Systemic regulation of serum calcium the calciotropic hormones parathyroid hormone calcitonin and dihydroxyvitamin D3

Serum calcium is maintained within tight limits (2.2-2.6 mmol 1_1), a process necessary for the maintenance of many cellular activities including neuromuscular function.

Parathyroid hormone (PTH) releases calcium into the blood by mobilizing bone mineral at times of calcium shortage. It binds to receptors on preosteo-blastic cells and signals the release of osteoclast activating factors. It also promotes calcium retention by the kidney and uptake from the gut.

Calcitonin is produced in response to a rise in serum calcium. It acts directly on osteoclasts and inhibits bone resorption.

Vitamin D metabolites regulate serum calcium and are essential for skeletal growth and development. The parent compound, vitamin D, is essentially inactive and requires two hydroxylation steps before gaining biological activity. The second of these hydroxylation steps is tightly regulated by PTH. Once formed, the most potent vitamin D metabolite, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), increases intestinal calcium absorption and promotes bone matrix mineralization. It also alters cell differentiation, influencing chondrocyte maturation within the growth plate and regulating gene expression in osteo-blastic cells.

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