CALCIUM METABOLISM

A total of 1 to 2 kg of calcium is present in the average adult, 98 of it in the skeleton. The calcium of the mineral phase at the surface of the crystals is in equilibrium with that in the ECF, but only a minor fraction of the total pool (~0.5) is exchangeable. In normal adults plasma levels range from 2.2 to 2.6 mmol/L (8.8 to 10.4 mg/dL). The calcium in plasma is present as three forms: free ions, ions bound to plasma proteins, and, to a small extent, diffusible complexes. The concentration of free calcium ions, averaging 1.2 mmol/L (4.8 mg/dL), influences many cellular functions and is subjected to tight hormonal control, especially through PTH (Chap. 341). The concentration of serum proteins is an important determinant of calcium ion concentration; most calcium ion is bound to albumin. Ionized calcium can be measured directly with the use of calcium-specific electrodes. If ionized calcium cannot be measured, certain approximations can be used to estimate the protein-bound and ionized fractions. One formula that approximates the amount of calcium bound to protein is

A simplified correction is sometimes used to assess whether the total serum calcium concentration is abnormal when serum proteins are low. The correction is to add 1 mg/dL to the serum calcium level for every 1 g/dL by which the serum albumin level is below 4.0 g/dL. If the serum calcium level, for example, is 7.8 mg/dL (a subnormal value) and the serum albumin level is only 3.0 g/dL, then the stated serum calcium level is corrected by adding 1 mg/dL; the corrected value of 8.8 mg/dL is within the normal range.

The concentration of calcium ions in the ECF is kept constant by processes that constantly add and remove calcium. Calcium enters the plasma via absorption from the intestinal tract and resorption of ions from the bone mineral. Calcium leaves the ECF via secretion into the gastrointestinal tract (~100 to 200 mg/d), urinary excretion (~50 to 300 mg/d), deposition in bone mineral, and losses in sweat (up to 100 mg/d). Bone resorption and formation are tightly coupled, with approximately 12 mmol (500 mg) calcium entering and leaving the skeleton daily (Fig. 340-3). Calcium ions inside the cell mediate a variety of cellular functions. The level of free calcium in the cell is very low, approximately 0.1 umol/L; thus, the gradient between plasma and intracellular free calcium is about 10,000 to 1. This gradient is tightly regulated by various channels and ion pumps.

The average dietary calcium intake for most adults in the United States is approximately 15 to 20 mmol/d (0.6 to 0.8 g/d). However, with heightened awareness of the role of adequate calcium intake for the prevention of osteoporosis, many adults on supplements have an intake of 20 to 37 mmol/d (0.8 to 1.5 g/d). Less than half of dietary calcium is absorbed in adults. Calcium absorption increases during periods of rapid growth in children, in pregnancy, and in lactation and decreases with advancing age. Most of the calcium is absorbed in the proximal small intestine, and the efficiency of absorption decreases in the more distal intestinal segments. Both active transport and diffusion-limited absorption are involved; the former is more important in the upper intestine and the latter in the lower intestine. Both processes are influenced by vitamin D (see below). All forms of calcium in the diet are not equally absorbed; calcium as the chloride is probably absorbed more efficiently than that in other preparations. Secretion of calcium into the intestinal lumen is constant and independent of absorption. If calcium availability in the diet is low [12 mmol/d (500 mg/d)], a positive calcium balance requires an efficiency of absorption 30 to 40.

The urinary calcium excretion of normal adults having an average calcium intake ranges between 2.5 and 10 mmol/d (100 and 400 mg/d). When the dietary calcium level is 5 mmol/d (200 mg/d), urinary calcium excretion is usually 5 mmol/d (200 mg/d). However, in most normal individuals, wide variations in dietary intake have little effect on urinary calcium. Hence, when the diet is low in calcium, the relative inefficiency of renal calcium conservation leads to a negative calcium balance unless calcium absorption is maximal (Fig. 340-3).

The amount of calcium in the urine is small compared with that filtered by the glomerulus [~150 to 250 mmol/d (6 to 10 g/d)] because the rates of reabsorption of the filtered calcium are high. Reabsorption takes place predominantly in the proximal tubule (~60) and in Henle's loop

(~25) and to a small extent in the distal tubule. The calcium-sensing receptor also plays a role in renal calcium excretion, though the mechanisms that regulate its function have not been fully defined (Chap. 341). The excretion of other electrolytes affects the urinary excretion of calcium. For example, urinary calcium is usually proportional to urinary sodium; sulfate also increases calcium excretion.

A deficiency of PTH or vitamin D, intestinal disease, or severe dietary calcium deprivation may provide challenges to calcium homeostasis that cannot be compensated adequately by renal calcium conservation, resulting in a negative calcium balance. Increased bone resorption may protect against ECF calcium depletion even in states of chronic negative calcium balance but only at the expense of progressive bone loss.

PATHOPHYSIOLOGY

A decrease in the concentration of free calcium ions in plasma results in increased neuromuscular irritability and tetany. This syndrome is characterized by peripheral and perioral paresthesia, carpal spasm, pedal spasm, anxiety, seizures, bronchospasm, laryngospasm, Chvostek's sign, Trousseau's sign, and Erb's sign, and lengthening of the QT interval of the electrocardiogram. In infants tetany may be manifested only by irritability and lethargy. The level of calcium ions that determines which features of tetany will be manifested varies among individuals. Tetany is also influenced by other components of the ECF; e.g., hypomagnesemia and alkalosis lower whereas hypokalemia and acidosis raise the threshold for tetany.

Increases in total serum calcium concentration are usually accompanied by increases in free calcium levels and may be associated with anorexia, nausea, vomiting, constipation, hypotonia, depression, and occasionally lethargy and coma. Persistent hypercalcemia, especially when accompanied by normal or elevated levels of serum phosphate, may cause ectopic deposition of a solid phase of calcium and phosphate in walls of blood vessels, connective tissue about the joints, gastric mucosa, cornea, and renal parenchyma. Hypercalcemia per se alters renal function in addition to the pathologic effects of calcium phosphate deposition.

PHOSPHORUS METABOLISM

Phosphorus is a major component of bone and of all other tissues and in some form is involved in almost all metabolic processes, including energy storage, membrane transport, membrane composition, and signal transduction. About 600 g of phosphorus is present in the normal adult, of which 85 is present in the crystalline structure of the skeleton.

