Parathyroid Hormone, Calcitonin, Calcium and Phosphate Metabolism, Vitamin D, Bone, and Teeth

Hypocalcemia Causes Nervous System Excitement and Tetany

When the extracellular fluid concentration of calcium ions falls below normal, the nervous system becomes progressively more excitable because of increased neuronal membrane permeability to sodium ions, allowing easy initiation of action potentials. At plasma calcium ion concentrations about 50% below normal, the peripheral nerve fibers become so excitable that they begin to discharge spontaneously, initiating trains of nerve impulses that pass to the peripheral skeletal muscles to elicit tetanic muscle contraction. Consequently, hypocalcemia causes tetany. It also occasionally causes seizures because of its action of increasing excitability in the brain.

Figure 80-2 shows tetany in the hand, which usually occurs before tetany develops in most other parts of the body. This is called carpopedal spasm.

Tetany ordinarily occurs when the blood concentration of calcium falls from its normal level of 9.4 mg/dl to about 6 mg/dl, which is only 35% below the normal calcium concentration, and it is usually lethal at about 4 mg/dl.

In laboratory animals, extreme hypocalcemia can cause other effects that are seldom evident in patients, such as marked dilation of the heart, changes in cellular enzyme activities, increased membrane permeability in some cells (in addition to nerve cells), and impaired blood clotting.

Hypercalcemia Depresses Nervous System and Muscle Activity

When calcium concentration in the body fluids rises above normal, the nervous system becomes depressed and reflex activities of the central nervous system are sluggish. Also, increased calcium ion concentration decreases the QT interval of the heart and causes lack of appetite and constipation, probably because of depressed contractility of the muscle walls of the gastrointestinal tract.

These depressive effects begin to appear when the blood level of calcium rises above about 12 mg/dl, and they can become marked as the calcium level rises above 15 mg/dl. When the calcium concentration rises above about 17 mg/dl in the blood, calcium phosphate crystals are likely to precipitate throughout the body; this condition is discussed later in connection with parathyroid poisoning.

Intestinal Absorption and Fecal Excretion of Calcium and Phosphate

The usual rates of intake are approximately 1000 mg/day each for calcium and phosphorus, about the amounts in 1 liter of milk. Normally, divalent cations such as calcium ions are poorly absorbed from the intestines. However, as discussed later, vitamin D promotes calcium absorption by the intestines, and about 35% (350 mg/day) of the ingested calcium is usually absorbed; the remaining calcium in the intestine is excreted in the feces. An additional 250 mg/day of calcium enters the intestines via secreted gastrointestinal juices and sloughed mucosal cells. Thus, about 90% (900 mg/day) of the daily intake of calcium is excreted in the feces ( Figure 80-3 ).

Intestinal absorption of phosphate occurs easily. Except for the portion of phosphate that is excreted in the feces in combination with nonabsorbed calcium, almost all the dietary phosphate is absorbed into the blood from the gut and later excreted in the urine.

Renal Excretion of Calcium and Phosphate

Approximately 10% (100 mg/day) of the ingested calcium is excreted in the urine. About 41% of the plasma calcium is bound to plasma proteins and is therefore not filtered by the glomerular capillaries. The remainder is combined with anions such as phosphate (9%) or ionized (50%) and filtered through the glomeruli into the renal tubules.

Normally, the renal tubules reabsorb 99% of the filtered calcium, and about 100 mg/day are excreted in the urine (see Chapter 30 for further discussion of renal calcium excretion). Approximately 90% of the calcium in the glomerular filtrate is reabsorbed in the proximal tubules, loops of Henle, and early distal tubules.

In the late distal tubules and early collecting ducts, reabsorption of the remaining 10% is more variable, depending on the calcium ion concentration in the blood.

When calcium concentration is low, this reabsorption is great, and almost no calcium is lost in the urine. Conversely, even a minute increase in blood calcium ion concentration above normal increases calcium excretion markedly. We shall see later in this chapter that the most important factor controlling this reabsorption of calcium in the distal portions of the nephron, and therefore controlling the rate of calcium excretion, is PTH.

