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Brief Overview of Calcium, Vitamin D, Parathyroid Hormone Metabolism, and Calcium-Sensing Receptor Function
SECTION II
PARATHYROID DISEASES AND CANCER
CHAPTER 6
Brief Overview of Calcium, Vitamin D, Parathyroid Hormone Metabolism, and Calcium-Sensing Receptor Function ALEXANDER SHIFRIN, MD, FACS, FACE, ECNU, FEBS (ENDOCRINE), FISS
CALCIUM METABOLISM Calcium, vitamin D, magnesium, phosphate, and parathyroid hormone (PTH) are involved in bone physiology and supplement each other in function. Calcium carries two major functions: it is the structural material to strengthen bones, and it serves as an important physiological and neuromuscular regulator. Bones serve as storage reservoirs for calcium. The mechanism of neuromuscular regulation by the calcium is very complex and depends on a number of factors. The plasma concentration of calcium must be kept within narrow limits, ranging between 8.8 mg/dL and 10.4 mg/dL (2.2e2.6 mmol/L), to prevent serious metabolic disequilibrium and severe physiological consequences to the body functions. The total body calcium content depends on the body’s weight. In an average size adult, the normal amount of calcium is approximately 1000e1300 g. Calcium content is particularly critical in the fetus and neonates. About 99% of the calcium is within bones, and the remaining 1% is either in the intracellular form or circulating in the plasma (extracellular). There are three main fractions of calcium in the plasma: the active ionized form of calcium (free calcium), protein bound, and complex. The ionized calcium consists of approximately 50% of the total plasma calcium, 40% of plasma calcium is bound to plasma proteins, mostly albumin, and 10% is complexed with anions such as bicarbonate, sulfate, phosphate, lactate, and citrate. The concentration of ionized calcium in normal circumstances is maintained between 1.1 and 1.3 mmol/L. This very narrow range is required to maintain normal neuromuscular activity.
Total serum concentration of calcium should be corrected to the plasma albumin level. Each 1 g/dL (10 g/L) of albumin binds about 0.8 mg/dL of calcium. The reduction in serum albumin concentration will lower the total calcium concentration without affecting the ionized calcium concentration and therefore without producing any signs or symptoms of hypocalcemia. However, hypocalcemia or hypercalcemia symptoms may develop if true (corrected or ionized) calcium level decreases or increases.1e6 Binding of calcium by albumin is pH dependent. The lower the pH, the less calcium binds to albumin (albumin has fewer binding sites for calcium). This results in the increase of free (unbound) calcium concentration. The higher the pH, the lower is the free calcium concentration.2,7 Calcium homeostasis involves the following mechanisms: calcium absorption in the gastrointestinal tract, excretion of the calcium within the renal tubules, and deposition into or removal of the calcium from the bone. Plasma calcium level is regulated by five main factors: PTH, active form of vitamin D (1,25(OH)2D), PTH-related peptide (PTHrP), serum phosphate, and fibroblast growth factor 23 (FGF23).3 Calcium is primarily controlled by PTH. It is the parathyroid hormone that maintains the concentration of unbound (free) calcium within a narrow range. Active form of vitamin D (1,25(OH)2D) also plays a significant role in calcium regulation, although in lesser extent than PTH. Calcitonin and magnesium play a role in calcium regulation as well. PTHrP factors influence the calcium transport across the membranes in the gastrointestinal tract and in the renal tubules.1e4
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Renal tubules Ca
transcellular
claudin 16 claudin 19
Mg
Renal cell Ca calbindin 28K
Blood vessel
NCX1
PMCA1b PMCA1,4
Mg
Ca Mg paracellular
TRPM 6/7
TRPV 6 Ca
1,25-(OH)2D
PMCA1b
NCX1
passive
1,25-(OH)2D
transcellular
calbindin D9K Na/K ATPase
PTH1R
Ca
Enterocyte
TRPM 6/7
TRPV 5
Intesne
Blood vessel FIG. 6.1 Calcium and magnesium transport. Calcium transport and magnesium transport occur by paracellular (passive) and transcellular (active) mechanisms. In the intestine, the mechanism of calcium absorption is by paracellular and transcellular transport. Paracellular transport occurs mostly in renal tubules, and it is facilitated by the protein claudin-16 and claudin-19. Transcellular (active) transport involves three steps: entry of calcium into the cell through TRPV5 (in renal tubules) and TRPV6 (in the intestine), binding of calcium to calbindins (calbindin 28K in renal tubules, and calbindin-D9K in enterocytes) that diffuse calcium to the basolateral membrane, and then transport calcium through the basolateral membrane via ATP-dependent Ca-ATPase (PMCA1b) and a Na/Ca exchanger (NCX1) (stimulated by PTH). There is also a mechanism of passive diffusion of calcium. Transcellular magnesium absorption facilitated by TRPM6 (in renal tubules and intestinal cells) and TRPM7 (more widely distributed), and at the basolateral membrane magnesium is transported by Na/K ATPase. Active vitamin D (1,25-(OH)2D) promotes calcium-transport of ATPase and TRPV6 through its receptors on duodenal cells and stimulates the expression of calbindin-D9K and calbindinD28K in renal tubules and enterocytes. PTH regulates NCX1 and influences the expression of TRPV5 in renal tubules. Ca, calcium; Mg, magnesium; Na/K ATPase, transports magnesium through basolateral membrane; NCX1, NAþ/Caþ-exchanger; PMCA1b, ATP-dependent calcium transporting ATPase (transports calcium through basolateral membrane); PTH, parathyroid hormone; PTH1R, PTH receptor (also called PTH/PTHrP); TRPM, transient receptor potential melastatin type; TRPV, transient receptor potential vanilloid type.
There are two mechanisms of calcium transport: paracellular (passive) and transcellular (active) (Fig. 6.1). Paracellular (passive) transfer occurs mostly in the renal tubules. Diet, such as high sodium, protein, or acid, increases calcium excretion. Seventy percent of passive reabsorption occurs in the proximal renal tubule in conjunction with sodium, 20% is in the loop of Henle, and 5%e10% in the distal tubule.1e3,6e8 Paracellular transfer depends on the concentration gradient, and is facilitated by proteins, called claudins, such as claudin-16 (paracellin-1) and claudin-19.3 For example, mutations in claudin-16 result in renal magnesium wasting syndrome and impaired claudinmediated paracellular resorption of magnesium and calcium.7,9,10 Transcellular (active) calcium transport in renal tubules is facilitated by transient receptor potential vanilloid type 5 (TRPV5)11,12 and calcium-
binding protein calbindin 28K.3 In conjunction with the calcium, renal tubules also facilitate passive transport of magnesium.1e3,6e8 In the intestine, the mechanism of calcium absorption is by passive (paracellular) and active (transcellular) transport. The main mechanism of absorption is active transport and it involves three steps: initial absorption from the intestinal lumen, transcellular transport, and transport of calcium across the basolateral membrane.1e3,7 Transcellular transport of calcium in the enterocyte is facilitated by the calcium-binding protein, calbindin-D9K,7,13 and by transient receptor potential vanilloid type (TRPV6).3,7,8 Furthermore, there are three steps involved in active calcium transport: entry of calcium into the cell through TRPV5 and TRPV6, binding of calcium to calbindin that diffuses it to the basolateral membrane, and transport of calcium
CHAPTER 6
Brief Overview of Calcium, Vitamin D, Parathyroid Hormone Metabolism
through the basolateral membrane via an ATPdependent Ca-ATPase (PMCA1b) and a Na/Ca exchanger (NCX1).7,8 TRPV5 and TRPV6 channels are under the negative calcium feedback mechanism, and are downregulated by calcium influx through these channels.7 NCX1 is expressed in different organs, including distal nephrons. It is under the direct stimulation of PTH and activated vitamin D (1,25-(OH)2D3), and it stimulates calcium reabsorption in distal nephron.7 NCX2 and NCX3 are only present in brain and skeletal muscle.7,14,15 PMCAs have high-affinity calcium efflux pumps that present in four different isoforms, PMCA1- 4. PMCA1 and PMCA4 (including PMCA1b) are isoforms expressed in the kidneys, while only PMCA1b is the predominant isoform expressed in the small intestine.7 The third mechanism is by passive diffusion or extrusion of calcium-calbindin complex vesicles. Active vitamin D stimulates calciumtransporting ATPase (PMCA1b) and TRPV6 through its receptors on duodenal cells and in small intestine.1,3,7,8 Duodenum and upper jejunum are principal sites for the active (transcellular) calcium absorption, whereas paracellular calcium absorption occurs throughout the entire length of the intestinal track.7,16 When food passes through the intestine, it transits the duodenum only for a short period, and the remaining time it passes through the distal part of the small intestine. When calcium intake is high, the passive (paracellular) mechanism is the predominant process of calcium absorption in the intestine. When calcium intake is low, the transcellular (active) calcium transport is the main mechanism of calcium absorption.7,17,18 It is important not to underestimate the influence of magnesium on calcium metabolism. Plasma magnesium is required for normal secretion of PTH. Hypomagnesemia results in inadequate PTH secretion, which cannot be corrected by calcium supplementation alone without adding magnesium. Normal plasma magnesium level is 0.7e1.2 mmol/L.3 The process of magnesium absorption is similar to the process of calcium absorption in the small intestine. In the renal tubules, the reabsorption of magnesium is mostly passive occurring in the ascending loop of Henle, along with calcium.3,8,19 Transcellular magnesium absorption is facilitated by two proteins, TRP melastatin type 6 (TRPM6) (in renal tubules and the intestinal cells) and TRP melastatin type 7 (TRPM7) (widely distributed, including the intestine), which are similar to the same proteins from the TRP channel family involved in calcium transport.3,8 At basolateral surface, magnesium is transported by the Na/K ATPase.3
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Blood vessel transcellular
Ca maternal-facing basement membrane
TRPV 6
Ca
Ca
Ca
calbindin 28K
calbindin D9K
Placenta fetal-facing basement membrane
PMCA
Blood vessel FIG. 6.2 Transcellular (active) calcium transport through the
placenta. Ca, calcium; PMCA, ATP-dependent CaeATPase; TRPV6, transient receptor potential vanilloid type 6.