In plasma from fasting subjects, most of the phosphorus is present as inorganic orthophosphate in concentrations of approximately 0.75 to 1.45 mmol/L (2.5 to 4.5 mg/dL). In contrast to calcium, of which ~50 is bound, only ~12 of the phosphorus in plasma is bound to proteins. Free HPO4

2 and NaHPO4 normally account for ~75 of the total phosphorus, and free H2PO4

accounts for ~10. Since so many species are present, depending on pH and other factors, concentrations are usually expressed in terms of elemental phosphorus, in units of mmol/L or mg/dL. The serum phosphorus, however, can vary based on age; young children have almost twice the serum phosphorus as adults due to the need for rapid skeletal mineralization. Postmenopausal women also have higher circulating phosphorus levels. After ingesting a meal containing carbohydrate, there is a decrease in serum phosphorus levels [by 0.3 to 0.5 mmol/dL (1.0 to 1.5 mg/dL)] in response to the increase in insulin secretion, which enhances cellular phosphorus uptake and utilization. An increase in serum pH will decrease serum phosphorus, whereas a decrease in pH increases phosphorus concentration. There is a circadian variation in phosphorus concentration even during a 24-h fast: the nadir occurs between 9:00 A.M. and noon followed by an increase to a plateau in the afternoon and another small peak after midnight.

Phosphorus is plentiful in the diet. Common sources include dairy products, meats, eggs, and carbonated beverages that contain phosphoric acid. Approximately 60 to 70 of phosphorus is passively absorbed in the small intestine (Fig. 340-4). 1,25(OH)2D enhances phosphorus

absorption along the entire small intestine, with the highest efficiency in the jejunum and ileum.

Chronic low phosphorus intake (2 mg/kg of body weight per day) decreases serum phosphorus levels. Low serum phosphorus stimulates the renal production of 1,25(OH)2D,

which, in turn, increases the efficiency intestinal absorption up to 80 to 90. 1,25(OH)2D also

decreases PTH secretion and, thereby reduces renal tubular loss of phosphorus.

The major control of phosphorus balance is exerted by the kidney. Approximately 90 of phosphorus in the circulation is filtered through the glomerulus and is largely absorbed by the proximal tubule such that only 10 to 15 of the filtered load is normally excreted. Urinary phosphorus excretion is reflective of dietary intake. Phosphorus absorption in the proximal tubule is coupled with sodium absorption. The primary regulation of phosphorus metabolism occurs in the distal convoluted tubule, and this mechanism is independent of sodium reabsorption. Volume expansion and decreased sodium reabsorption increase phosphorus clearance.

HYPOPHOSPHATEMIA

Causes Although there are many potential causes for hypophosphatemia (Table 340-1), the most common etiologies include: (1) decreased intestinal phosphorus absorption, either due to vitamin D deficiency or the presence of a phosphorus-binding antacid; (2) urinary losses that are PTH- or alcohol-mediated; and (3) a shift of phosphorus from extracellular to intracellular compartments due to exogenous administration of insulin or consumption of nutrients that stimulate insulin release (e.g., carbohydrates). Increased renal clearance of phosphorus occurs in primary hyperparathyroidism, vitamin D deficiency, vitamin D-resistant and D-dependent rickets, hyperglycemic states, and oncogenic osteomalacia. In vitamin D deficiency, serum phosphorus is low because of decreased intestinal absorption as well as secondary hyperparathyroidism, which increases phosphorus losses in the urine. In X-linked hypophosphatemic rickets, there is a genetic defect in the PHEX gene, which encodes a neutral endopeptidase presumed to degrade the phosphaturia hormone known as phosphatonin. The disorder is associated with a severe renal leak of phosphorus into the urine. In addition, there is a defect in hypophosphatemia-mediated stimulation of 25(OH)D-1-hydroxylase, resulting in decreased intestinal phosphorus absorption. Acidosis and hyperglycemic states associated with polyuria also cause excessive phosphorus loss in the urine. Ketoacidosis enhances intracellular and organic phosphorus degradation, thereby releasing large amounts of inorganic phosphorus into the circulation that is cleared into the urine. In ketosis, the serum phosphorus is often normal because of the continuous shift of phosphorus from intracellular to extracellular pools. However, when the ketosis is corrected, hypophosphatemia is apparent because of the return of phosphorus into the intracellular compartment (Chap. 333). A severe, acquired form of hypophosphatemia, oncogenic osteomalacia, is associated with vascular, mesenchymal tumors such as hemangiopericytomas but occasionally also with small cell lung cancer, prostate cancer, and other malignant tumors. It is likely that these tumors secrete a substance similar or identical to phosphatonin. The phosphorus levels in these patients are usually extremely low [0.4 to 0.5 mmol/L (1.2 to 1.5 mg/dL)], and the 1,25(OH)2D levels are low or undetectable. The

disorder is associated with severe fatigue, muscle weakness, and unrelenting bone discomfort.

Alcohol abuse is the most common cause of severe hypophosphatemia, which is caused by poor dietary intake of phosphorus, ethanol-enhanced urinary excretion of inorganic phosphorus, the use of calcium- or aluminum-containing antacids, and vomiting. Hypophosphatemia may transiently worsen with refeeding. Alcoholics may also have associated calcium and vitamin D deficiency and secondary hyperparathyroidism, which enhances phosphorus-wasting in the urine. Alcoholic ketoacidosis induces marked phosphaturia. Intense hyperventilation for prolonged periods may depress serum phosphorus levels due to associated alkalosis. Rapid correction of chronic respiratory acidosis has also been associated with hypophosphatemia and can lead to diaphragm weakness and an exacerbation of respiratory failure. Advanced leukemia with blast crisis (leukocyte counts usually 100,000) may cause severe hypophosphatemia; the likely cause is a rapid uptake of phosphorus into the rapidly dividing cells.

Laboratory and Clinical Findings Serum phosphorus levels should be determined in a fasting state. Mild hypophosphatemia is not usually associated with clinical symptoms. In severe hypophosphatemia [0.3 mmol/L (1.0 mg/dL)], multiple organ systems are affected. Patients become irritable, apprehensive, and hyperventilate, resulting in complaints of muscle

weakness, numbness, and paresthesia. In the most severe form, they are confused or obtunded and suffer from seizures and coma, which can ultimately lead to death. This metabolic encephalopathy is often associated with slowing of the electroencephalogram.

Phosphorus is essential for muscle function because of the need for large amounts of ATP and creatine phosphate. Patients with severe hypophosphatemia often complain of fatigue, muscle weakness, myalgia, and myopathy. Hypophosphatemia can cause rhabdomyolysis, which is particularly common in chronic alcoholics or during alcohol withdrawal. Rhabdomyolysis can be precipitated during treatment for diabetic ketoacidosis or by hyperalimentation or refeeding in a malnourished patient. Cardiomyopathy can also occur, resulting in reduced cardiac output, impaired pressor responsiveness to catecholamines, hypotension, and ventricular arrhythmias. Restoration of phosphorus deficits can result in prompt reversal. Severe muscle weakness can lead to respiratory insufficiency.