Renal phosphate excretion is controlled by an overflow mechanism, as explained in Chapter 30 . That is, when phosphate concentration in the plasma is below the critical value of about 1 mmol/L, all the phosphate in the glomerular filtrate is reabsorbed and no phosphate is lost in the urine. Above this critical concentration, however, the rate of phosphate loss is directly proportional to the additional increase. Thus, the kidneys regulate the phosphate concentration in the extracellular fluid by altering the rate of phosphate excretion in accordance with the plasma phosphate concentration and the rate of phosphate filtration by the kidneys.

However, as discussed later in this chapter, PTH can greatly increase phosphate excretion by the kidneys, thereby playing an important role in the control of plasma phosphate and calcium concentrations.

Organic Matrix of Bone

The organic matrix of bone is 90% to 95% collagen fibers, and the remainder is a homogeneous gelatinous medium called ground substance . The collagen fibers extend primarily along the lines of tensional force and give bone its powerful tensile strength.

The ground substance is composed of extracellular fluid plus proteoglycans, especially chondroitin sulfate and hyaluronic acid . These proteoglycans help control the deposition of calcium salts and are important in bone repair after injury, although some of their functions are still unclear.

Bone Salts

The crystalline salts deposited in the organic matrix of bone are composed principally of calcium and phosphate. The formula for the major crystalline salt, known as hydroxyapatite, is as follows:

Each crystal—about 400 angstroms (Å) long, 10 to 30 Å thick, and 100 Å—is shaped like a long, flat plate. The relative ratio of calcium to phosphorus can vary markedly under different nutritional conditions, with the calcium to phosphorus ratio on a weight basis varying between 1.3 and 2.0.

Magnesium, sodium, potassium, and carbonate ions are also present among the bone salts, although x-ray diffraction studies fail to show definite crystals formed by them. Therefore, they are believed to be conjugated to the hydroxyapatite crystals rather than organized into distinct crystals of their own. This ability of many types of ions to conjugate to bone crystals extends to many ions normally foreign to bone, such as strontium, uranium, plutonium, the other transuranic elements, lead, gold, and other heavy metals. Deposition of radioactive substances in the bone can cause prolonged irradiation of the bone tissues, and if a sufficient amount is deposited, an osteogenic sarcoma (bone cancer) may eventually develop.

Tensile and Compressional Strength of Bone

Each collagen fiber of cortical (compact) bone is composed of repeating periodic segments every 640 Å along its length; hydroxyapatite crystals lie adjacent to each segment of the fiber and are bound tightly to it. This intimate bonding prevents “shear” in the bone; that is, it prevents the crystals and collagen fibers from slipping out of place, which is essential in providing strength to the bone. In addition, the segments of adjacent collagen fibers overlap, also causing hydroxyapatite crystals to be overlapped like bricks keyed to one another in a brick wall.

The collagen fibers of bone, like those of tendons, have great tensile strength, whereas the calcium salts have great compressional strength. These combined properties plus the degree of bondage between the collagen fibers and the crystals provide a bony structure that has both extreme tensile strength and compressional strength.

Hydroxyapatite Does Not Precipitate in Extracellular Fluid Despite Supersaturation of Calcium and Phosphate Ions

The concentrations of calcium and phosphate ions in extracellular fluid are considerably greater than those required to cause precipitation of hydroxyapatite. However, inhibitors are present in almost all tissues of the body, as well as in plasma, to prevent precipitation; one such inhibitor is pyrophosphate . Therefore, hydroxyapatite crystals fail to precipitate in normal tissues except in bone despite the state of supersaturation of the ions.

Mechanism of Bone Calcification

The initial stage in bone production is secretion of collagen molecules (called collagen monomers ) and ground substance (mainly proteoglycans) by osteoblasts . The collagen monomers polymerize rapidly to form collagen fibers; the resultant tissue becomes osteoid, a cartilage-like material differing from cartilage in that calcium salts readily precipitate in it. As the osteoid is formed, some of the osteoblasts become entrapped in the osteoid and become quiescent. At this stage, they are called osteocytes.