During pregnancy, calcium is actively transported across the placenta from the maternal circulation to the fetal circulation in late gestation by the syncytiotrophoblast cells (the epithelial layer separating the maternal and fetal circulation)7,20 (Fig. 6.2). TRPV6 expression appears to be predominant over the TRPV5 expression.21 Calcium diffuses across these cells by binding with both calbindin-D9K and calbindinD28K. Then, calcium is actively transported through the fetal-facing basement membrane, specifically by a calcium-ATPase (PMCA).22,23 The last 7 days of gestation expression of calbindin-D9K increases more than 100-fold, and PMCA over 2-fold.24
CALCIUM-SENSING RECEPTOR Calcium binds to the calcium-sensing receptor (CaSR) on the parathyroid glands and alters PTH secretion. In addition to calcium, magnesium also binds to CaSR and influences the PTH secretion in a similar manner as calcium. CaSR is present and functioning not only in the parathyroid glands but in other organs as well. For example, in the gastrointestinal tract CaSR plays the role of “food sensors” by stimulating the parietal and G cells of the stomach to secrete gastric acid and gastrin. CaSR influences the exocrine pancreas secretion, regulates fluid retention in the large intestine, and affects intestinal motility through its expression in the myenteric and submucosal plexus of the enteric nervous system. The CaSR is also present in tissues such as renal
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tubules, bone, and cartilage. In the kidneys, it regulates the release of renin that modulates blood pressure and fluid balance.1,3,4 In bones, it regulates the mineralization and controls the differentiation, proliferation, and activity of osteoblasts. CaSR also has a role in the central and peripheral nervous system. Lastly, it is expressed in the breast tissue, ovaries, uterus, testes, and prostate. Mutations of the CaSR result in either activation or inactivation of the receptor. Deactivating, heterogeneous mutations of the CaSR gene result in familial hypocalciuric hypercalcemia (FHH). It causes the disabling of calcium-dependent inhibition of renal calcium reabsorption, leads to hypocalciuria, with altering of calcium-dependent feedback inhibition of PTH secretion, resulting in mild elevation in the serum PTH level and associated hypercalcemia.1,3,4,25 Deactivating homozygous or compound heterozygous mutations of the CaSR gene in neonates presents as neonatal severe hyperparathyroidism (NSHPT) and results in severe hypercalcemia, marked elevations in serum PTH level, and near-total loss of calciummediated feedback control of PTH secretion.25,55 This results in skeletal demineralization and pathological fractures.26,27 The bone loss will be reversed by the parathyroidectomy.28 There are two activating mutations of the CaSR: autosomal-dominant hypocalcemia (ADH) and Bartter Syndrome Type 5. ADH is a benign chronic condition, incidentally found on routine blood work.29 It presents with a longstanding history of paresthesia, intermittent fasciculations, and childhood seizures. The diagnosis is made by the presence of hypocalcemia with inappropriately normal or very low serum PTH levels, and enhanced inhibition of renal calcium reabsorption resulting in hypercalciuria. Associated finding could include hypomagnesemia, hyperphosphatemia, and hypocalciuria.30 Other findings include a reduction of the glomerular filtration rate of ionized calcium but with an intact renal calcium reabsorption mechanism.31,32 Bartter Syndrome Type 5 results from severe activating mutations of the CaSR with the development of the renal salt wasting syndrome, hypocalcemia, suppressed serum PTH levels, hypocalciuria, hyperphosphatemia, ectopic mineralization, and cataracts. CaSR activation on the contraluminal membrane of the thick ascending limb disables NaCl reabsorption. CaSR normally promotes phosphate retention by suppressing renal phosphate excretion and PTH-induced inhibition of phosphate reabsorption.33e36 This syndrome is treated by giving negative modulators of the CaSR, called calcilytics.37,38
VITAMIN D METABOLISM Vitamin D (Calciferol) is a fat-soluble vitamin. It functions in a role of a hormone or prohormone because of its regulatory role in calcium homeostasis and bone metabolism. It has a much lower effect on calcium homeostasis compared to PTH. Vitamin D is received in two different forms: vitamin D-2 (ergocalciferol) and vitamin D-3 (cholecalciferol). Eighty percent of the body’s source of vitamin D comes from sunlight as vitamin D3, and the rest of it comes from food as vitamin D2. Both are metabolized in a similar manner, and they are equally potent. Ultraviolet light of wavelength 270e300 nm is adsorbed by the melanocytes. Its action is directed at the cholesterol precursor, 7-dehydroxycholesterol, where it breaks the B ring of the steroid molecule, creating a secosteroid. Then, by body heat action, it is converted to cholecalciferol (Vitamin D3). The peak of vitamin D3 synthesis is 6 weeks after maximal exposure to sunlight.1e3 Darker-skinned individuals require a sixfold greater amount of sunlight exposure to synthesize the same amount of Vitamin D as a lighter-skinned individual.39 Vitamin D2 is received form plants and food such as fish, liver oil, fatty fish, egg yolk, liver, and milk fortified with vitamin D-2.1e3,40 It is synthesized from ergosterol and structurally different from cholecalciferol. The process of generation of the active form of vitamin D involves three steps: synthesis, conversion to the major circulating form of vitamin D, 25-hydroxyvitamin D (25(OH)D) in the liver, and lastly conversion by the 1a-hydroxylase to the biologically active form of vitamin D, 1,25(OH)2D in the kidney (Fig. 6.3). After the synthesis, vitamin D binds to the vitamin D-binding protein (DBP), passes through the liver, where it is metabolized by cytochrome 450 enzyme to 25(OH)D, and circulates in plasma bound to DBP protein. Passing through the kidney, 25(OH) Vitamin D is metabolized by 25-hydroxyvitamin D 1 a-hydroxylase into its active hormone, 1,25(OH)2D. Activity of 1a-hydroxylase is controlled by PTH via its cAMP protein kinase action, and by hypocalcemia. Vitamin D is stored in the liver and adipose tissue. Obese people have more vitamin D into their fat store and lower circulating level of vitamin D. The primary action of 1,25(OH)2D is to stimulate the formation of calcium-binding proteins within the intestinal epithelial cells that helps absorption of calcium in the intestine41e46 (Fig. 6.3). It also helps in phosphorus absorption. 1,25(OH)2D stimulates the expression of calbindin-D9K and NCX1in renal tubules, calbindin-D28K, PMCA1b, and enterocytes.7
CHAPTER 6
Brief Overview of Calcium, Vitamin D, Parathyroid Hormone Metabolism
67
sun
80%
20% UVB
skin
plants, yeast
7-dehydroxycholesterol to secosteroid
food, milk Vitamin D2 (ergocalciferol)
Vitamin D3 (cholecalciferol) DBP
Liver: 25-Hydroxylase DBP
25(OH)D 25(OH)D - DBP
PTH hypocalcemia
Kidney: 25-hydroxyvitamin D 1α-hydroxylase
Intesne: formaon of CaBP, absorpon of Ca & Phos bone
1,25(OH)2D
kidney
FIG. 6.3 Vitamin D metabolism. Ca, calcium; CaBP, calcium binding protein; DBP, vitamin D-binding protein; Phos, phosphorus; PTH, parathyroid hormone.
