Calcium plays an important role in intracellular and extracellular metabolism controlling such processes as nerve conduction, muscle contraction, coagulation, electrolyte and enzyme regulation, and hormone release. Calcium metabolism, in turn, is tightly regulated by a series of hormones that affect not only the entry of calcium into the extracellular space from bone and the GI tract but also control its excretion from the kidneys.
Calcium hemostasis
Ninety-eight percent of body calcium is found in the skeleton; this is closely related to the extracellular concentration of calcium. Intracellular calcium is less than extracellular calcium by a factor of 100,000. Intracellular processes, including the activity of many enzymes, cell division, and exocytosis, are controlled by intracellular calcium. The primary mediator of the intracellular effects of calcium is the calcium-binding regulatory protein, calmodulin.
Plasma calcium is maintained despite its large movements across the gut, bone, kidney, and cells. Changes in calcium ions usually are accompanied by changes in total calcium in the ECF. In plasma, calcium exists in 3 different forms: (1) 50% as ionized or the biologically active form, (2) 45% bound to plasma proteins (mainly albumin), and (3) 5% complexed to phosphate and citrate. Because the proportion of bound calcium varies little within individuals, in the absence of severe acidosis or alkalosis, the amount of albumin is the major factor determining the amount of calcium that is bound.
Very little evidence suggests that intracellular stores of calcium contribute in any way to plasma calcium homeostasis. An exception is in the parathyroid gland, in which the intracellular concentration increases in response to changes in extracellular concentration, which in turn alters the rate of parathyroid hormone (PTH) secretion. Any decrease in extracellular calcium ion concentration leads to an increase in PTH secretion. PTH increases distal renal tubular reabsorption of calcium within minutes and stimulates osteoclast activity, with release of calcium from the skeleton within 1-2 hours. More prolonged PTH elevation stimulates 1alpha-hydroxylase activity in the proximal tubular cells, which leads to 1,25-dihydroxyvitamin D (1,25(OH)2 D3) production. All these mechanisms help to maintain the serum calcium level within normal limits.
A normal serum calcium level is 8-10 mg/dL (2-2.5 mmol/L) with some inter-laboratory variation in the reference range, and hypercalcemia is defined as a serum calcium level greater than 10.5 mg/dL (>2.5 mmol/L). Hypercalcemia may be classified based on total serum and ionized calcium levels, as follows:
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Mild: Total Ca 10.5-11.9 mg/dL (2.5-3 mmol/L) or ionized Ca 5.6-8 mg/dL (1.4-2 mmol/L)
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Moderate: Total Ca 12-13.9 mg/dL (3-3.5 mmol/L) or ionized Ca 8-10 mg/dL (2-2.5 mmol/L)
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Hypercalcemic crisis: Total Ca 14-16 mg/dL (3.5-4 mmol/L) or ionized Ca 10-12 mg/dL (2.5-3 mmol/L)
Only 1-2% of total body calcium is in the exchangeable form in circulation, and the rest forms part of the skeleton. Only one half of the exchangeable calcium is in the active ionized form, with the remainder bound to albumin, globulin, and other inorganic molecules. Protein binding of calcium is influenced by pH with metabolic acidosis leading to increased ionized calcium from reduced protein binding, and alkalosis leading to reduced ionized calcium from increased protein binding. Because calcium binds to albumin and only the unbound (free or ionized) calcium is biologically active, the serum level must be adjusted for abnormal albumin levels.
For every 1-g/dL drop in serum albumin below 4 g/dL, measured serum calcium decreases by 0.8 mg/dL. Therefore, to correct for an albumin level of less than 4 g/dL, one should add 0.8 to the measured value of calcium for each 1-g/dL decrease in albumin. Without this correction, an abnormally high serum calcium level may appear to be normal.
A patient with a serum calcium level of 10.3 mg/dL but an albumin level of 3 g/dL appears to have a normal serum calcium level. However, when corrected for the low albumin, the real serum calcium value is 11.1 mg/dL (10.3 + 0.8), a more obviously abnormal level. Alternatively, serum free (ionized) calcium levels can be directly measured, negating the need for correction for albumin. Corrected calcium can be calculated using the following formula:
Corrected Ca = (4 - plasma albumin in g/dL) × 0.8 + serum calcium
Mild cases of hypercalcemia can be asymptomatic and are more often diagnosed incidentally from routine blood tests. Because calcium metabolism normally is tightly controlled by the body, even mild persistent elevations above normal signal disease and should be investigated.
Calcium is controlled by 2 mechanisms: (1) controlling or major regulatory hormones and (2) influencing hormones. Controlling or major regulatory hormones include PTH, calcitonin, and vitamin D. The image below reviews vitamin D metabolism. In the kidney, vitamin D and PTH stimulate the activity of the epithelial calcium channel and the calcium-binding protein (ie, calbindin) to increase active transcellular calcium absorption in the distal convoluted tubule. [4] Influencing hormones include thyroid hormones, growth hormone, and adrenal and gonadal steroids.
Vitamin D metabolism.
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Role of the calcium-sensing receptor
The calcium-sensing receptor (CaSR) is a G protein–coupled receptor, which allows the parathyroid chief cells, the thyroidal C cells, and the ascending limb of the loop of Henle (renal tubular epithelial cells) to respond to changes in the extracellular calcium concentration. The ability of the CaSR to sense the serum Ca++ is essential for the appropriate regulation of PTH secretion by the parathyroid glands and for the regulation of passive paracellular calcium absorption in the loop of Henle. Calcitonin secretion and renal tubular calcium reabsorption also are directly regulated by the action of Ca++ on the calcium receptor. [5]
The CaSR gene is located on band 3q13-q21 and encodes a 1078 amino acid protein. CaSR is expressed in many tissues. Three uncommon human disorders are due to abnormalities of the CaSR gene: familial benign hypocalciuric hypercalcemia, neonatal severe hyperparathyroidism, and autosomal dominant hypocalcemia with hypercalciuria. [6, 7]