Within a few days after the osteoid is formed, calcium salts begin to precipitate on the surfaces of the collagen fibers. The precipitates first appear at intervals along each collagen fiber, forming minute nidi that rapidly multiply and grow over a period of days and weeks into the finished product, hydroxyapatite crystals.

The initial calcium salts to be deposited are not hydroxyapatite crystals but amorphous compounds (noncrystalline), a mixture of salts such as CaHPO ₄ × 2H ₂ O, Ca ₃ (PO ₄ ) ₂ × 3H ₂ O, and others. Then, by a process of substitution and addition of atoms, or reabsorption and reprecipitation, these salts are converted into the hydroxyapatite crystals over a period of weeks or months. A few percent may remain permanently in the amorphous form, which is important because these amorphous salts can be absorbed rapidly when there is a need for extra calcium in the extracellular fluid.

Although the mechanism that causes calcium salts to be deposited in the osteoid is not fully understood, regulation of this process appears to depend to a great extent on pyrophosphate, which inhibits hydroxyapatite crystallization and calcification of the bone. The levels of pyrophosphate, in turn, are regulated by at least three other molecules. One of the most important of these molecules is a substance called tissue-nonspecific alkaline phosphatase (TNAP), which breaks down pyrophosphate and keeps its levels in check so that bone calcification can occur as needed. TNAP is secreted by osteoblasts into the osteoid to neutralize pyrophosphate, and once pyrophosphate has been neutralized, the natural affinity of the collagen fibers for calcium salts causes hydroxyapatite crystallization. The importance of TNAP in bone mineralization is illustrated by the finding that mice with genetic deficiency of TNAP, which causes pyrophosphate levels to rise too high, are born with soft bones that are not adequately calcified.

The osteoblast also secretes at least two other substances that regulate bone calcification: (1) nucleotide pyrophosphatase phosphodiesterase 1 (NPP1), which produces pyrophosphate outside the cells, and (2) ankylosis protein (ANK), which contributes to the extracellular pool of pyrophosphate by transporting it from the interior to the surface of the cell. Deficiencies of NPP1 or ANK cause decreased extracellular pyrophosphate and excessive calcification of bone, such as bone spurs, or even calcification of other tissues such as tendons and ligaments of the spine, which occurs in people with a form of arthritis called ankylosing spondylitis .

Precipitation of Calcium in Nonosseous Tissues Under Abnormal Conditions

Although calcium salts usually do not precipitate in normal tissues besides bone, under abnormal conditions, they can precipitate. For example, they precipitate in arterial walls in arteriosclerosis and cause the arteries to become bonelike tubes. Likewise, calcium salts frequently deposit in degenerating tissues or in old blood clots. Presumably, in these cases, the inhibitor factors that normally prevent deposition of calcium salts disappear from the tissues, thereby allowing precipitation.

Deposition of Bone by the Osteoblasts

Bone is continually being deposited by osteoblasts, and it is continually being resorbed where osteoclasts are active ( Figure 80-5 ). Osteoblasts are found on the outer surfaces of the bones and in the bone trabecular cavities. A small amount of osteoblastic activity occurs continually in all living bones (on ≈4% of all surfaces at any given time in an adult), so at least some new bone is being formed constantly.

Resorption of Bone—Function of the Osteoclasts

Bone is also being continually resorbed in the presence of osteoclasts, which are large, phagocytic, multinucleated cells (containing as many as 50 nuclei) that are derivatives of monocytes or monocyte-like cells formed in the bone marrow. The osteoclasts are normally active on less than 1% of the bone surfaces of an adult and, as discussed later, PTH controls the bone resorptive activity of osteoclasts.

Histologically, bone absorption occurs immediately adjacent to the osteoclasts. The mechanism of this resorption is believed to be the following: The osteoclasts send out villus-like projections toward the bone, forming a ruffled border adjacent to the bone ( Figure 80-6 ). The villi secrete two types of substances: (1) proteolytic enzymes, released from the lysosomes of the osteoclasts, and (2) several acids, including citric acid and lactic acid, released from the mitochondria and secretory vesicles. The enzymes digest or dissolve the organic matrix of the bone, and the acids cause dissolution of the bone salts. The osteoclastic cells also imbibe minute particles of bone matrix and crystals by phagocytosis, eventually also dissoluting these particles and releasing the products into the blood.