Optimal serum level of 25(OH) vitamin D is between 30 and 50 ng/mL.41e46 Levels below 20 ng/mL are considered insufficient and suboptimal for skeletal health. Levels below 10 ng/mL are considered to be severely deficient. Deficiency may be the result of low dietary intake, malabsorption, target organ resistance, or impaired 1a-hydroxylation of 25(OH)D, although high level, above 50 ng/mL, may cause toxicity.41 At high concentrations, the stimulation of osteoblasts to produce cytokines causes an increased production of osteoclasts that lead to bone resorption.40
PARATHYROID HORMONE METABOLISM Parathyroid glands play a key role in maintaining the extracellular calcium concentration. The parathyroid gland is composed of two types of cells: chief and the oxyphil.47 Parathyroid chief cells, the main type of parathyroid gland cells, produce PTH. Oxyphil cells produce PTHrP, calcitriol, and some other factors.48 Sensitivity of the sestamibi scan depends on a radioisotope that is retained in mitochondria-rich cells, which are predominantly oxyphil cells (those that do not produce PTH). This explains the reason why the sestamibi scan is not always accurate in the identification of parathyroid adenoma. When the content of parathyroid oxyphil cells in the parathyroid adenoma is greater than
25%, the sensitivity of the sestamibi scan in the late phase of the test is much higher (sensitivity of about 78% for adenoma with more than 25% of oxyphil cells content vs. sensitivity of 33% for adenoma with 1%e25% of oxyphil cells, and sensitivity of 0% for adenoma with no oxyphil cells).49 PTH is 84 amino acid polypeptide synthesized from 115-amino acid polypeptide pre-pro-PTH within chief cells of the parathyroid gland. PTH synthesis is constant, and secretion is continuous through the parathyroid cell membrane, rather than sporadic, with circadian dynamics and in a pulsatile fashion.1e3,6 Almost no hormone is stored within the glands themselves. Among 84 amino acids, only 34 amino acid terminals are required for full activity of the hormone. During episodes of hypocalcemia, PTH is secreted within seconds by exocytosis. The half-life of the PTH is approximately 3 to 4 min in the bloodstream. Maier et al. studied PTH hormone elimination kinetics during a parathyroidectomy and reported that PTH half-life was 3.43 min, while ionized calcium started to decrease only at 30 min after the adenoma was removed. PTH elimination occurs in the kidneys and in the liver. Once secreted, it is rapidly taken up mostly by the liver, cleaved into fragments, and cleared by the kidneys. The current basis for modern assays measuring of intact PTH level is the identification of these fragments in the
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blood stream. In the liver, PTH degrades through two mechanisms. First, by enzymatic reaction in the Kupffer cells follow a MichaeliseMentgen kinetic (the bestknown models of enzyme kinetics). Second, PTH diś tributes into the space of Disse (the perisinusoidal space in between a hepatocyte and a sinusoid) and intracellular space, taken up by hepatocytes where it is modulating their glucose and amino acid metabolism and does not reenter the circulation. Forty percent of PTH metabolism occurs in the kidneys, where it is filtered and then reabsorbed in the proximal tubular cells, where it is degraded without reentering the circulation. PTH works through two receptors: PTH1R and PTH2R. PTH1R (also called PTH/PTHrP receptor) binds two molecules, the PTH and PTHrP, but only, the PTH molecule binds to the PTH2R. PTH1R is predominantly expressed in bones and kidneys. PTH2R only presents in the central nervous system. The main function of PTH is to regulate calcium homeostasis. The principal target organs for PTH are bones and kidneys (through PTH1R).1e7,50,51 In bones, under physiological circumstances, PTH promotes bone formation via receptors on the osteoblasts. During hypocalcemia, PTH stimulates bone resorption to maintain normal calcium balance and restore normocalcemia. PTH alters the balance between the expression of the receptor activator of nuclear factor kappa-B ligand (RANKL) produced by osteoclast, and the receptor osteoprotegerin (OPG) produced by osteoblasts. PTH has either a catabolic or anabolic effect, depending on the dose and periodicity of the PTH signal. Catabolic effect on bones develops with continuous exposure to PTH, as in primary hyperparathyroidism. PTH is changing the balance between RANKL and OPG in favor of bone resorption and demineralization: the encoding for RANKL increases, and encoding for OPG mRNA decreases.1e6 This effect of PTH on RANKL and OPG diminishes in about 1 year after a successful parathyroidectomy.6,52 Anabolic effect develops with low doses, intermittent PTH secretion.6 In the proximal tubules of the kidneys nephrons, the main action of the PTH is activation of 25-hydroxyvitamin D 1a-hydroxylase that converts 25-hydroxyvitamin D (25(OH)D) to its active form, 1,25(OH)2D, which then facilitates the absorption of both calcium and phosphate from the intestines.2,3,7,53,54 In distal tubules, it promotes reabsorption of calcium and magnesium and excretion of phosphate, by action through PTH1R receptor.3 This mechanism of PTH binding to PTH1R involves stimulation of adenylyl cyclase and increasing cyclic AMP (cAMP) concentrations that activate phospholipase C pathway.2 PTH also has the effect on bicarbonate and
amino acid reabsorption in the proximal tubule. This results in hyperparathyroidism-related mild form of Fanconi syndrome (the generalized dysfunction of the proximal tubule presented with polyuria, hypokalemia, glycosuria, hypophosphatemia, and low molecular weight proteinuria). This is also resolved when hyperparathyroidism is reversed.1,3,6
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18. Bronner F, Pansu D. Nutritional aspects of calcium absorption. J Nutr. 1999;129(9e12). 19. Schweigel M, Martens H. Magnesium transport in the gastrointestinal tract. Front Biosci. 2000;5:D666eD677. 20. Faulk WP, McIntyre JA. Immunological studies of human trophoblast: markers, subsets and functions. Immunol Rev. 1983;75:139e175. 21. Peng JB, Brown EM, Hediger MA. Structural conservation of the genes encoding CaT1, CaT2, and related cation channels. Genomics. 2001;76:99e109. 22. Belkacemi L, Gariepy G, Mounier C, Simoneau L, Lafond J. Calbindin-D9k (CaBP9k) localization and levels of expression in trophoblast cells from human term placenta. Cell Tissue Res. 2004;315:107e117. 23. Belkacemi L, Simoneau L, Lafond J. Calcium-binding proteins: distribution and implication in mammalian placenta. Endocrine. 2002;19:57e64. 24. Glazier JD, Atkinson DE, Thornburg KL, et al. Gestational changes in Ca2þ transport across rat placenta and mRNA for calbindin9K and Ca(2þ)-ATPase. Am J Physiol Regul Integr Comp Physiol. 1992;263:R930eR935. 25. Conigrave AD. The calcium-sensing receptor and the parathyroid: past, present, future. Front Physiol. 2016;7:563. 26. Pollak MR, Chou YH, Marx SJ, et al. Familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism. Effects of mutant gene dosage on phenotype. J Clin Investig. 1994;93:1108e1112. 27. Brown EM, Pollak M, Seidman CE, et al. Calcium-ionsensing cell-surface receptors. N Engl J Med. 1995;333: 234e240. 28. Marx SJ, Lasker RD, Brown EM, et al. Secretory dysfunction in parathyroid cells from a neonate with severe primary hyperparathyroidism. J Clin Endocrinol Metab. 1986;62: 445e449. 29. Pearce SH, Williamson C, Kifor O, et al. A familial syndrome of hypocalcemia with hypercalciuria due to mutations in the calcium-sensing receptor. N Engl J Med. 1996;335:1115e1122. 30. Tan YM, Cardinal J, Franks AH, et al. Autosomal dominant hypocalcemia: a novel activating mutation (E604K) in the cysteine-rich domain of the calcium-sensing receptor. J Clin Endocrinol Metab. 2003;88:605e610. 31. Thakker RV. Diseases associated with the extracellular calcium-sensing receptor. Cell Calcium. 2004;35:275e282. 32. Egbuna OI, Brown EM. Hypercalcaemic and hypocalcaemic conditions due to calcium-sensing receptor mutations. Best Pract Res Clin Rheumatol. 2008;22:129e148. 33. Riccardi D, Brown E. Physiology and pathophysiology of the calcium-sensing receptor in the kidney. Am J Physiol Renal Physiol. 2010;298:F485eF499. 34. Riccardi D, Traebert M, Ward DT, et al. Dietary phosphate and parathyroid hormone alter the expression of the calcium-sensing receptor (CaR) and the Naþ-dependent Pi transporter (NaPi-2) in the rat proximal tubule. Pflügers Archiv. 2000;441:379e387. 35. Ba J, Brown D, Friedman PA. Calcium-sensing receptor regulation of PTH-inhibitable proximal tubule phosphate transport. Am J Physiol. 2003;285:F1233eF1243.
