Hormonal Control of Calcium & Phosphate Metabolism & Physiology of Bone PDF

Chapter 21 Hormonal Control of Calcium & Phosphate Metabolism & the Physiology of Bone Introduction Calcium is an essential intracellular signaling molecule and also plays a variety of extracellular functions, thus the control of body calcium concentrations is...

Chapter 21 Hormonal Control of Calcium & Phosphate Metabolism & the Physiology of Bone Introduction Calcium is an essential intracellular signaling molecule and also plays a variety of extracellular functions, thus the control of body calcium concentrations is vitally important. The components of the system that maintains calcium homeostasis include cell types that sense changes in extracellular calcium and release calcium- regulating hormones, and the targets of these hormones, including the kidneys, bones, and intestine, that respond with changes in calcium mobilization, excretion, or uptake. Three hormones are primarily concerned with the regulation of calcium homeostasis. Parathyroid hormone (PTH) is secreted by the parathyroid glands. Its main action is to mobilize calcium from bone and increase urinary phosphate excretion. 1,25-Dihydroxycholecalciferol is a steroid hormone formed from vitamin D by successive hydroxylations in the liver and kidneys. Its primary action is to increase calcium absorption from the intestine. Calcitonin, a calcium-lowering hormone that in mammals is secreted primarily by cells in the thyroid gland, inhibits bone resorption. Although the role of calcitonin seems to be relatively minor, all three hormones probably operate in concert to maintain the constancy of the calcium level in the body fluids. Phosphate homeostasis is likewise critical to normal body function, particularly given its inclusion in adenosine triphosphate (ATP), its role as a biological buffer, and its role as a modifier of proteins, thereby altering their functions. Many of the systems that regulate calcium homeostasis also contribute to that of phosphate, albeit sometimes in a reciprocal manner. CALCIUM & PHOSPHORUS METABOLISM Calcium The body of a young adult human contains about 1100 g (27.5 moles) of calcium. Ninety-nine percent of the calcium is in the skeleton. Plasma calcium, normally at a concentration of around 10 mg/dL (5 mEq/L, 2.5 mmol/L), is partly bound to protein and partly diffusible (Table 21–1). It is the free, ionized calcium (Ca2+) in the body fluids that is a vital second messenger and is necessary for blood coagulation, muscle contraction, and nerve function. A decrease in extracellular Ca2+ exerts a net excitatory effect on nerve and muscle cells in vivo. The result is hypocalcemic tetany, which is characterized by extensive spasms of skeletal muscle, involving especially the muscles of the extremities and the larynx. Laryngospasm can become so severe that the airway is obstructed and fatal asphyxia is produced. Ca2+ also plays an important role in blood clotting, but in vivo, fatal tetany would occur before compromising the clotting reaction.  Because the extent of Ca2+ binding by plasma proteins is proportional to the plasma protein level, it is important to know the plasma protein level when evaluating the total plasma calcium. Other electrolytes and pH also affect the free Ca2+ level. Thus, for example, symptoms of tetany appear at higher total calcium levels if the patient hyperventilates, thereby increasing plasma pH. Plasma proteins are more ionized when the pH is high, providing more protein anions to bind with Ca2+. The calcium in bone is of two types: a readily exchangeable reservoir and a much larger pool of stable calcium that is only slowly exchangeable. Two independent but interacting homeostatic systems affect the calcium in bone. One is the system that regulates plasma Ca2+, providing for the movement of about 500 mmol of Ca2+ per day into and out of the readily exchangeable pool in the bone (Figure 21–1). The other system involves bone remodeling by the constant interplay of bone resorption and deposition. However, the Ca2+ interchange between plasma and this stable pool of bone calcium is only about 7.5 mmol/d. The overall transportation process of Ca2+ across the brush border of intestinal epithelial cells to the bloodstream is regulated by 1,25-dihydroxycholecalciferol. Plasma Ca2+ is filtered in the kidneys, but 98–99% of the filtered Ca2+ is reabsorbed. About 60% of the reabsorption occurs in the proximal tubules and the remainder in the ascending limb of the loop of Henle and the distal tubule. Distal tubular reabsorption is regulated by PTH. PHOSPHORUS Phosphate is found in ATP, cyclic adenosine monophosphate (cAMP), 2,3- diphosphoglycerate, many proteins, and other