As discussed later, PTH stimulates osteoclast activity and bone resorption, but this process occurs through an indirect mechanism. The bone-resorbing osteoclast cells do not have PTH receptors. Instead, the osteoblasts signal osteoclast precursors to form mature osteoclasts. Two osteoblast proteins responsible for this signaling are receptor activator for nuclear factor κB ligand (RANKL) and macrophage colony-stimulating factor, which are both necessary for formation of mature osteoclasts.

PTH binds to receptors on the adjacent osteoblasts, stimulating synthesis of RANKL, which is also called osteoprotegerin ligand (OPGL). RANKL binds to its receptors (RANK) on preosteoclast cells, causing them to differentiate into mature multinucleated osteoclasts. The mature osteoclasts then develop a ruffled border and release enzymes and acids that promote bone resorption.

Osteoblasts also produce osteoprotegerin (OPG), sometimes called osteoclastogenesis inhibitory factor , a cytokine that inhibits bone resorption. OPG acts as a “decoy,” binding to RANKL and preventing it from interacting with its receptor, thereby inhibiting differentiation of preosteoclasts into mature osteoclasts that resorb bone. OPG opposes the bone resorptive activity of PTH, and mice with a genetic deficiency of OPG have severe decreases in bone mass compared with mice that have normal OPG formation.

Although the factors regulating OPG are not well understood, vitamin D and PTH appear to stimulate production of mature osteoclasts through the dual action of inhibiting OPG production and stimulating RANKL formation. Glucocorticoids also promote osteoclast activity and bone resorption by increasing RANKL production and decreasing formation of OPG. On the other hand, the hormone estrogen stimulates OPG production. The balance of OPG and RANKL produced by osteoblasts therefore plays a major role in determining osteoclast activity and bone resorption.

The therapeutic importance of the OPG-RANKL pathway is currently being exploited. Novel drugs that mimic the action of OPG by blocking the interaction of RANKL with its receptor appear to be useful for treating bone loss in postmenopausal women and in some patients with bone cancer.

Bone Deposition and Resorption Are Normally in Equilibrium

Except in growing bones, the rates of bone deposition and resorption are normally equal, so the total mass of bone remains constant. Osteoclasts usually exist in small but concentrated masses, and once a mass of osteoclasts begins to develop, it usually eats away at the bone for about 3 weeks, creating a tunnel that ranges in diameter from 0.2 to 1 millimeter and is several millimeters long. At the end of this time, the osteoclasts disappear, the tunnel is invaded by osteoblasts, and new bone begins to develop. Bone deposition continues for several months, with the new bone being laid down in successive layers of concentric circles (lamellae) on the inner surfaces of the cavity until the tunnel is filled. Deposition of new bone ceases when the bone begins to encroach on the blood vessels supplying the area. The canal through which these vessels run, called the haversian canal, is all that remains of the original cavity. Each new area of bone deposited in this way is called an osteon, as shown in Figure 80-7 .

Value of Continual Bone Remodeling

The continual deposition and resorption of bone have several important functions. First, bone ordinarily adjusts its strength in proportion to the degree of bone stress. Consequently, bones thicken when subjected to heavy loads. Second, even the shape of the bone can be rearranged for proper support of mechanical forces by deposition and resorption of bone in accordance with stress patterns. Third, because old bone becomes relatively brittle and weak, new organic matrix is needed as the old organic matrix degenerates. In this manner, the normal toughness of bone is maintained. Indeed, the bones of children, in whom the rates of deposition and absorption are rapid, show little brittleness in comparison with the bones of the elderly, in whom the rates of deposition and resorption are slow.