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36. Riccardi D, Valenti G. Localization and function of the renal calcium-sensing receptor. Nat Rev Nephrol. 2016;12: 414e425. 37. Mayr B, Glaudo M, Schöfl C. Activating calcium-sensing receptor mutations: prospects for future treatment with calcilytics. Trends Endocrinol Metabol. 2016;27:643e652. 38. Nemeth EF, Goodman WG. Calcimimetic and calcilytic drugs: feats, flops, and futures. Calcif Tissue Int. 2016;98: 341e358. 39. Lo CW, Paris PW, Holick MF. Indian and Pakistani immigrants have the same capacity as Caucasians to produce vitamin D in response to ultraviolet irradiation. Am J Clin Nutr. 1986;44(5):683e685. 40. Winter W, Kleerekoper M. Bone and mineral metabolism. In: Risteli J, Risteli L, Burtis CA, Ashwood ER, Bruns D, eds. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 5th ed. San Diego, CA: Elsevier Publishing; 2012:1765. 41. Sanders KM, Stuart AL, Williamson EJ, et al. Annual highdose oral vitamin D and falls and fractures in older women: a randomized controlled trial. J Am Med Assoc. 2010;303(18):1815e1822. 42. Dawson-Hughes B. Vitamin D deficiency in adults: definition, clinical manifestations, and treatment. In: Drezner MK, Rosen CJ, eds. UpToDate. Waltham, MA: UpToDate; 2016 (updated: March, 2016). 43. Dawson-Hughes B, Harris SS, Krall EA, Dallal GE. Effect of calcium and vitamin D supplementation on bone density in men and women 65 years of age or older. N Engl J Med. 1997;337(10):670e676. 44. Chapuy MC, Pamphile R, Paris E, et al. Combined calcium and vitamin D3 supplementation in elderly women: confirmation of reversal of secondary hyperparathyroidism and hip fracture risk: the Decalyos II study. Osteoporos Int. 2002;13(3):257e264. 45. Trivedi DP, Doll R, Khaw KT. Effect of four monthly oral vitamin D3 (cholecalciferol) supplementation on fractures and mortality in men and women living in the community: randomised double blind controlled trial. BMJ. 2003;326(7387):469. 46. Bilezikian JP, Brandi ML, Eastell R, et al. Guidelines for the management of asymptomatic primary hyperparathyroidism: summary statement from the Fourth International Workshop. Clin Endocrinol Metabol. 2014;99(10): 3561e3569. 47. Isono H, Shoumura S, Emura S. Ultrastructure of the parathyroid gland. Histol Histopathol. 1990;5(1):95e112. 48. Ritter CS, Haughey BH, Miller B, Brown AJ. Differential gene expression by oxyphil and chief cells of human parathyroid glands. J Clin Endocrinol Metab. 2012;97(8): E1499eE1505. 49. Carpentier A, Jeannotte S, Verreault J, et al. Preoperative localization of parathyroid lesions in hyperparathyroidism: relationship between technetium-99m-MIBI uptake and oxyphil cell content. J Nucl Med. 1998;39(8):1441e1444. 50. Zindel D, Engel S, Bottrill AR, et al. Identification of key phosphorylation sites in PTH1R that determine arrestin3 binding and fine-tune receptor signaling. Biochem J. 2016;473(22):4173e4192.
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51. Maier GW, Kreis ME, Renn W, Pereira PL, Häring HU, Becker HD. Parathyroid hormone after adenectomy for primary hyperparathyroidism. A study of peptide hormone elimination kinetics in humans. J Clin Endocrinol Metab. 1998;83(11):3852e3856. 52. Stilgren LS, Rettmer E, Eriksen EF, Hegedus L, BeckNielsen H, Abrahamsen B. Skeletal changes in osteoprotegerin and receptor activator of nuclear factor-kappaB ligand mRNA levels in primary hyperparathyroidism: effect of parathyroidectomy and association with bone metabolism. Bone. 2004;35:256e265.
53. El-Hajj Fuleihan G, Brown EM. Parathyroid hormone secretion and action. In: Rosen CJ, ed. UpToDate. Waltham, MA: UpToDate; 2014. 54. Winter WE, Harris NS. Calcium biology and disorders. In: Clarke W, ed. Con- Temporary Practice in Clinical Chemistry. 2nd ed. Washington, DC: AACC Press; 2011:506. 55. Ward BK, Magno AL, Davis EA, et al. Functional deletion of the calcium-sensing receptor in a case of neonatal severe hyperparathyroidism. J Clin Endocrinol Metab. 2004; 89(8):3721e3730.
PARATHYROID DISEASES AND CANCER
CHAPTER 6
Brief Overview of Calcium, Vitamin D, Parathyroid Hormone Metabolism, and Calcium-Sensing Receptor Function ALEXANDER SHIFRIN, MD, FACS, FACE, ECNU, FEBS (ENDOCRINE), FISS
CALCIUM METABOLISM Calcium, vitamin D, magnesium, phosphate, and parathyroid hormone (PTH) are involved in bone physiology and supplement each other in function. Calcium carries two major functions: it is the structural material to strengthen bones, and it serves as an important physiological and neuromuscular regulator. Bones serve as storage reservoirs for calcium. The mechanism of neuromuscular regulation by the calcium is very complex and depends on a number of factors. The plasma concentration of calcium must be kept within narrow limits, ranging between 8.8 mg/dL and 10.4 mg/dL (2.2e2.6 mmol/L), to prevent serious metabolic disequilibrium and severe physiological consequences to the body functions. The total body calcium content depends on the body’s weight. In an average size adult, the normal amount of calcium is approximately 1000e1300 g. Calcium content is particularly critical in the fetus and neonates. About 99% of the calcium is within bones, and the remaining 1% is either in the intracellular form or circulating in the plasma (extracellular). There are three main fractions of calcium in the plasma: the active ionized form of calcium (free calcium), protein bound, and complex. The ionized calcium consists of approximately 50% of the total plasma calcium, 40% of plasma calcium is bound to plasma proteins, mostly albumin, and 10% is complexed with anions such as bicarbonate, sulfate, phosphate, lactate, and citrate. The concentration of ionized calcium in normal circumstances is maintained between 1.1 and 1.3 mmol/L. This very narrow range is required to maintain normal neuromuscular activity.