vital compounds in the body. Phosphorylation and dephosphorylation of proteins are involved in the regulation of cell function. Therefore, it is not surprising that, like calcium, phosphate metabolism is closely regulated. Total body phosphorus is 500–800 g (16.1–25.8 moles), 85– 90% of which is in the skeleton. Total plasma phosphorus is about 12 mg/dL, with two-thirds of this total in organic compounds and the remaining inorganic phosphorus (Pi) mostly in PO43–, HPO42–, and H2PO4–. The amount of phosphorus normally entering bone is about 3 mg (97 μmol)/kg/d, with an equal amount leaving via reabsorption. Pi in the plasma is filtered in the glomeruli, and 85–90% of the filtered Pi is reabsorbed. Active transport in the proximal tubule accounts for most of the reabsorption and involves two related sodium-dependent Pi cotransporters, NaPi-IIa and NaPi-IIc. NaPi-IIa is powerfully inhibited by PTH, which causes its internalization and degradation and thus a reduction in renal Pi reabsorption. Pi is absorbed in the duodenum and small intestine. Many stimuli that increase Ca2+ absorption, including 1,25-dihydroxycholecalciferol, also increase Pi absorption. VITAMIN D & THE HYDROXYCHOLECALCIFEROLS CHEMISTRY The active transport of Ca2+ and PO43- from the intestine is increased by a metabolite of vitamin D. The term “vitamin D” is used to refer to a group of closely related sterols produced by the action of ultraviolet light on certain provitamins (Figure 21–2). Vitamin D3, which is also called cholecalciferol, is produced in the skin of mammals from 7-dehydrocholesterol by the action of sunlight. The reaction involves the rapid formation of previtamin D3, which is then converted more slowly to vitamin D3. Vitamin D3 and its hydroxylated derivatives are transported in the plasma bound to a globulin, vitamin D-binding protein (DBP). Vitamin D3 is also ingested in the diet. Vitamin D3 is metabolized by enzymes that are members of the cytochrome P450 (CYP) superfamily. In the liver, vitamin D3 is converted to 25- hydroxycholecalciferol (calcidiol, 25-OHD3). The 25-hydroxycholecalciferol is converted in the cells of the proximal tubules of the kidneys to the more active metabolite 1,25-dihydroxycholecalciferol, which is also called calcitriol or 1,25- (OH)2D3. 1,25-Dihydroxycholecalciferol is also made in the placenta, in keratinocytes in the skin, and in macrophages. The normal plasma level of 25- hydroxycholecalciferol is about 30 ng/mL, and that of 1,25- dihydroxycholecalciferol is about 0.03 ng/mL (approximately 100 pmol/L). The less active metabolite 24,25-dihydroxycholecalciferol is also formed in the kidneys (Figure 21–2). In addition to increasing Ca2+ absorption from the intestine, 1,25- dihydroxycholecalciferol facilitates Ca2+ reabsorption in the kidneys (proximal tubules), increases the synthetic activity of osteoblasts, and is necessary for normal calcification of matrix. The stimulation of osteoblasts brings about a secondary increase in the activity of osteoclasts. REGULATION OF SYNTHESIS The formation of 25-hydroxycholecalciferol does not appear to be stringently regulated. However, the formation of 1,25-dihydroxycholecalciferol in the kidneys, which is catalyzed by the renal 1α-hydroxylase, is regulated in a feedback manner by plasma Ca2+ and PO43+ (Figure 21–3). When the plasma Ca2+ level is high, little 1,25-dihydroxycholecalciferol is produced, and the kidneys produce the relatively inactive metabolite 24,25-dihydroxycholecalciferol instead. This effect of Ca2+ on production of 1,25-dihydroxycholecalciferol is the mechanism that brings about adaptation of Ca2+ absorption from the intestine. Conversely, expression of 1α- hydroxylase is stimulated by PTH, and when the plasma Ca2+ level is low, PTH secretion is increased. The production of 1,25-dihydroxycholecalciferol is also increased by low plasma PO43– levels and inhibited by high plasma PO43– levels, by a direct inhibitory effect of PO43– on the 1α-hydroxylase. Additional control of 1,25- dihydroxycholecalciferol formation results from a direct negative feedback effect of the metabolite on 1α-hydroxylase, a positive feedback action on the formation of 24,25-dihydroxycholecalciferol, and a direct action on the parathyroid gland to inhibit PTH expression. THE PARATHYROID GLANDS ANATOMY Humans usually have four parathyroid glands: two embedded in the superior poles of the thyroid and two in its inferior poles (Figure 21–4). Each parathyroid gland is a richly vascularized disk, about 3 × 6 × 2 mm, containing two distinct types of cells (Figure 21–5). The abundant chief cells, which contain a prominent Golgi apparatus plus endoplasmic reticulum and secretory granules, synthesize and