Control of the Rate of Bone Deposition by Bone “Stress”

Bone is deposited in proportion to the compressional load that the bone must carry. For example, the bones of athletes become considerably heavier than those of nonathletes. Also, if a person has one leg in a cast but continues to walk on the opposite leg, the bone of the leg in the cast becomes thin and as much as 30% decalcified within a few weeks, whereas the opposite bone remains thick and normally calcified. Therefore, continual physical stress stimulates osteoblastic deposition and calcification of bone.

Bone stress also determines the shape of bones under certain circumstances. For example, if a long bone of the leg breaks in its center and then heals at an angle, the compression stress on the inside of the angle causes increased deposition of bone. Increased resorption occurs on the outer side of the angle where the bone is not compressed. After many years of increased deposition on the inner side of the angulated bone and resorption on the outer side, the bone can become almost straight, especially in children because of the rapid remodeling of bone at younger ages.

Repair of a Fracture Activates Osteoblasts

Fracture of a bone in some way maximally activates all the periosteal and intraosseous osteoblasts involved in the break. Also, immense numbers of new osteoblasts are formed almost immediately from osteoprogenitor cells, which are bone stem cells in the surface tissue lining bone, called the “bone membrane.” Therefore, within a short time, a large bulge of osteoblastic tissue and new organic bone matrix, followed shortly by the deposition of calcium salts, develops between the two broken ends of the bone. This area is called a callus.

Many orthopedic surgeons use the phenomenon of bone stress to accelerate fracture healing. This acceleration is achieved through use of special mechanical fixation apparatuses for holding the ends of the broken bone together so that the patient can continue to use the bone immediately. This use causes stress on the opposed ends of the broken bones, which accelerates osteoblastic activity at the break and often shortens convalescence.

Cholecalciferol (Vitamin D ₃ ) Is Formed in the Skin

Several compounds derived from sterols belong to the vitamin D family, and they all perform similar functions. Vitamin D ₃ (also called cholecalciferol ) is the most important of these compounds and is formed in the skin as a result of irradiation of 7-dehydrocholesterol, a substance normally in the skin, by ultraviolet rays from the sun. Consequently, appropriate exposure to the sun prevents vitamin D deficiency. The additional vitamin D compounds that we ingest in food are identical to the cholecalciferol formed in the skin, except for the substitution of one or more atoms that do not affect their function.

Cholecalciferol Is Converted to 25-Hydroxycholecalciferol in the Liver

The first step in the activation of cholecalciferol is to convert it to 25-hydroxycholecalciferol, which occurs in the liver. The process is limited because the 25-hydroxycholecalciferol has a feedback inhibitory effect on the conversion reactions. This feedback effect is extremely important for two reasons.

First, the feedback mechanism precisely regulates the concentration of 25-hydroxycholecalciferol in the plasma, an effect that is shown in Figure 80-9 . Note that intake of vitamin D ₃ can increase many times, and yet the concentration of 25-hydroxycholecalciferol remains nearly normal. This high degree of feedback control prevents excessive action of vitamin D when intake of vitamin D ₃ is altered over a wide range.

Second, this controlled conversion of vitamin D ₃ to 25-hydroxycholecalciferol conserves the vitamin D stored in the liver for future use. Once vitamin D ₃ is converted, the 25-hydroxycholecalciferol persists in the body for only a few weeks, whereas in the vitamin D form, it can be stored in the liver for many months.

Formation of 1,25-Dihydroxycholecalciferol in the Kidneys and Its Control by Parathyroid Hormone

Figure 80-8 also shows the conversion in the proximal tubules of the kidneys of 25-hydroxycholecalciferol to 1 , 25-dihydroxycholecalciferol . This latter substance is by far the most active form of vitamin D because the previous products in the scheme of Figure 80-8 have less than 1/1000 of the vitamin D effect. Therefore, in the absence of the kidneys, vitamin D loses almost all its effectiveness.

Note also in Figure 80-8 that the conversion of 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol requires PTH. In the absence of PTH, almost none of the 1,25-dihydroxycholecalciferol is formed. Therefore, PTH exerts a potent influence in determining the functional effects of vitamin D in the body.