurological and/or neurodevelopmental effects from exposure to PCBs also have been hypothesized to involve interference with calcium homeostatic mechanisms and intracellular second messenger systems by PCB congeners that are not effective Ah receptor agonists (see reviews by Kodavanti and Tilson 1997 Tilson and Kodavanti 1998 Tilson et al. 1998). In agreement with structure-activity relationships observed for PCB effects on dopamine levels in pheochromocytoma cells (Shain et al. 1991), 2,2 -diCB altered intracellular calcium homeostasis in cultured rat cerebellar granule cells (increased free calcium levels and inhibited calcium... 

There is another important member of the Ca homeostatic system. While vitamin D and PTH act together to increase the calcium level of the blood, calcitonin, a hormone of the thyroid gland, lowers the level of Ca + by promoting deposition of calcium in bone by the osteoblasts. The overall effect is to hold the concentration of Ca + in the blood at 2.2-... 

Pig. 5. Diagramatic repiesentation of calcium homeostatic mechanisms involving the vitamin D endocrine system located in the kidney. Note that the calcium sensing organs are the para-thryoid glands (hypocalcemia) and the C-cells of the thyroid (hypercalcemia). Note that the vitamin D hormone acts by itself on the intestine and together with parathyroid hormone in bone and kidney... 

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