secrete PTH. The less abundant and larger oxyphil cells contain oxyphil granules and large numbers of mitochondria in their cytoplasm. In humans, few oxyphil cells are seen before puberty, and thereafter they increase in number with age. SYNTHESIS & METABOLISM OF PTH Human PTH is a linear polypeptide with a molecular weight of 9500 that contains 84 amino acid residues. It is synthesized as part of a larger molecule containing 115 amino acid residues (preproPTH). On entry of preproPTH into the endoplasmic reticulum, a leader sequence is removed from the amino terminal to form the 90- amino-acid polypeptide proPTH. Six additional amino acid residues are removed from the amino terminal of proPTH in the Golgi apparatus, and the 84-amino-acid polypeptide PTH is packaged in secretory granules and released as the main secretory product of the chief cells. The normal plasma level of intact PTH is 10–55 pg/mL. The half-life of PTH is approximately 10 min, and the secreted polypeptide is rapidly cleaved by the Kupfer cells in the liver into fragments that are probably biologically inactive. PTH and these fragments are then cleared by the kidneys. ACTIONS PTH acts directly on bone to increase bone resorption and mobilize Ca2+. In addition to increasing plasma Ca2+, PTH increases phosphate excretion in the urine and thereby depresses plasma phosphate levels. This phosphaturic action is due to a decrease in reabsorption of phosphate via effects on NaPi-IIa in the proximal tubules. PTH also increases reabsorption of Ca2+ in the distal tubules, although Ca2+ excretion in the urine is often increased in hyperparathyroidism because the increase in the load of filtered calcium overwhelms the effect on reabsorption. PTH also increases the formation of 1,25-dihydroxycholecalciferol, and this increases Ca2+ absorption from the intestine. On a longer time scale, PTH stimulates both osteoblasts and osteoclasts. REGULATION OF SECRETION Circulating Ca2+ acts directly on the parathyroid glands in a negative feedback manner to regulate the secretion of PTH. When the plasma Ca2+ level is high, PTH secretion is inhibited and Ca2+ is deposited in the bones. When it is low, secretion is increased and Ca2+ is mobilized from the bones. 1,25-Dihydroxycholecalciferol acts directly on the parathyroid glands to decrease preproPTH mRNA. Increased plasma phosphate stimulates PTH secretion by lowering plasma levels of free Ca2+ and inhibiting the formation of 1,25- dihydroxycholecalciferol. Magnesium is required to maintain normal parathyroid secretory responses. Impaired PTH release along with diminished target organ responses to PTH account for the hypocalcemia that occasionally occurs in magnesium deficiency. CALCITONIN ORIGIN, SECRETION & METABOLISM Calcitonin is a Ca2+-lowering hormone. In mammals, calcitonin is produced by the parafollicular cells of the thyroid gland, which are also known as the clear or C cells. Calcitonin secretion is increased when the thyroid gland is exposed to a plasma calcium level of approximately 9.5 mg/dL. Above this level, plasma calcitonin is directly proportional to plasma calcium. β-Adrenergic agonists, dopamine, and estrogens also stimulate calcitonin secretion. Gastrin, cholecystokinin (CCK), glucagon, and secretin have also been reported to stimulate calcitonin secretion, with gastrin being the most potent stimulus. Thus, the plasma calcitonin level is elevated in Zollinger–Ellison syndrome and in pernicious anemia. However, the dose of gastrin needed to stimulate calcitonin secretion is supraphysiologic and not seen after eating in normal individuals, so dietary calcium in the intestine probably does not induce secretion of a calcium-lowering hormone prior to the calcium being absorbed. In any event, the actions of calcitonin are short-lived because it has a half- life of less than 10 min in humans. ACTIONS Receptors for calcitonin are found in bones and the kidneys. Calcitonin lowers circulating calcium and phosphate levels. It exerts its calcium-lowering effect by inhibiting bone resorption. This action is direct, and calcitonin inhibits the activity of osteoclasts in vitro. It also increases Ca2+ excretion in the urine. The exact physiologic role of calcitonin is uncertain. The calcitonin content of the human thyroid is low, and after thyroidectomy, bone density and plasma Ca2+ level are normal as long as the parathyroid glands are intact. In addition, after thyroidectomy, there are only transient abnormalities of Ca2+ homeostasis when a Ca2+ load is injected. This may be explained in part by secretion of calcitonin from tissues other than the thyroid. However, there is general agreement that the hormone has little long-term effect on the plasma