Calcium homeostasis - Calcium metabolism - Calcium levels

Aug 20, 2014 Home › Calcium deficiency symptoms › Calcium levels, homeostasis and metabolism.

What is calcium homeostasis or metabolism?

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Calcium homeostasis or metabolism refers to the controlled regulation of calcium level in the extracellular fluid within a narrow range. The extracellular fluid levels of the mineral must be maintained to achieve optimal functionality. Any increase in the levels above the regulated range may cause disorders like hypercalcemia; decrease in levels will cause hypocalcemia and osteoporosis.

Calcium is the most abundant mineral in the human body and contributes to many biochemical processes including functioning of the neuromotor systems and blood coagulation. The biological processes that contribute to its homeostasis are intestinal absorption pathways, bone remodeling processes, renal reabsorption and excretion processes and endocrine metabolism and regulation.

Calcium homeostasis

The predominant endocrine regulators of calcium homeostasis are parathyroid hormone (PTH) and active vitamin D, and, to a lesser extent, calcitonin. Fibroblast growth factor 23 (FGF23), transmembrane protein enzyme (klotho) and transient receptor potential cation channel subfamily V member 5 (TRPV5), also have roles in the mineral homeostasis in serum levels. The interplay between the biological and biochemical factors in the metabolism is described below.

Calcium absorption

For new bone formation and growth calcium has to be taken through food. Many factors inhibit or aid in the absorption of the mineral in the intestine. By stimulating production of active vitamin D (1,25-dihydroxyvitamin D3), parathyroid Hormone (PTH) enhances the intestinal absorption of the mineral. Calcium transport mechanism accounts for the major portion of the mineral absorbed from the intestine, whereas passive diffusion accounts for less than 20% of the absorption.

Total and free calcium homeostasis

Nearly 99% of the body's calcium is present in bones, mostly as hydroxyapatite crystals.

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Nearly 1% is present in the extracellular fluid (ECF) for maintaining total serum levels (8.8 to 10.4 mg/dL) of the mineral. Of this more than 40% is albumin bound. Less than 60% of the calcium is ionized and free available for metabolism. However in clinical laboratory serum analysis, it technically difficult to analyze the free ionic form and only the total serum levels are determined. The ionized mineral maintained by homeostasis is assumed to be half of the total serum level.

Bone remodeling process and homeostasis

Bone metabolism is a continuous process. Old bones are resorbed and new bone is formed to keep pace with the growth and repairs. Further bone is the source for maintaining calcium homeostasis in case of lowered levels in the serum. When the serum concentration of the mineral increases, to maintain the homeostasis, the mineral is actively removed from the serum by deposition on to the bones and also by excretion.

Parathyroid Hormone (PTH) in metabolism

Parathyroid Hormone (PTH) is the main player in bringing about the homeostasis or metabolism in the serum levels of the mineral.

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PTH by stimulating the osteoclasts causes the bone resorption and increase in serum levels of the mineral. The calcium-sensing receptors (CaSRs) on the surface of the parathyroid cells as well as in the bone and kidney are activated by the ionized serum levels of the mineral.

The PTH secretion is controlled by negative feedback from the serum levels. PTH also stimulates production of hormonally active form of vitamin D in the kidney. PTH also decreases the excretion and increases reabsorption of this mineral by the kidneys for the mineral metabolism.

Active vitamin D in metabolism

Along with PTH, active vitamin D (1,25-dihydroxyvitamin D3), is also required in the metabolism of the bone remodelling, especially resorption. Active vitamin D increases the dietary absorption of the mineral. It also increases the renal reabsorption of calcium in the urine. The source vitamin D is food. In the skin vitamin D is formed after exposure to sunlight/ultraviolet light.

Calcitonin and homeostasis

The parafollicular cells (C-cells) of the thyroid gland secrete calcitonin, a 32-amino acid polypeptide hormone regulator of homeostasis.

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The main function of calcitonin in the homeostasis is to lower the calcium levels in serum. The action of calcitonin is antagonistic to PTH on bone reabsorption. In metabolism of bones, calcitonin increases the deposition of the mineral on to the bones and soft tissues. The concentration levels of the calcium ions in the extracellular fluids is the stimulus for calcitonin secretion and metabolism.

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References: 1.Lumachi F, Motta R, Cecchin D, Ave S, Camozzi V, Basso SM, Luisetto G. Calcium metabolism & hypercalcemia in adults. Curr Med Chem. 2011;18(23):3529-36. 2.Munro Peacock. Calcium Metabolism in Health and Disease. CJASN January 2010 vol. 5 no. Supplement 1 S23-S30. 3.Gregory R. Mundya and Theresa A. Guise. Hormonal Control of Calcium Homeostasis. Clinical Chemistry August 1999 vol. 45 no. 8 1347-1352.

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