Total serum concentration of calcium should be corrected to the plasma albumin level. Each 1 g/dL (10 g/L) of albumin binds about 0.8 mg/dL of calcium. The reduction in serum albumin concentration will lower the total calcium concentration without affecting the ionized calcium concentration and therefore without producing any signs or symptoms of hypocalcemia. However, hypocalcemia or hypercalcemia symptoms may develop if true (corrected or ionized) calcium level decreases or increases.1e6 Binding of calcium by albumin is pH dependent. The lower the pH, the less calcium binds to albumin (albumin has fewer binding sites for calcium). This results in the increase of free (unbound) calcium concentration. The higher the pH, the lower is the free calcium concentration.2,7 Calcium homeostasis involves the following mechanisms: calcium absorption in the gastrointestinal tract, excretion of the calcium within the renal tubules, and deposition into or removal of the calcium from the bone. Plasma calcium level is regulated by five main factors: PTH, active form of vitamin D (1,25(OH)2D), PTH-related peptide (PTHrP), serum phosphate, and fibroblast growth factor 23 (FGF23).3 Calcium is primarily controlled by PTH. It is the parathyroid hormone that maintains the concentration of unbound (free) calcium within a narrow range. Active form of vitamin D (1,25(OH)2D) also plays a significant role in calcium regulation, although in lesser extent than PTH. Calcitonin and magnesium play a role in calcium regulation as well. PTHrP factors influence the calcium transport across the membranes in the gastrointestinal tract and in the renal tubules.1e4
Advances in Treatment and Management in Surgical Endocrinology. https://doi.org/10.1016/B978-0-323-66195-9.00006-6 Copyright © 2020 Elsevier Inc. All rights reserved.
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Renal tubules Ca
transcellular
claudin 16 claudin 19
Mg
Renal cell Ca calbindin 28K
Blood vessel
NCX1
PMCA1b PMCA1,4
Mg
Ca Mg paracellular
TRPM 6/7
TRPV 6 Ca
1,25-(OH)2D
PMCA1b
NCX1
passive
1,25-(OH)2D
transcellular
calbindin D9K Na/K ATPase
PTH1R
Ca
Enterocyte
TRPM 6/7
TRPV 5
Intesne
Blood vessel FIG. 6.1 Calcium and magnesium transport. Calcium transport and magnesium transport occur by paracellular (passive) and transcellular (active) mechanisms. In the intestine, the mechanism of calcium absorption is by paracellular and transcellular transport. Paracellular transport occurs mostly in renal tubules, and it is facilitated by the protein claudin-16 and claudin-19. Transcellular (active) transport involves three steps: entry of calcium into the cell through TRPV5 (in renal tubules) and TRPV6 (in the intestine), binding of calcium to calbindins (calbindin 28K in renal tubules, and calbindin-D9K in enterocytes) that diffuse calcium to the basolateral membrane, and then transport calcium through the basolateral membrane via ATP-dependent Ca-ATPase (PMCA1b) and a Na/Ca exchanger (NCX1) (stimulated by PTH). There is also a mechanism of passive diffusion of calcium. Transcellular magnesium absorption facilitated by TRPM6 (in renal tubules and intestinal cells) and TRPM7 (more widely distributed), and at the basolateral membrane magnesium is transported by Na/K ATPase. Active vitamin D (1,25-(OH)2D) promotes calcium-transport of ATPase and TRPV6 through its receptors on duodenal cells and stimulates the expression of calbindin-D9K and calbindinD28K in renal tubules and enterocytes. PTH regulates NCX1 and influences the expression of TRPV5 in renal tubules. Ca, calcium; Mg, magnesium; Na/K ATPase, transports magnesium through basolateral membrane; NCX1, NAþ/Caþ-exchanger; PMCA1b, ATP-dependent calcium transporting ATPase (transports calcium through basolateral membrane); PTH, parathyroid hormone; PTH1R, PTH receptor (also called PTH/PTHrP); TRPM, transient receptor potential melastatin type; TRPV, transient receptor potential vanilloid type.
There are two mechanisms of calcium transport: paracellular (passive) and transcellular (active) (Fig. 6.1). Paracellular (passive) transfer occurs mostly in the renal tubules. Diet, such as high sodium, protein, or acid, increases calcium excretion. Seventy percent of passive reabsorption occurs in the proximal renal tubule in conjunction with sodium, 20% is in the loop of Henle, and 5%e10% in the distal tubule.1e3,6e8 Paracellular transfer depends on the concentration gradient, and is facilitated by proteins, called claudins, such as claudin-16 (paracellin-1) and claudin-19.3 For example, mutations in claudin-16 result in renal magnesium wasting syndrome and impaired claudinmediated paracellular resorption of magnesium and calcium.7,9,10 Transcellular (active) calcium transport in renal tubules is facilitated by transient receptor potential vanilloid type 5 (TRPV5)11,12 and calcium-
binding protein calbindin 28K.3 In conjunction with the calcium, renal tubules also facilitate passive transport of magnesium.1e3,6e8 In the intestine, the mechanism of calcium absorption is by passive (paracellular) and active (transcellular) transport. The main mechanism of absorption is active transport and it involves three steps: initial absorption from the intestinal lumen, transcellular transport, and transport of calcium across the basolateral membrane.1e3,7 Transcellular transport of calcium in the enterocyte is facilitated by the calcium-binding protein, calbindin-D9K,7,13 and by transient receptor potential vanilloid type (TRPV6).3,7,8 Furthermore, there are three steps involved in active calcium transport: entry of calcium into the cell through TRPV5 and TRPV6, binding of calcium to calbindin that diffuses it to the basolateral membrane, and transport of calcium
CHAPTER 6
Brief Overview of Calcium, Vitamin D, Parathyroid Hormone Metabolism
through the basolateral membrane via an ATPdependent Ca-ATPase (PMCA1b) and a Na/Ca exchanger (NCX1).7,8 TRPV5 and TRPV6 channels are under the negative calcium feedback mechanism, and are downregulated by calcium influx through these channels.7 NCX1 is expressed in different organs, including distal nephrons. It is under the direct stimulation of PTH and activated vitamin D (1,25-(OH)2D3), and it stimulates calcium reabsorption in distal nephron.7 NCX2 and NCX3 are only present in brain and skeletal muscle.7,14,15 PMCAs have high-affinity calcium efflux pumps that present in four different isoforms, PMCA1- 4. PMCA1 and PMCA4 (including PMCA1b) are isoforms expressed in the kidneys, while only PMCA1b is the predominant isoform expressed in the small intestine.7 The third mechanism is by passive diffusion or extrusion of calcium-calbindin complex vesicles. Active vitamin D stimulates calciumtransporting ATPase (PMCA1b) and TRPV6 through its receptors on duodenal cells and in small intestine.1,3,7,8 Duodenum and upper jejunum are principal sites for the active (transcellular) calcium absorption, whereas paracellular calcium absorption occurs throughout the entire length of the intestinal track.7,16 When food passes through the intestine, it transits the duodenum only for a short period, and the remaining time it passes through the distal part of the small intestine. When calcium intake is high, the passive (paracellular) mechanism is the predominant process of calcium absorption in the intestine. When calcium intake is low, the transcellular (active) calcium transport is the main mechanism of calcium absorption.7,17,18 It is important not to underestimate the influence of magnesium on calcium metabolism. Plasma magnesium is required for normal secretion of PTH. Hypomagnesemia results in inadequate PTH secretion, which cannot be corrected by calcium supplementation alone without adding magnesium. Normal plasma magnesium level is 0.7e1.2 mmol/L.3 The process of magnesium absorption is similar to the process of calcium absorption in the small intestine. In the renal tubules, the reabsorption of magnesium is mostly passive occurring in the ascending loop of Henle, along with calcium.3,8,19 Transcellular magnesium absorption is facilitated by two proteins, TRP melastatin type 6 (TRPM6) (in renal tubules and the intestinal cells) and TRP melastatin type 7 (TRPM7) (widely distributed, including the intestine), which are similar to the same proteins from the TRP channel family involved in calcium transport.3,8 At basolateral surface, magnesium is transported by the Na/K ATPase.3
65
Blood vessel transcellular
Ca maternal-facing basement membrane
TRPV 6
Ca
Ca
Ca
calbindin 28K
calbindin D9K
Placenta fetal-facing basement membrane
PMCA
Blood vessel FIG. 6.2 Transcellular (active) calcium transport through the
placenta. Ca, calcium; PMCA, ATP-dependent CaeATPase; TRPV6, transient receptor potential vanilloid type 6.