Erythrocytes and leukocytes are highly dependent on phosphorus for their function. Chronic hypophosphatemia decreases 2,3-bisphosphoglycerate and ATP, enhancing oxygen dissociation from hemoglobin and leading to tissue hypoxia. Hypophosphatemia causes impaired phagocytosis and opsonization and, therefore, increases susceptibility to bacterial and fungal infections.

Chronic hypophosphatemia causes a mineralization defect of the skeleton. In children, this causes rickets. In adults, chronic hypophosphatemia (often due to vitamin D deficiency) causes osteomalacia (see below). Patients with severe renal phosphorus-wasting and severe chronic hypophosphatemia may have marked fatigue, muscle weakness, and severe bone pain, especially of their long bones and ribcage.

TREATMENT

Mild hypophosphatemia usually resolves spontaneously when the underlying cause is corrected. Oral phosphorus replacement is sufficient if serum phosphorus is 0.3 mmol/L (1 mg/dL) and the patient is asymptomatic. Milk is an excellent source of phosphorus as it contains 1 g of inorganic phosphorus per liter. Carbonated beverages that contain phosphoric acid provide another source of phosphorus, especially for patients with lactase deficiency. Pharmaceutical preparations of phosphorus, such as Neutraphos or KPhos, contain sodium and potassium salts of phosphate. Depending on the degree of hypophosphatemia, up to 3 g/d can be given in four to six divided doses per 24 h. These doses usually do not cause diarrhea; 5 g/d will induce diarrhea.

For severe hypophosphatemia, with serum phosphorus levels 0.2 to 0.3 mmol/L (0.5 to 1.0 mg/dL), 3 g/d of phosphorus may be required over several days to replete body stores. In patients with severe symptomatic hypophosphatemia who are unable to eat, intravenous phosphorus can be given, up to 1 g in 1 L of fluid over 8 to 12 h. Some caution is necessary when giving phosphorus intravenously because of the potential for precipitating soft tissue calcification. A serum calcium serum phosphorus product 70 markedly increases the risk of soft tissue calcification and nephrocalcinosis. Patients with chronic hypophosphatemia caused by inherited or acquired renal phosphorus leak require vigilance when receiving high doses of oral phosphorus. Transiently elevated serum phosphorus levels can decrease ionized calcium levels, resulting in chronic stimulation of the parathyroid gland and leading to autonomous, persistent hyperplasia of the parathyroid glands. Thus, it is best to give frequent divided doses of phosphorus (four to six times a day), equaling a total of 2 to 3 g/d.

Phosphorus should not be given intramuscularly or subcutaneously because it can cause soft tissue necrosis and severe discomfort. Intravenous sodium or potassium phosphate, 15 mmol (0.465 g) of elemental phosphorus given in 100 mL of 0.9 saline over 60 min, elevates serum phosphorus levels by an average of 0.6 to 1.2 mmol/L (1.75 to 3.8 mg/dL).

HYPERPHOSPHATEMIA

In adults, hyperphosphatemia is defined as a serum phosphorus level 1.6 mmol/L (5 mg/dL). In children, this level is much higher. The most common causes of hyperphosphatemia are

acute and chronic renal failure (Table 340-2). In renal failure, the loss of tubular function impairs phosphorus excretion. This results in a cascade of events that can also affect calcium and phosphorus metabolism. The increase in serum phosphorus levels reduces serum calcium levels and the production of 1,25(OH)2D, leading to decreased intestinal calcium absorption

and secondary hyperparathyroidism. Patients with pseudohypoparathyroidism and tumoral calcinosis also have decreased renal phosphorus clearance that results in hyperphosphatemia. Hypothyroidism reduces renal phosphorus excretion and may increase circulating concentrations of phosphorus. Vitamin D intoxication, due to excessive ingestion of either vitamin D or one of its analogues, can cause hyperphosphatemia along with hypercalcemia. Severe hypothermia, crush injuries, nontrauma rhabdomyolysis, tumoral calcinosis, and cytotoxic therapy of hematologic malignancies such as acute lymphoblastic leukemia can be associated with hyperphosphatemia. The serum phosphorus level can be artifactually elevated due to hemolysis of the blood sample. Thrombocytosis and multiple myeloma can cause spuriously elevated serum phosphorus levels due to thrombocytolysis.

Laboratory and Clinical Findings A rapid elevation of serum phosphorus can cause hypocalcemia and symptoms of neuromuscular irritability and tetany. Chronic hyperphosphatemia in association with normocalcemia can result in nephrocalcinosis and soft tissue calcification.

TREATMENT

In addition to treating the underlying disorder, dietary phosphorus intake should be limited by restricting carbonated beverages containing phosphoric acid and decreasing milk and dairy product consumption. The dietary intake of phosphorus should be between 600 and 1000 mg a day with modest protein restriction. For control of chronic hyperphosphatemia, usually in patients with chronic renal failure, oral aluminum hydroxide or aluminum carbonate gels are indicated. Prolonged use of aluminum-containing compounds is not recommended because of aluminum toxicity causing adynamic bone disease, proximal myopathy, encephalopathy, and anemia. When hyperphosphatemia is due to vitamin D intoxication, calcium salts are contraindicated because the high efficiency of calcium absorption can lead to severe hypercalcemia, soft tissue calcification, and nephrocalcinosis.

VITAMIN D

Vitamin D is a hormone rather than a classic vitamin, since with adequate exposure to sunlight, no dietary supplements are needed. Vitamin D exerts its physiologic effects on bone, intestine, kidney, and the parathyroid glands to modulate calcium and phosphorus metabolism. The active principle of vitamin D is synthesized under metabolic control via successive hydroxylations in the liver and kidney and is transported through the blood to its main target tissues (the small intestine and bone), where it regulates calcium homeostasis.

PHOTOBIOGENESIS

Vitamin D3 is a derivative of 7-dehydrocholesterol (provitamin D3), the immediate precursor of

cholesterol. When skin is exposed to sunlight or certain artificial light sources, the ultraviolet radiation enters the epidermis and causes transformation of 7-dehydrocholesterol to vitamin D3.

Wavelengths between 290 and 315 nm are absorbed by the conjugated double bonds at C5

and C7 of 7-dehydrocholesterol to produce previtamin D3 (Fig. 340-5). Vitamin D3 is made in

the skin from the previtamin for many hours after a single sun exposure (Fig. 340-5). Once vitamin D3 is synthesized, it is translocated from the epidermis into the circulation by the vitamin

D-binding protein. Melanin in the skin competes with 7-dehydrocholesterol for ultraviolet photons and thus can limit the synthesis of previtamin D3. The photochemical isomerization of

previtamin D3 and vitamin D3 to biologically inert products appears to be more important in

preventing excessive production of previtamin D3 and vitamin D3 during prolonged exposure to

the sun.