Ca2+ level in adult animals and humans. Further, unlike PTH and 1,25-dihydroxycholecalciferol, calcitonin does not appear to be involved in phosphate homeostasis. Moreover, patients with medullary carcinoma of the thyroid have a very high circulating calcitonin level but no symptoms directly attributable to the hormone, and their bones are essentially normal. No syndrome due to calcitonin deficiency has been described. More hormone is secreted in young individuals, and it may play a role in skeletal development. In addition, it may protect the bones of the mother from excess calcium loss during pregnancy. Bone formation in the infant and lactation are major drains on Ca2+ stores, and plasma concentrations of 1,25-dihydroxycholecalciferol are elevated in pregnancy. They would cause bone loss in the mother if bone resorption were not simultaneously inhibited by an increase in the plasma calcitonin level. SUMMARY OF CALCIUM HOMEOSTATIC MECHANISMS The actions of the three principal hormones that regulate the plasma concentration of Ca2+ can now be summarized. PTH increases plasma Ca2+ by mobilizing this ion from bone. It increases Ca2+ reabsorption in the kidney, but this may be offset by the increase in filtered Ca2+. It also increases the formation of 1,25- dihydroxycholecalciferol. 1,25-Dihydroxycholecalciferol increases Ca2+ absorption from the intestine and increases Ca2+ reabsorption in the kidneys. Calcitonin inhibits bone resorption and increases the amount of Ca2+ in the urine. EFFECTS OF OTHER HORMONES & HUMORAL AGENTS ON CALCIUM METABOLISM Calcium metabolism is affected by various hormones in addition to 1,25- dihydroxycholecalciferol, PTH, and calcitonin. Glucocorticoids lower plasma Ca2+ levels by inhibiting osteoclast formation and activity, but over long periods they cause osteoporosis by decreasing bone formation and increasing bone resorption. They decrease bone formation by inhibiting protein synthesis in osteoblasts. They also decrease the absorption of Ca2+ and PO43– from the intestine and increase the renal excretion of these ions. The decrease in plasma Ca2+ concentration also increases the secretion of PTH, and bone resorption is facilitated. Growth hormone increases Ca2+ excretion in the urine, but it also increases intestinal absorption of Ca2+, and this effect may be greater than the effect on excretion, with a resultant positive calcium balance. Insulin-like growth factor I (IGF-I) generated by the action of growth hormone stimulates protein synthesis in bone. As noted previously, thyroid hormones may cause hypercalcemia, hypercalciuria, and, in some instances, osteoporosis. Estrogens prevent osteoporosis by inhibiting the stimulatory effects of certain cytokines on osteoclasts. Insulin increases bone formation, and there is significant bone loss in untreated diabetes. BONE PHYSIOLOGY Bone is a special form of connective tissue with a collagen framework impregnated with Ca2+ and PO43– salts, particularly hydroxyapatites, which have the general formula Ca10(PO4)6(OH)2. Bone is also involved in overall Ca2+ and PO43– homeostasis. It protects vital organs, and the rigidity it provides permits locomotion and the support of loads against gravity. Old bone is constantly being resorbed and new bone formed, permitting remodeling that allows it to respond to the stresses and strains that are put upon it. It is a living tissue that is well vascularized and has a total blood flow of 200–400 mL/min in adult humans. STRUCTURE There are two types of bone: compact or cortical bone, which makes up the outer layer of most bones (Figure 21–8) and accounts for 80% of the bone in the body; and trabecular or spongy bone inside the cortical bone, which makes up the remaining 20% of bone in the body. In compact bone, the surface-to-volume ratio is low, and bone cells lie in lacunae. They receive nutrients by way of canaliculi that ramify throughout the compact bone (Figure 21–8). Trabecular bone is made up of spicules or plates, with a high surface to volume ratio and many cells sitting on the surface of the plates. Nutrients diffuse from bone extracellular fluid (ECF) into the trabeculae, but in compact bone, nutrients are provided via haversian canals (Figure 21–8), which contain blood vessels. Around each haversian canal, collagen is arranged in concentric layers, forming cylinders called osteons or haversian systems. The protein in bone matrix is over 90% type I collagen, which is also the major structural protein in tendons and skin. This collagen, which weight for weight is as strong as steel, is made up of a triple helix of three polypeptides bound tightly together. Two of these are identical α1 polypeptides encoded by one gene, and one