During pregnancy, calcium is actively transported across the placenta from the maternal circulation to the fetal circulation in late gestation by the syncytiotrophoblast cells (the epithelial layer separating the maternal and fetal circulation)7,20 (Fig. 6.2). TRPV6 expression appears to be predominant over the TRPV5 expression.21 Calcium diffuses across these cells by binding with both calbindin-D9K and calbindinD28K. Then, calcium is actively transported through the fetal-facing basement membrane, specifically by a calcium-ATPase (PMCA).22,23 The last 7 days of gestation expression of calbindin-D9K increases more than 100-fold, and PMCA over 2-fold.24
CALCIUM-SENSING RECEPTOR Calcium binds to the calcium-sensing receptor (CaSR) on the parathyroid glands and alters PTH secretion. In addition to calcium, magnesium also binds to CaSR and influences the PTH secretion in a similar manner as calcium. CaSR is present and functioning not only in the parathyroid glands but in other organs as well. For example, in the gastrointestinal tract CaSR plays the role of “food sensors” by stimulating the parietal and G cells of the stomach to secrete gastric acid and gastrin. CaSR influences the exocrine pancreas secretion, regulates fluid retention in the large intestine, and affects intestinal motility through its expression in the myenteric and submucosal plexus of the enteric nervous system. The CaSR is also present in tissues such as renal
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tubules, bone, and cartilage. In the kidneys, it regulates the release of renin that modulates blood pressure and fluid balance.1,3,4 In bones, it regulates the mineralization and controls the differentiation, proliferation, and activity of osteoblasts. CaSR also has a role in the central and peripheral nervous system. Lastly, it is expressed in the breast tissue, ovaries, uterus, testes, and prostate. Mutations of the CaSR result in either activation or inactivation of the receptor. Deactivating, heterogeneous mutations of the CaSR gene result in familial hypocalciuric hypercalcemia (FHH). It causes the disabling of calcium-dependent inhibition of renal calcium reabsorption, leads to hypocalciuria, with altering of calcium-dependent feedback inhibition of PTH secretion, resulting in mild elevation in the serum PTH level and associated hypercalcemia.1,3,4,25 Deactivating homozygous or compound heterozygous mutations of the CaSR gene in neonates presents as neonatal severe hyperparathyroidism (NSHPT) and results in severe hypercalcemia, marked elevations in serum PTH level, and near-total loss of calciummediated feedback control of PTH secretion.25,55 This results in skeletal demineralization and pathological fractures.26,27 The bone loss will be reversed by the parathyroidectomy.28 There are two activating mutations of the CaSR: autosomal-dominant hypocalcemia (ADH) and Bartter Syndrome Type 5. ADH is a benign chronic condition, incidentally found on routine blood work.29 It presents with a longstanding history of paresthesia, intermittent fasciculations, and childhood seizures. The diagnosis is made by the presence of hypocalcemia with inappropriately normal or very low serum PTH levels, and enhanced inhibition of renal calcium reabsorption resulting in hypercalciuria. Associated finding could include hypomagnesemia, hyperphosphatemia, and hypocalciuria.30 Other findings include a reduction of the glomerular filtration rate of ionized calcium but with an intact renal calcium reabsorption mechanism.31,32 Bartter Syndrome Type 5 results from severe activating mutations of the CaSR with the development of the renal salt wasting syndrome, hypocalcemia, suppressed serum PTH levels, hypocalciuria, hyperphosphatemia, ectopic mineralization, and cataracts. CaSR activation on the contraluminal membrane of the thick ascending limb disables NaCl reabsorption. CaSR normally promotes phosphate retention by suppressing renal phosphate excretion and PTH-induced inhibition of phosphate reabsorption.33e36 This syndrome is treated by giving negative modulators of the CaSR, called calcilytics.37,38
VITAMIN D METABOLISM Vitamin D (Calciferol) is a fat-soluble vitamin. It functions in a role of a hormone or prohormone because of its regulatory role in calcium homeostasis and bone metabolism. It has a much lower effect on calcium homeostasis compared to PTH. Vitamin D is received in two different forms: vitamin D-2 (ergocalciferol) and vitamin D-3 (cholecalciferol). Eighty percent of the body’s source of vitamin D comes from sunlight as vitamin D3, and the rest of it comes from food as vitamin D2. Both are metabolized in a similar manner, and they are equally potent. Ultraviolet light of wavelength 270e300 nm is adsorbed by the melanocytes. Its action is directed at the cholesterol precursor, 7-dehydroxycholesterol, where it breaks the B ring of the steroid molecule, creating a secosteroid. Then, by body heat action, it is converted to cholecalciferol (Vitamin D3). The peak of vitamin D3 synthesis is 6 weeks after maximal exposure to sunlight.1e3 Darker-skinned individuals require a sixfold greater amount of sunlight exposure to synthesize the same amount of Vitamin D as a lighter-skinned individual.39 Vitamin D2 is received form plants and food such as fish, liver oil, fatty fish, egg yolk, liver, and milk fortified with vitamin D-2.1e3,40 It is synthesized from ergosterol and structurally different from cholecalciferol. The process of generation of the active form of vitamin D involves three steps: synthesis, conversion to the major circulating form of vitamin D, 25-hydroxyvitamin D (25(OH)D) in the liver, and lastly conversion by the 1a-hydroxylase to the biologically active form of vitamin D, 1,25(OH)2D in the kidney (Fig. 6.3). After the synthesis, vitamin D binds to the vitamin D-binding protein (DBP), passes through the liver, where it is metabolized by cytochrome 450 enzyme to 25(OH)D, and circulates in plasma bound to DBP protein. Passing through the kidney, 25(OH) Vitamin D is metabolized by 25-hydroxyvitamin D 1 a-hydroxylase into its active hormone, 1,25(OH)2D. Activity of 1a-hydroxylase is controlled by PTH via its cAMP protein kinase action, and by hypocalcemia. Vitamin D is stored in the liver and adipose tissue. Obese people have more vitamin D into their fat store and lower circulating level of vitamin D. The primary action of 1,25(OH)2D is to stimulate the formation of calcium-binding proteins within the intestinal epithelial cells that helps absorption of calcium in the intestine41e46 (Fig. 6.3). It also helps in phosphorus absorption. 1,25(OH)2D stimulates the expression of calbindin-D9K and NCX1in renal tubules, calbindin-D28K, PMCA1b, and enterocytes.7
CHAPTER 6
Brief Overview of Calcium, Vitamin D, Parathyroid Hormone Metabolism
67
sun
80%
20% UVB
skin
plants, yeast
7-dehydroxycholesterol to secosteroid
food, milk Vitamin D2 (ergocalciferol)
Vitamin D3 (cholecalciferol) DBP
Liver: 25-Hydroxylase DBP
25(OH)D 25(OH)D - DBP
PTH hypocalcemia
Kidney: 25-hydroxyvitamin D 1α-hydroxylase
Intesne: formaon of CaBP, absorpon of Ca & Phos bone
1,25(OH)2D
kidney
FIG. 6.3 Vitamin D metabolism. Ca, calcium; CaBP, calcium binding protein; DBP, vitamin D-binding protein; Phos, phosphorus; PTH, parathyroid hormone.