Aging decreases the capacity of the skin to produce vitamin D3; this capacity is reduced more

than fourfold after age 70. Topical sunscreens can reduce or prevent cutaneous production of vitamin D3 by absorbing the solar radiation responsible for previtamin D3 synthesis in the skin.

Other factors that affect the cutaneous synthesis of vitamin D3 include altitude, geographic

location, time of day, and area exposed. Latitude has profound effects on the cutaneous synthesis of vitamin D3. As the zenith angle of the sun increases with approaching winter, more

of the high-energy ultraviolet photons responsible for formation of the previtamin are absorbed by the ozone layer. In an area such as Boston (42N), the absorption of these photons is so complete that essentially no vitamin D3 is made in the skin between the months of November

through February.

When the entire body is exposed to sufficient sunlight to cause mild erythema, the increase in the blood vitamin D is approximately equivalent to consuming oral doses of 10,000 to 25,000 international units (1 IU 0.025 ug) of vitamin D. Only when skin irradiation is insufficient to produce the required quantities of vitamin D3 is dietary supplementation needed to prevent

skeletal mineralization defects. The fortification of milk and some cereals with either crystalline vitamin D2 (Fig. 340-5) or vitamin D3 should prevent rickets and osteomalacia. A survey of the

vitamin D content in milk from the United States and western Canada revealed, however, that 71 did not contain 80 to 120 of the amount of vitamin D on the label and that ~15 of skim milk did not contain detectable vitamin D.

In 1997, the Food and Nutrition Board for the Institute of the Medicine recommended 200 IU/d as the adequate intake of vitamin D for neonates, children, and adults up to 50 years. For adults 51 to 70 and 71 years, the committee recommended 400 and 600 IU/d, respectively. In the absence of adequate sunlight exposure, all children and adults require at least 400 to 600 IU/d.

METABOLISM

In the liver, vitamin D is metabolized to 25-hydroxyvitamin D [25(OH)D] by hepatic mitochondrial and/or microsomal enzyme(s) (Fig. 340-5). 25(OH)D is one of the major circulating metabolites, and its half-life is about 21 days. The concentrations of 25(OH)D and some of its metabolites in the serum are measured using competitive binding assays. The normal serum 25(OH)D concentration varies among different laboratories from 20 to 200 nmol/L (8 to 80 ng/mL). Individuals exposed to excessive sunlight may have concentrations of 25(OH)D up to 250 nmol/L (100 ng/mL) without adverse effects on calcium metabolism. The serum 25(OH)D levels usually reflect both 25-hydroxyvitamin D2 [25(OH)D2] and 25-

hydroxyvitamin D3 [25(OH)D3]. The ratio of these two 25-hydroxylated derivatives depends on

the relative amounts of vitamins D2 or D3 present in the diet and the amount of previtamin D3

produced by exposure to sunlight.

The hepatic 25-hydroxylation of vitamin D is regulated by a product feedback mechanism. This regulation, however, is not tight; an increase in dietary intake or endogenous production of vitamin D3 increases 25(OH)D levels in the serum. The levels can rise to 1200 nmol/L (500

ng/mL) when the intake of vitamin D is excessive. Serum 25(OH)D levels are reduced in severe chronic liver disease (Table 340-5). 25(OH)D is probably not biologically active at physiologic levels in vivo but is active in vitro at high concentrations.

After formation in the liver, 25(OH)D is bound by the vitamin D-binding protein and transported to the kidney for an additional stereospecific hydroxylation on either C1 or C24 (Fig. 340-5). The

kidney plays a pivotal role in the metabolism of 25(OH)D to the biologically active metabolite. The renal mitochondrial 25(OH)D-1-hydroxylase activity is enhanced by hypocalcemia to increase the rate of conversion of 25(OH)D to 1,25(OH)2D. Hypocalcemia may not control this

hydroxylation directly, however. Any decrease in the serum concentration of calcium below normal is a stimulus for increased secretion of PTH, which increases the synthesis of 1,25(OH)2D in the renal proximal convoluted tubule. The renal production of 1,25(OH)2D

enhances the effects of PTH in lowering circulating concentrations (and presumably renal

intracellular concentrations) of phosphate (Fig. 340-6). 1,25(OH)2D also influences the renal

metabolism of 25(OH)D by diminishing 25(OH)D-1-hydroxylase activity and enhancing the metabolism of 25(OH)D to 24R,25-dihydroxyvitamin D [24,25(OH)2D].

24,25(OH)2D is normally present in serum at a concentration of 1 to 10 nmol/L (0.5 to 5.0

ng/mL). 24,25(OH)2D is also a substrate for renal 25(OH)D-1-hydroxylase and is converted to

1,24R,25-trihydroxyvitamin D [1,24,25(OH)3D], which, in turn, is metabolized to the biologically

inactive substance calcitroic acid (Fig. 340-5). Cultured cells that possess nuclear receptors for 1,25(OH)2D, such as chondrocytes, skin keratinocytes and fibroblasts, and intestinal and

melanoma cells, also metabolize 25(OH)D to 24,25(OH)2D. Studies of the vitamin D-24-

hydroxylase null mice indicate that the major role of 24-hydroxylation is in the regulation of levels of 1,25(OH)2D.

PHYSIOLOGY

1,25(OH)2D, produced by the kidney and the placenta, is the only known important metabolite

of vitamin D; the potential roles of other metabolites have not been clarified. 1,25(OH)2D bound

to a vitamin D-binding protein is delivered to various target organs, where the free form is taken up by cells and transported to a specific nuclear receptor protein. The vitamin D receptor (VDR) belongs to the nuclear receptor superfamily of steroid-retinoid-thyroid hormone-vitamin D transcription regulatory factors (Chap. 327). The VDR interacts with the retinoic acid X receptor (RXR) to form a heterodimeric (RXR-VDR) complex that binds to specific DNA sequences, termed the vitamin D response elements (VDREs). After 1,25(OH)2D binds to the receptor, it

induces conformational changes that result in the recruitment of a multitude of transcriptional coactivators that stimulate the transcription of target genes. In the intestine, the activated VDR stimulates calcium-binding protein synthesis; in bone, it stimulates production of osteocalcin, osteopontin, and alkaline phosphatase. 1,25(OH)2D also may have nonnuclear effects on its

target tissues; 1,25(OH)2D increases the transport of calcium from the extracellular to

intracellular space, and it can mobilize calcium from intracellular calcium pools and enhance phosphatidylinositol metabolism. In the intestine, the net effect of 1,25(OH)2D is to stimulate

calcium and phosphate transport from the lumen of the small intestine into the circulation (Fig. 340-6). The effect of 1,25(OH)2D on the enhancement of bone resorption is synergistic with that

of PTH. Mature osteoclasts do not possess receptors for either PTH or 1,25(OH)2D. Both PTH

and 1,25(OH)2D interact with their specific receptors on osteoblasts or stromal fibroblasts to

induce the production of RANK ligand on the osteoblast's cell surface. As described above, the RANK ligand interacts with the RANK receptor on immature osteoclasts, stimulating immature osteoclastic precursors to differentiate into mature osteoclasts. The role of 1,25(OH)2D in the

renal handling of calcium and phosphorus remains uncertain. Whatever the role of extraintestinal VDRs may be, the compelling evidence is that the phenotype of VDR null mice is corrected in the setting of normal mineral ion homeostasis. Thus the skeletal consequences of VDR ablation are the result of impaired intestinal calcium absorption and/or the accompanying secondary hyperparathyroidism and hypophosphatemia.

