Parathyroid hormone physiological functions

It is now well established that l,25-(OH)2D3 is the active hormonal form of vitamin D3 [32], The production of l,25-(OH)2D3 in the kidney is regulated by dietary calcium and phosphate and also by changes in serum calcium and parathyroid hormone, which clearly highlight the hormonal nature of this compound. Functionally, the three classical actions of l,25-(OH)2D3 are to stimulate intestinal calcium and independently phosphate absorption, the mobilization of calcium from bone, and increase renal reabsorption of calcium. The focus of this review will be to explore the most recent concepts of vitamin D in regard to its metabolism and physiology, and with respect to the medicinal applications of vitamin D3 metabolites and analogues. 

There is evidence that 24-hydroxycalcidiol has physiological functions distinct from those of calcitriol, and the regulation of the 24-hydroxylase suggests that it functions to provide a metabolically active product, as well as diverting calcidiol away from calcitriol synthesis (Henry, 2001). Studies of knockout mice lacking the 24-hydroxylase show that 24-hydroxycalcidiol has a role in both in-tramembranous bone formation during development and the suppression of parathyroid hormone secretion (St-Arnaud, 1999 van Leeuwen et al., 2001). 

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... 

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