Optimal serum level of 25(OH) vitamin D is between 30 and 50 ng/mL.41e46 Levels below 20 ng/mL are considered insufficient and suboptimal for skeletal health. Levels below 10 ng/mL are considered to be severely deficient. Deficiency may be the result of low dietary intake, malabsorption, target organ resistance, or impaired 1a-hydroxylation of 25(OH)D, although high level, above 50 ng/mL, may cause toxicity.41 At high concentrations, the stimulation of osteoblasts to produce cytokines causes an increased production of osteoclasts that lead to bone resorption.40
PARATHYROID HORMONE METABOLISM Parathyroid glands play a key role in maintaining the extracellular calcium concentration. The parathyroid gland is composed of two types of cells: chief and the oxyphil.47 Parathyroid chief cells, the main type of parathyroid gland cells, produce PTH. Oxyphil cells produce PTHrP, calcitriol, and some other factors.48 Sensitivity of the sestamibi scan depends on a radioisotope that is retained in mitochondria-rich cells, which are predominantly oxyphil cells (those that do not produce PTH). This explains the reason why the sestamibi scan is not always accurate in the identification of parathyroid adenoma. When the content of parathyroid oxyphil cells in the parathyroid adenoma is greater than
25%, the sensitivity of the sestamibi scan in the late phase of the test is much higher (sensitivity of about 78% for adenoma with more than 25% of oxyphil cells content vs. sensitivity of 33% for adenoma with 1%e25% of oxyphil cells, and sensitivity of 0% for adenoma with no oxyphil cells).49 PTH is 84 amino acid polypeptide synthesized from 115-amino acid polypeptide pre-pro-PTH within chief cells of the parathyroid gland. PTH synthesis is constant, and secretion is continuous through the parathyroid cell membrane, rather than sporadic, with circadian dynamics and in a pulsatile fashion.1e3,6 Almost no hormone is stored within the glands themselves. Among 84 amino acids, only 34 amino acid terminals are required for full activity of the hormone. During episodes of hypocalcemia, PTH is secreted within seconds by exocytosis. The half-life of the PTH is approximately 3 to 4 min in the bloodstream. Maier et al. studied PTH hormone elimination kinetics during a parathyroidectomy and reported that PTH half-life was 3.43 min, while ionized calcium started to decrease only at 30 min after the adenoma was removed. PTH elimination occurs in the kidneys and in the liver. Once secreted, it is rapidly taken up mostly by the liver, cleaved into fragments, and cleared by the kidneys. The current basis for modern assays measuring of intact PTH level is the identification of these fragments in the
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blood stream. In the liver, PTH degrades through two mechanisms. First, by enzymatic reaction in the Kupffer cells follow a MichaeliseMentgen kinetic (the bestknown models of enzyme kinetics). Second, PTH diś tributes into the space of Disse (the perisinusoidal space in between a hepatocyte and a sinusoid) and intracellular space, taken up by hepatocytes where it is modulating their glucose and amino acid metabolism and does not reenter the circulation. Forty percent of PTH metabolism occurs in the kidneys, where it is filtered and then reabsorbed in the proximal tubular cells, where it is degraded without reentering the circulation. PTH works through two receptors: PTH1R and PTH2R. PTH1R (also called PTH/PTHrP receptor) binds two molecules, the PTH and PTHrP, but only, the PTH molecule binds to the PTH2R. PTH1R is predominantly expressed in bones and kidneys. PTH2R only presents in the central nervous system. The main function of PTH is to regulate calcium homeostasis. The principal target organs for PTH are bones and kidneys (through PTH1R).1e7,50,51 In bones, under physiological circumstances, PTH promotes bone formation via receptors on the osteoblasts. During hypocalcemia, PTH stimulates bone resorption to maintain normal calcium balance and restore normocalcemia. PTH alters the balance between the expression of the receptor activator of nuclear factor kappa-B ligand (RANKL) produced by osteoclast, and the receptor osteoprotegerin (OPG) produced by osteoblasts. PTH has either a catabolic or anabolic effect, depending on the dose and periodicity of the PTH signal. Catabolic effect on bones develops with continuous exposure to PTH, as in primary hyperparathyroidism. PTH is changing the balance between RANKL and OPG in favor of bone resorption and demineralization: the encoding for RANKL increases, and encoding for OPG mRNA decreases.1e6 This effect of PTH on RANKL and OPG diminishes in about 1 year after a successful parathyroidectomy.6,52 Anabolic effect develops with low doses, intermittent PTH secretion.6 In the proximal tubules of the kidneys nephrons, the main action of the PTH is activation of 25-hydroxyvitamin D 1a-hydroxylase that converts 25-hydroxyvitamin D (25(OH)D) to its active form, 1,25(OH)2D, which then facilitates the absorption of both calcium and phosphate from the intestines.2,3,7,53,54 In distal tubules, it promotes reabsorption of calcium and magnesium and excretion of phosphate, by action through PTH1R receptor.3 This mechanism of PTH binding to PTH1R involves stimulation of adenylyl cyclase and increasing cyclic AMP (cAMP) concentrations that activate phospholipase C pathway.2 PTH also has the effect on bicarbonate and
amino acid reabsorption in the proximal tubule. This results in hyperparathyroidism-related mild form of Fanconi syndrome (the generalized dysfunction of the proximal tubule presented with polyuria, hypokalemia, glycosuria, hypophosphatemia, and low molecular weight proteinuria). This is also resolved when hyperparathyroidism is reversed.1,3,6
REFERENCES 1. Allgrove J. Physiology of calcium, phosphate and magnesium. Endocr Dev. 2009;16:8e31. 2. Song L. Calcium and bone metabolism indices. Adv Clin Chem. 2017;82:1e46. 3. Allgrove J. Physiology of calcium, phosphate, magnesium and vitamin D. Endocr Dev. 2015;28:7e32. 4. Schreckenberg R, Schlüter KD. Calcium sensing receptor expression and signalling in cardiovascular physiology and disease. Vasc Pharmacol. 2018;107:35e42. 5. Hoenderop JG, Bindels RJ. Calciotropic and magnesiotropic TRP channels. Physiology. 2008;23:32e40. 6. Silva BC, Bilezikian JP. Parathyroid hormone: anabolic and catabolic actions on the skeleton. Curr Opin Pharmacol. June 2015;22:41e50. 7. Hoenderop JGJ, Nilius B, Bindels RJM. Calcium absorption across epithelia. Physiol Rev. 2005;85:373e422. 8. Lameris AL, Nevalainen PI, Reijnen D, et al. Segmental transport of Ca2⁺ and Mg2⁺ along the gastrointestinal tract. Am J Physiol Gastrointest Liver Physiol. 2015;308(3): G206eG216. 9. Simon DB, Lu Y, Choate KA, et al. Paracellin-1, a renal tight junction protein required for paracellular Mg2þ resorption. Science. 1999;285:103e106. 10. Wong V, Goodenough DA. Paracellular channels. Science. 1999;285:62. 11. Montell C, Birnbaumer L, Flockerzi V, et al. A unified nomenclature for the superfamily of TRP cation channels. Mol Cell. 2002;9:229e231. 12. Peng JB, Chen XZ, Berger UV, et al. Molecular cloning and characterization of a channel-like transporter mediating intestinal calcium absorption. J Biol Chem. 1999;274: 22739e22746. 13. Yamagishi N, Yukawa YA, Ishiguro N, et al. Expression of calbindin-D9k messenger ribonucleic acid in the gastrointestinal tract of dairy cattle. J Vet Med A Physiol Pathol Clin Med. 2002;49:461e465. 14. Li Z, Matsuoka S, Hryshko LV, et al. Cloning of the NCX2 isoform of the plasma membrane Na -Ca2 exchanger. J Biol Chem. 1994;269:17434e17439. 15. Nicoll DA, Quednau BD, Qui Z, Xia YR, Lusis AJ, Philipson KD. Cloning of a third mammalian Na -Ca2 exchanger, NCX3. J Biol Chem. 1996;271:24914e24921. 16. Bronner F, Pansu D, Stein WD. An analysis of intestinal calcium transport across the rat intestine. Am J Physiol Gastrointest Liver Physiol. 1986;250:G561eG569. 17. Bronner F. Mechanisms of intestinal calcium absorption. J Cell Biochem. 2003;88:387e393.