Receptors for 1,25(OH)2D are also present in cells not classically considered target organs for

this hormone, including skin, breast, pituitary, parathyroids, pancreatic beta cells, gonads, brain, skeletal muscle, circulating monocytes, and activated B and T lymphocytes. Although its physiologic role in these cells remains to be determined, 1,25(OH)2D inhibits proliferation of

keratinocytes and fibroblasts, stimulates terminal differentiation of keratinocytes, induces monocytes to produce interleukin (IL)1 and to differentiate into macrophages and osteoclast-like cells, inhibits the production of PTH, and inhibits the production of IL-2 and immunoglobulin by activated T and B lymphocytes, respectively.

In addition, a variety of tumor cell lines, including lines derived from breast carcinomas, melanomas, and promyeloblasts, possess receptors for 1,25(OH)2D. Tumor cell lines that have

1,25(OH)2D receptors respond to the hormone by decreasing the rate of proliferation and

enhancing differentiation. For example, when malignant receptor-positive human promyelocytic cells (HL-60) are exposed to 1,25(OH)2D, the cells mature into functioning macrophages within

1 week. Although calcitriol [1,25(OH)2D] is not useful for the treatment of leukemia, the

antiproliferative effects of calcitriol and its analogue calcipotriene provide the rationale for their use in the treatment of psoriasis.

1,25(OH)2D regulates PTH synthesis by negative feedback (Fig. 340-6). This effect is the

rationale for giving 1,25(OH)2D3, and its less calcemic-inducing analogue 19-nor-1,25-

dihydroxyvitamin D3 (Fig. 340-7), to lower circulating levels of PTH in patients with chronic renal

failure (Chap. 341).

The principal physiologic mechanism regulating the production of 1,25(OH)2D appears to

involve changes in serum extracellular calcium concentrations that result in reciprocal changes in secretion of PTH, the latter controlling, possibly through actions on serum or tissue phosphorus levels, the rate of 1,25(OH)2D production. Other factors that enhance 1,25(OH)2D

production include estrogen, prolactin, and growth hormone. Humans adapt to increased calcium requirements during growth, pregnancy, and lactation by increasing the efficiency of intestinal calcium absorption, possibly by enhancing 25(OH)D-1-hydroxylase activity. During the first two trimesters of pregnancy, the levels of 1,25(OH)2D increase in proportion to the

concentration of the vitamin D-binding protein; levels of free 1,25(OH)2D do not change. During

the last trimester, the need for calcium for mineralization of the fetal skeleton is met by an increase in the concentrations of free 1,25(OH)2D and enhanced maternal intestinal calcium

absorption.

Most measurements of circulating 1,25(OH)2D in various physiologic or pathologic states utilize

a receptor/competitive binding assay. Serum levels of vitamin D and 25(OH)D vary with the season and with vitamin D intake, whereas levels of 1,25(OH)2D appear to be unaltered by

seasonal variation, by increases in dietary vitamin D, or by exposure to sunlight (Table 340-6); as long as vitamin D supplies and circulating concentrations of 25(OH)D are sufficient, metabolic influences control the renal 25(OH)D-1-hydroxylase to ensure a closely regulated circulating concentration of 1,25(OH)2D. The serum concentration of 1,25(OH)2D ranges from

40 to 160 pmol/L (16 to 65 pg/mL), and its serum half-life is from 3 to 6 h.

PHARMACOLOGY

Casual exposure to sunlight provides most people with adequate vitamin D. In elderly individuals, exposure of hands, face, and arms to a suberythemal dose of sunlight two to three times a week is usually adequate. A variety of over-the-counter vitamin preparations contain 400 IU of either vitamin D2 (ergocalciferol) or vitamin D3 (cholecalciferol). More potent

preparations of vitamin D (calciferol) are available in capsule and tablet form (50,000 IU), as oil (500,000 IU/mL), and in oral solution (8000 IU/mL). A single oral dose of 50,000 IU of vitamin D2 increases the circulating concentration of vitamin D from 25 nmol/L (10 ng/mL) to 130 to

260 nmol/L (50 to 100 ng/mL) within 12 to 24 h; the plasma half-life is about 2 days. Serum concentrations of 25(OH)D and 1,25(OH)2D are not changed by these doses of vitamin D. For

treatment of vitamin D deficiency, 50,000 IU of vitamin D once a week for 8 weeks raises the circulating concentration of 25(OH)D into the normal range; in the presence of secondary hyperparathyroidism, the circulating concentrations of 1,25(OH)2D can increase to supranormal

levels [up to 600 pmol/L (250 pg/mL)]. 25(OH)D3 (calcifediol) available in capsules containing

either 20 or 50 ug may be useful in treating vitamin D deficiency [low 25(OH)D concentrations] in patients with severe liver dysfunction. Pharmacologic doses are used to treat disorders of 25(OH)D metabolism; in pharmacologic doses, 25(OH)D3 is believed to act via interaction with

the VDR. Calcitriol is available in capsules containing 0.25 or 0.5 ug and as a solution for intravenous use (1.0 and 2.0 ug/mL). Calcitriol is efficacious in a variety of disorders (Chap. 341), but even low doses can cause hypercalcemia, leading to attempts to develop analogues with less calcemic activity. Two such calcitriol analogues have been approved in the United

States for the treatment of renal osteodystrophy; 19-nor-1,25-dihydroxyvitamin D2, and 24-epi-

1,25-dihydroxyvitamin D2 (Fig. 340-7). 1-Hydroxyvitamin D3 [1(OH)D3] is a potent

1,25(OH)2D3 agonist that is used in Europe and Japan. The structure of this analogue is

identical to that of the natural renal hormone with the exception that it lacks a C25 OH. In

humans, this analogue is rapidly metabolized by the liver to 1,25(OH)2D3. Topical preparations

of calcitriol (3 ug/g) in Europe and calcipotriene (50 ug/g) in Europe and the United States are used for the treatment of psoriasis. When applied over a large surface area, both can potentially cause hypercalcemia and hypercalciuria. Oral calcitriol is also effective for psoriasis and psoriatic arthritis.