is an α2 polypeptide encoded by a different gene. Collagens make up a family of structurally related proteins that maintain the integrity of many different organs. Over 40 collagen genes that contribute to at least 28 distinct trimeric collagens have so far been identified. BONE GROWTH During fetal development, most bones are modeled in cartilage and then transformed into bone by ossification (enchondral bone formation). The exceptions are the clavicles, the mandibles, and certain bones of the skull in which mesenchymal cells form bone directly (intramembranous bone formation). During growth, specialized areas at the ends of each long bone (epiphyses) are separated from the shaft of the bone by a plate of actively proliferating cartilage, the epiphysial plate (Figure 21–9). The bone increases in length as this plate lays down new bone on the end of the shaft. The width of the epiphysial plate is proportional to the rate of growth. The width is affected by several hormones, but most markedly by the pituitary growth hormone and IGF-I. Linear bone growth can occur as long as the epiphyses are separated from the shaft of the bone, but such growth ceases after the epiphyses unite with the shaft (epiphysial closure). The cartilage cells stop proliferating, become hypertrophic, and secrete vascular endothelial growth factor (VEGF), leading to vascularization and ossification. The epiphyses of the various bones close in an orderly temporal sequence, the last epiphyses closing after puberty. The normal age at which each of the epiphyses closes is known, and the “bone age” of a young individual can be determined by radiographing the skeleton and noting which epiphyses are open and which are closed. The periosteum is a dense fibrous, vascular, and innervated membrane that covers the surface of bones. This layer consists of an outer layer of collagenous tissue and an inner layer of fine elastic fibres that can include cells that have the potential to contribute to bone growth. The periosteum covers all surfaces of the bone except for those capped with cartilage (eg, at the joints) and serves as a site of attachment of ligaments and tendons. As one ages, the periosteum becomes thinner and loses some of its vasculature. This renders bones more susceptible to injury and disease. BONE FORMATION & RESORPTION The cells responsible for bone formation are osteoblasts and the cells responsible for bone resorption are osteoclasts. Osteoblasts are modified fibroblasts. Their early development from the mesenchyme is the same as that of fibroblasts, with extensive growth factor regulation. Later, ossification-specific transcription factors, such as runt-related transcription factor 2 (Runx2; also known as core binding factor subunit alpha-1), contribute to their differentiation. Osteoclasts, on the other hand, are members of the monocyte family. Osteoclasts erode and absorb previously formed bone. They become attached to bone via integrins in a membrane extension called the sealing zone. This creates an isolated area between the bone and a portion of the osteoclast. Proton pumps (ie, H+- dependent ATPases) then move from endosomes into the cell membrane apposed to the isolated area, and they acidify the area to approximately pH 4.0. The acidic pH dissolves hydroxyapatite, and acid proteases secreted by the cell break down collagen, forming a shallow depression in the bone (Figure 21–10). The products of digestion are then endocytosed and move across the osteoclast by transcytosis, with release into the interstitial fluid. The collagen breakdown products have pyridinoline structures, and pyridinolines can be measured in the urine as an index of the rate of bone resorption.  Throughout life, bone is being constantly resorbed and new bone is being formed. The calcium in bone turns over at a rate of 100% per year in infants and 18% per year in adults. Bone remodeling is mainly a local process carried out in small areas by populations of cells called bone-remodeling units. First, osteoclasts resorb bone, and then osteoblasts lay down new bone in the same general area. This cycle takes about 100 days. Modeling drifts also occur in which the shapes of bones change as bone is resorbed in one location and added in another. Osteoclasts tunnel into cortical bone followed by osteoblasts, whereas trabecular bone remodeling occurs on the surface of the trabeculae. About 5% of the bone mass is being remodeled by about 2 million bone-remodeling units in the human skeleton at any one time. The renewal rate for bone is about 4% per year for compact bone and 20% per year for trabecular bone. The remodeling is related in part to the stresses and strains imposed on the skeleton by gravity.