CHAPTER 6
Brief Overview of Calcium, Vitamin D, Parathyroid Hormone Metabolism
18. Bronner F, Pansu D. Nutritional aspects of calcium absorption. J Nutr. 1999;129(9e12). 19. Schweigel M, Martens H. Magnesium transport in the gastrointestinal tract. Front Biosci. 2000;5:D666eD677. 20. Faulk WP, McIntyre JA. Immunological studies of human trophoblast: markers, subsets and functions. Immunol Rev. 1983;75:139e175. 21. Peng JB, Brown EM, Hediger MA. Structural conservation of the genes encoding CaT1, CaT2, and related cation channels. Genomics. 2001;76:99e109. 22. Belkacemi L, Gariepy G, Mounier C, Simoneau L, Lafond J. Calbindin-D9k (CaBP9k) localization and levels of expression in trophoblast cells from human term placenta. Cell Tissue Res. 2004;315:107e117. 23. Belkacemi L, Simoneau L, Lafond J. Calcium-binding proteins: distribution and implication in mammalian placenta. Endocrine. 2002;19:57e64. 24. Glazier JD, Atkinson DE, Thornburg KL, et al. Gestational changes in Ca2þ transport across rat placenta and mRNA for calbindin9K and Ca(2þ)-ATPase. Am J Physiol Regul Integr Comp Physiol. 1992;263:R930eR935. 25. Conigrave AD. The calcium-sensing receptor and the parathyroid: past, present, future. Front Physiol. 2016;7:563. 26. Pollak MR, Chou YH, Marx SJ, et al. Familial hypocalciuric hypercalcemia and neonatal severe hyperparathyroidism. Effects of mutant gene dosage on phenotype. J Clin Investig. 1994;93:1108e1112. 27. Brown EM, Pollak M, Seidman CE, et al. Calcium-ionsensing cell-surface receptors. N Engl J Med. 1995;333: 234e240. 28. Marx SJ, Lasker RD, Brown EM, et al. Secretory dysfunction in parathyroid cells from a neonate with severe primary hyperparathyroidism. J Clin Endocrinol Metab. 1986;62: 445e449. 29. Pearce SH, Williamson C, Kifor O, et al. A familial syndrome of hypocalcemia with hypercalciuria due to mutations in the calcium-sensing receptor. N Engl J Med. 1996;335:1115e1122. 30. Tan YM, Cardinal J, Franks AH, et al. Autosomal dominant hypocalcemia: a novel activating mutation (E604K) in the cysteine-rich domain of the calcium-sensing receptor. J Clin Endocrinol Metab. 2003;88:605e610. 31. Thakker RV. Diseases associated with the extracellular calcium-sensing receptor. Cell Calcium. 2004;35:275e282. 32. Egbuna OI, Brown EM. Hypercalcaemic and hypocalcaemic conditions due to calcium-sensing receptor mutations. Best Pract Res Clin Rheumatol. 2008;22:129e148. 33. Riccardi D, Brown E. Physiology and pathophysiology of the calcium-sensing receptor in the kidney. Am J Physiol Renal Physiol. 2010;298:F485eF499. 34. Riccardi D, Traebert M, Ward DT, et al. Dietary phosphate and parathyroid hormone alter the expression of the calcium-sensing receptor (CaR) and the Naþ-dependent Pi transporter (NaPi-2) in the rat proximal tubule. Pflügers Archiv. 2000;441:379e387. 35. Ba J, Brown D, Friedman PA. Calcium-sensing receptor regulation of PTH-inhibitable proximal tubule phosphate transport. Am J Physiol. 2003;285:F1233eF1243.
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36. Riccardi D, Valenti G. Localization and function of the renal calcium-sensing receptor. Nat Rev Nephrol. 2016;12: 414e425. 37. Mayr B, Glaudo M, Schöfl C. Activating calcium-sensing receptor mutations: prospects for future treatment with calcilytics. Trends Endocrinol Metabol. 2016;27:643e652. 38. Nemeth EF, Goodman WG. Calcimimetic and calcilytic drugs: feats, flops, and futures. Calcif Tissue Int. 2016;98: 341e358. 39. Lo CW, Paris PW, Holick MF. Indian and Pakistani immigrants have the same capacity as Caucasians to produce vitamin D in response to ultraviolet irradiation. Am J Clin Nutr. 1986;44(5):683e685. 40. Winter W, Kleerekoper M. Bone and mineral metabolism. In: Risteli J, Risteli L, Burtis CA, Ashwood ER, Bruns D, eds. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 5th ed. San Diego, CA: Elsevier Publishing; 2012:1765. 41. Sanders KM, Stuart AL, Williamson EJ, et al. Annual highdose oral vitamin D and falls and fractures in older women: a randomized controlled trial. J Am Med Assoc. 2010;303(18):1815e1822. 42. Dawson-Hughes B. Vitamin D deficiency in adults: definition, clinical manifestations, and treatment. In: Drezner MK, Rosen CJ, eds. UpToDate. Waltham, MA: UpToDate; 2016 (updated: March, 2016). 43. Dawson-Hughes B, Harris SS, Krall EA, Dallal GE. Effect of calcium and vitamin D supplementation on bone density in men and women 65 years of age or older. N Engl J Med. 1997;337(10):670e676. 44. Chapuy MC, Pamphile R, Paris E, et al. Combined calcium and vitamin D3 supplementation in elderly women: confirmation of reversal of secondary hyperparathyroidism and hip fracture risk: the Decalyos II study. Osteoporos Int. 2002;13(3):257e264. 45. Trivedi DP, Doll R, Khaw KT. Effect of four monthly oral vitamin D3 (cholecalciferol) supplementation on fractures and mortality in men and women living in the community: randomised double blind controlled trial. BMJ. 2003;326(7387):469. 46. Bilezikian JP, Brandi ML, Eastell R, et al. Guidelines for the management of asymptomatic primary hyperparathyroidism: summary statement from the Fourth International Workshop. Clin Endocrinol Metabol. 2014;99(10): 3561e3569. 47. Isono H, Shoumura S, Emura S. Ultrastructure of the parathyroid gland. Histol Histopathol. 1990;5(1):95e112. 48. Ritter CS, Haughey BH, Miller B, Brown AJ. Differential gene expression by oxyphil and chief cells of human parathyroid glands. J Clin Endocrinol Metab. 2012;97(8): E1499eE1505. 49. Carpentier A, Jeannotte S, Verreault J, et al. Preoperative localization of parathyroid lesions in hyperparathyroidism: relationship between technetium-99m-MIBI uptake and oxyphil cell content. J Nucl Med. 1998;39(8):1441e1444. 50. Zindel D, Engel S, Bottrill AR, et al. Identification of key phosphorylation sites in PTH1R that determine arrestin3 binding and fine-tune receptor signaling. Biochem J. 2016;473(22):4173e4192.
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SECTION II
Parathyroid Diseases and Cancer
51. Maier GW, Kreis ME, Renn W, Pereira PL, Häring HU, Becker HD. Parathyroid hormone after adenectomy for primary hyperparathyroidism. A study of peptide hormone elimination kinetics in humans. J Clin Endocrinol Metab. 1998;83(11):3852e3856. 52. Stilgren LS, Rettmer E, Eriksen EF, Hegedus L, BeckNielsen H, Abrahamsen B. Skeletal changes in osteoprotegerin and receptor activator of nuclear factor-kappaB ligand mRNA levels in primary hyperparathyroidism: effect of parathyroidectomy and association with bone metabolism. Bone. 2004;35:256e265.
53. El-Hajj Fuleihan G, Brown EM. Parathyroid hormone secretion and action. In: Rosen CJ, ed. UpToDate. Waltham, MA: UpToDate; 2014. 54. Winter WE, Harris NS. Calcium biology and disorders. In: Clarke W, ed. Con- Temporary Practice in Clinical Chemistry. 2nd ed. Washington, DC: AACC Press; 2011:506. 55. Ward BK, Magno AL, Davis EA, et al. Functional deletion of the calcium-sensing receptor in a case of neonatal severe hyperparathyroidism. J Clin Endocrinol Metab. 2004; 89(8):3721e3730.