When vitamin D is chemically manipulated to rotate the A ring through 180, the C3 -OH

assumes a geometric position that mimics the C1 -OH (Fig. 340-7). These compounds, called

pseudo-1-hydroxyvitamin D analogues, include the clinically useful dihydrotachysterol (DHT). This analogue is less effective in stimulating intestinal calcium transport on a weight basis than either vitamin D or 1,25(OH)2D. Because it does not require 1-hydroxylation to be active on

intestinal calcium transport, it is 3 to 10 times more potent than vitamin D in disease states that impair renal 25(OH)D-1-hydroxylase, such as hypoparathyroidism and chronic renal failure. Dihydrotachysterol is efficiently metabolized in the liver to 25-hydroxy-DHT, which is the biologically active form.

PARATHYROID HORMONE

PHYSIOLOGY

The primary function of PTH is to maintain the extracellular fluid (ECF) calcium concentration within a narrow normal range. The hormone acts directly on bone and kidney and indirectly on intestine through its effects on synthesis of 1,25(OH)2D to increase serum calcium

concentrations; in turn, PTH production is closely regulated by the concentration of serum ionized calcium. This feedback system is the critical homeostatic mechanism for maintenance of ECF calcium. Any tendency toward hypocalcemia, as might be induced by calcium-deficient diets, is counteracted by an increased secretion of PTH. This in turn (1) acts to increase the rate of dissolution of bone mineral, thereby increasing the flow of calcium from bone into blood; (2) reduces the renal clearance of calcium, returning more of the calcium filtered at the glomerulus into ECF; and (3) increases the efficiency of calcium absorption in the intestine. Immediate control of blood calcium is probably due to effects of the hormone on bone and, to a lesser extent, on renal calcium clearance. Maintenance of steady-state calcium balance, on the other hand, probably results from the effects of 1,25(OH)2D on calcium absorption (Chap.

340). The renal actions of the hormone are exerted at multiple sites and include inhibition of phosphate transport (proximal tubule), increased reabsorption of calcium (distal tubule), and stimulation of the renal 25(OH)D-1-hydroxylase. As much as 12 mmol (500 mg) calcium is transferred between the ECF and bone each day (a large amount in relation to the total ECF calcium pool), and PTH has a major effect on this transfer. The homeostatic role of the hormone can preserve calcium concentration in blood acutely at the cost of bone destruction.

PTH has multiple actions on bone, some direct and some indirect. It increases the rate of calcium release from bone into blood acutely; PTH-mediated changes in bone calcium release can be seen within minutes. The chronic effects of PTH are to increase the number of bone cells, both osteoblasts and osteoclasts, and to increase the remodeling of bone; these effects are apparent within hours after the hormone is given and persist for hours after PTH is withdrawn. Continuous exposure to elevated levels of PTH for days (as in hyperparathyroidism or long-term infusions in animals) leads to increased osteoclast-mediated bone resorption. However, the administration of PTH intermittently over days in animals or osteoporotic patients leads to a net stimulation of bone formation rather than bone breakdown. Striking increases, especially in trabecular bone in the spine and hip, have been reported with the use of PTH in combination with estrogen. PTH as monotherapy caused a highly significant reduction in fracture incidence in a worldwide placebo-controlled trial.

Osteoblasts (or stromal cell precursors), which have PTH receptors, are crucial to this bone-forming effect of PTH; osteoclasts, which appear to lack PTH receptors, mediate bone breakdown. PTH-mediated stimulation of osteoclasts is believed to be indirect, acting in part through cytokines released from osteoblasts to activate osteoclasts; in experimental studies of bone resorption in vitro, osteoblasts must be present for PTH to activate osteoclasts to resorb bone. The nature of the cytokines that stimulate osteoclasts is a subject of major interest. Insulin-like growth factor 1, interleukin 6, granulocyte-macrophage colony stimulating factor, and possibly other agents are candidates, but the definitive messenger(s) has not been determined. Direct cell-to-cell contact between osteoblasts (stromal cells) and osteoclast precursors is also key to osteoclast function. Cell-associated ligands and receptors, as well as soluble decoy receptors, are involved in these interactions (Chap. 340).

STRUCTURE

PTH is an 84-amino-acid single-chain peptide. The amino acid sequence of PTH has been characterized in multiple mammalian species, revealing marked conservation in the amino-terminal portion, which is critical for many biologic actions of the molecule. Synthetic fragments of the amino-terminal sequence as small as 1-14 residues are sufficient to activate the major receptor (see below). Biologic roles for the carboxyl-terminal region of PTH are under investigation; a separate receptor may exist for this region of the molecule. Fragments shortened or modified at the amino terminus still bind to the PTH receptor but lose the capacity to stimulate biologic responses. For example, the peptide composed of sequences 7-34 is a competitive inhibitor of active hormone binding to receptors in vitro but is a weak inhibitor in vivo.

BIOSYNTHESIS, SECRETION, AND METABOLISM

Synthesis Parathyroid cells have multiple methods of adapting to increased needs for PTH production. Most rapid (within minutes) is secretion of preformed hormone in response to hypocalcemia. Second, within hours, changes in gene activity and increased PTH mRNA are induced by sustained hypocalcemia. Finally, protracted challenge leads within days to cellular replication to increase gland mass.

PTH is initially synthesized as a larger molecule (preproparathyroid hormone, consisting of 115 amino acids), which is then reduced in size by a second cleavage (proparathyroid hormone, 90 amino acids) before secretion as the 84-amino-acid peptide. The hydrophobic regions of the preproparathyroid hormone serve a role in guiding transport of the polypeptide from sites of synthesis on polyribosomes through the endoplasmic reticulum to secretory granules. In one kindred with hypoparathyroidism, a mutation in the preprotein region of the gene disrupts this critical hydrophobic sequence and interferes with hormone secretion.

Studies with cloned and expressed PTH genes in vitro have demonstrated DNA regions involved in transcriptional control, including sites for interaction and regulation by the vitamin D receptor, as well as sites through which ambient calcium regulates transcription. Suppression of PTH gene activity at the transcriptional level by calcium is nearly maximal at physiologic concentrations; hypercalcemia results in no significant change. Hypocalcemia, however, increases transcriptional activity within hours. 1,25(OH)2D3 strongly suppresses PTH gene

transcription, though not when chronic hypocalcemia is induced experimentally in animals. In patients with renal failure, however, intravenous administration of supraphysiologic levels of 1,25(OH)2D3 or analogues of the active metabolite can dramatically suppress PTH

overproduction, which is sometimes difficult to control due to severe secondary hyperparathyroidism. Control over hormone stores is exerted by variation in the rates of proteolytic destruction of preformed hormone under the control of ECF calcium; high calcium increases and low calcium inhibits the proteolytic destruction of hormone stores. Regulation of hormone precursor processing and proteolytic destruction of preformed hormone (posttranslational regulation of hormone production) is an important mechanism for mediating rapid (minutes) changes in hormone availability.

Regulation of PTH Secretion PTH secretion increases steeply to a maximum value of five times the basal rate of secretion as calcium concentration falls from normal to the range of 1.9

to 2.0 mmol/L (7.5 to 8.0 mg/dL) (measured as total calcium). The ionized fraction of blood calcium is the important determinant of hormone secretion. Magnesium may influence hormone secretion in the same direction as calcium. It is unlikely, however, that physiologic variations in magnesium concentration affect PTH secretion. Severe intracellular magnesium deficiency impairs PTH secretion (see below).

The level of ECF calcium controls PTH secretion by interaction with a calcium sensor, a GPCR for which Ca2+ ions act as the ligand (see below). This receptor is a member of a distinctive subfamily of the GPCR superfamily that is characterized by a large extracellular domain suitable for "clamping" the small-molecule ligand. Stimulation of the receptor by high calcium levels leads to suppression of PTH secretion. The intracellular signals generated by the active receptor appear to be inositol triphosphate (IP3) and diacylglycerol (DAG) formed by

activation of phospholipase. The receptor is present in parathyroid glands and the calcitonin-secreting cells (C cells) of the thyroid, brain, and kidney. Genetic evidence has revealed a key biologic role for the calcium-sensing receptor in parathyroid gland responsiveness to calcium and, unexpectedly, in renal calcium clearance. Point mutations associated with loss of function cause a syndrome resembling hyperparathyroidism (FHH) but with hypocalciuria. On the other hand, gain-of-function mutations cause a form of hypocalcemia resembling hypoparathyroidism (see below).

Metabolism The secreted form of PTH is indistinguishable by immunologic criteria and by molecular size from the 84-amino-acid peptide (PTH 1-84) extracted from glands. However, much of the immunoreactive material found in the circulation is smaller than the extracted or secreted hormone. The principal circulating fragments of immunoreactive hormone lack a portion of the critical amino-terminal sequence required for biologic activity and, hence, are biologically inactive fragments (so-called middle- and carboxyl-terminal fragments). Much of the proteolysis of hormone occurs in the liver and kidney. However, fragments corresponding to the middle- and carboxyl-terminal portions have also been detected in effluent blood from the parathyroids and in the peripheral circulation; there is no convincing evidence, however, for circulating amino-terminal fragments. Peripheral metabolism of PTH does not appear to be regulated by physiologic states (high versus low calcium, etc.); hence peripheral metabolism of hormone, although responsible for rapid clearance of secreted hormone, appears to be a high-capacity, metabolically invariant catabolic process.

The rate of clearance of the secreted 84-amino-acid peptide from blood is more rapid than the rate of clearance of the biologically inactive fragment(s) corresponding to the middle- and carboxyl-terminal regions of PTH. Consequently, the interpretation of PTH immunoassays is influenced by the nature of the peptide fragments detected by the antibodies. Before the introduction of double-antibody assays designed to detect intact, biologically active hormone, most immunoassays also measured biologically inert long-lived fragments. Changes in the rate of production or clearance of fragments therefore alter the concentration of immunoreactive hormone.

Although the problems inherent in PTH measurements have been largely circumvented by use of double-antibody assays that detect only the intact molecule, new evidence has revealed the existence of a hitherto unappreciated larger PTH fragment that may affect the interpretation of most currently available double-antibody assays as well. A large amino-terminally truncated form of PTH, possibly PTH(7-84), is present in normal and uremic individuals in addition to PTH(1-84). The concentration of the putative 7-84 fragment relative to that of intact PTH(1-84) is higher with induced hypercalcemia (e.g., in uremic patients) than in eucalcemic or hypocalcemic conditions. The large fragment almost certainly cannot have (on the basis of structure-activity studies with PTH discussed above) much, if any, of the biologic potency of PTH. The suggestion that the PTH(7-84)-like fragment might act as an inhibitor of PTH action remains to be clarified. The identification of this fragment has clinical significance, particularly in renal failure, as efforts to prevent secondary hyperparathyroidism by a variety of measures (vitamin D analogues, higher calcium intake, and phosphate-lowering strategies) may have led to oversuppression of biologically active intact PTH when the presence of the amino-terminally truncated PTH was not appreciated. The role, if any, of excessive PTH suppression due to inaccurate measurement of PTH in adynamic bone disease in renal failure (see below) is unknown. Newer assays with extreme amino-terminal epitopes are being studied intensively.

PARATHYROID HORMONE-RELATED PROTEIN

The paracrine factor termed PTHrP is responsible for most instances of hypercalcemia of malignancy, a syndrome that resembles hyperparathyroidism. Many different cell types produce PTHrP, including brain, pancreas, heart, lung, mammary tissue, placenta, endothelial cells, and smooth muscle. In fetal animals, PTHrP directs transplacental calcium transfer, and high concentrations of PTHrP are produced in mammary tissue and secreted into milk. Human and bovine milk, for example, contain very high concentrations of the hormone; the biologic significance of the latter is unknown. PTHrP may also play a role in uterine contraction and other biologic functions, still being clarified in other tissue sites.

PTH and PTHrP, although distinctive products of different genes, exhibit considerable functional and structural homology (Fig. 341-1) and may have evolved from a shared ancestral gene. The structure of the gene for human PTHrP, however, is more complex than that of PTH, containing multiple exons and multiple sites for alternate splicing patterns during formation of the mRNA. Protein products of 141, 139, and 173 amino acids are produced, and other molecular forms may result from tissue-specific degradation at accessible internal cleavage sites. The biologic roles of these various molecular species and the nature of the circulating forms of PTHrP are unclear. It is uncertain whether PTHrP circulates at any significant level in normal human adults; as a paracrine factor, PTHrP may be produced, act, and be destroyed locally within tissues. In adults PTHrP appears to have little influence on calcium homeostasis, except in disease states, when large tumors, especially of the squamous cell type, lead to massive overproduction of the hormone (Fig. 341-1).

PTH AND PTHRP HORMONE ACTION

Because PTHrP shares a significant homology with PTH in the critical amino terminus, it binds to and activates the PTH/PTHrP receptor, indistinguishably from effects seen with PTH. The 500-amino-acid PTH/PTHrP receptor (also known as the PTH1 receptor) belongs to a subfamily of GPCR that includes those for glucagon, secretin, and vasoactive intestinal peptide. The extracellular regions are involved in hormone binding, and the intracellular domains, after hormone activation, bind G protein subunits to transduce hormone signaling into cellular responses through stimulation of second messengers (Fig. 341-2). A second PTH receptor (PTH2 receptor) is expressed in brain, pancreas, and several other tissues. Its amino acid sequence and the pattern of its binding and stimulatory response to PTH and PTHrP differ from those of the PTH1 receptor. The PTH/PTHrP receptor responds equivalently to PTH and PTHrP, whereas the PTH2 receptor responds only to PTH. The endogenous ligand and the physiologic significance of this receptor are not completely defined.

The PTH1 and PTH2 receptors can be traced backward in evolutionary time to fish. The zebrafish PTH1 and PTH2 receptors exhibit the same selective responses to PTH and PTHrP as do the human PTH1 and PTH2 receptors. The evolutionary conservation of structure and function suggests unique biologic roles for these receptors. Recently, a 39-amino-acid hypothalamic peptide, tubular infundibular peptide (TIP-39), has been characterized and is a likely natural ligand of the PTH2 receptor.

G proteins of the Gs class link the PTH/PTHrP receptor to adenylate cyclase, an enzyme that

generates cyclic AMP, leading to activation of protein kinase A. Coupling to G proteins of the Gq

class links hormone action to phospholipase C, an enzyme that generates inositol phosphates (e.g., IP3) and DAG, leading to activation of protein kinase C and intracellular calcium release

(Fig. 341-2). Studies using the cloned PTH/PTHrP receptor confirm that it can be coupled to more than one G protein and second-messenger kinase pathway, apparently explaining the multiplicity of pathways stimulated by PTH. Incompletely characterized second-messenger responses may be independent of phospholipase C or adenylate cyclase stimulation (the latter, however, is the strongest and best characterized second messenger signaling pathway for PTH).

The details of the biochemical steps by which an increased intracellular concentration of cyclic AMP, IP3, DAG, and intracellular Ca2+ lead to ultimate changes in ECF calcium and

phosphate ion translocation or bone cell function are unknown. Stimulation of protein kinases (A and C) and calcium transport channels is associated with a variety of hormone-specific tissue responses. These responses include inhibition of phosphate and bicarbonate transport, stimulation of calcium transport, and activation of renal 1-hydroxylase in the kidney. The responses in bone include effects on collagen synthesis; increased alkaline phosphatase, ornithine decarboxylase, citrate decarboxylase, and glucose-6-phosphate dehydrogenase activities; DNA, protein, and phospholipid synthesis; and calcium and phosphate transport. Ultimately, these biochemical events lead to an integrated hormonal response in bone turnover and calcium homeostasis.

PTH also activates Na+/Ca2+ exchanges in renal distal tubular sites and stimulates translocation of preformed calcium transport channels, moving them from the interior to the apical surface to mediate increased tubular uptake of calcium. PTH-dependent stimulation of phosphate excretion (blocking reabsorptionthe opposite effect from actions on calcium in the kidney) involves the sodium-dependent phosphate cotransporter, NPT-2, lowering its apical membrane content (and therefore function). Similar shifts may be involved in other renal tubular transport effects of PTH.

PTHrP exerts important developmental influences on fetal bone development and in adult physiology. A homozygous knockout of the PTHrP gene (or the gene for the PTH receptor) in mice causes a lethal deformity in which animals are born with severe skeletal deformities resembling chondrodysplasia (Fig. 341-3).

CALCITONIN (See also Chap. 339)

Calcitonin is a hypocalcemic peptide hormone that in several mammalian species acts as the physiologic antagonist to PTH. Calcitonin seems to be of limited physiologic significance in humans, at least in calcium homeostasis, as contrasted with a clearly definable role in calcium metabolism in many other mammalian species. Calcitonin is of medical significance, however, because of its role as a tumor marker in sporadic and hereditary cases of medullary carcinoma and its medical use as an adjunctive treatment in severe hypercalcemia and in Paget's disease of bone.

The hypocalcemic activity of calcitonin is accounted for primarily by inhibition of osteoclast-mediated bone resorption and secondarily by stimulation of renal calcium clearance. These effects are mediated by receptors on osteoclasts and renal tubular cells. Calcitonin exerts additional effects through receptors present in brain, gastrointestinal tract, and the immune system. The hormone, for example, exerts analgesic effects directly on cells in the hypothalamus and related structures, possibly by interacting with receptors for related peptide hormones, such as calcitonin gene-related peptide (CGRP) or amylin. The latter ligands have specific high-affinity receptors and also can bind to and trigger calcitonin receptors. The calcitonin receptors are homologous in structure to the PTH/PTHrP receptor.

The thyroid is the major source of the hormone, and the cells involved in calcitonin synthesis arise from neural crest tissue. During embryogenesis, these cells migrate into the ultimobranchial body, derived from the last branchial pouch. In submammalian vertebrates, the ultimobranchial body constitutes a discrete organ, anatomically separate from the thyroid gland; in mammals, the ultimobranchial gland fuses with and is incorporated into the thyroid gland.

The naturally occurring calcitonins consist of a peptide chain of 32 amino acids. There is considerable sequence variability among species. Calcitonin from salmon is 10 to 100 times more potent than mammalian forms in lowering serum calcium in animals. Calcitonin is synthesized as a precursor molecule that is four times larger than calcitonin itself. Analysis of the sequence of the coding portions of the gene for rat calcitonin indicates that at least two peptides flank calcitonin. It is likely (by analogy with the common precursor for adrenocorticotropic hormone and endorphin) that these peptides, of still uncharacterized biologic function, are released along with calcitonin.

There are two calcitonin genes, and , located on chromosome 11 in the general region of the -globulin and PTH genes; the transcriptional control of these genes is complex. Two different mRNA molecules are transcribed from the gene; one is translated into the precursor for calcitonin, and the other message is translated into an alternative product, CGRP. CGRP is synthesized wherever the calcitonin mRNA is expressed, e.g., in medullary carcinoma of the thyroid. The , or CGRP-2, gene is transcribed into the mRNA for CGRP in the central nervous system (CNS); this gene does not produce calcitonin, however. CGRP has cardiovascular actions and may serve as a neurotransmitter or play a developmental role in the CNS.

The secretion of calcitonin is under the direct control of blood calcium. The circulating level of calcitonin in humans is lower than that in many other species. In humans, changes in calcium and phosphate metabolism are not seen despite extreme variations in calcitonin production; no definite effects are attributable to calcitonin deficiency (totally thyroidectomized patients receiving only replacement thyroxine) or excess (patients with medullary carcinoma of the thyroid, a calcitonin-secreting tumor) (Chap. 339). Although there are no obvious abnormalities in calcium metabolism in patients with elevated calcitonin levels, bone remodeling is chronically suppressed. Calcitonin has been a useful pharmacologic agent to suppress bone resorption in Paget's disease (Chap. 343), has had limited use in the treatment of osteoporosis (Chap. 342), and is useful in early phases of treatment of severe hypercalcemia (see below).