1. Parathyroid Hormone, Calcitonin, Calcium and Phosphate Metabolism
Prof. dr. Zoran Valić
Department of Physiology
University of Split School of Medicine

2. Calcium and Phosphate Regulation in the ECF and Plasma
Ca+2 in extracellular fluid (ECF) – 2,4 mmol/L
normally precisely regulated (few %)
key role of Ca+2 in many physiologic processes: muscle contraction, blood clotting, transmission of nerve impulses
hypercalcemia – depression;/ hypocalcemia – excitetion (tetany)
0.1% Ca+2 in ECF, 1% in cells, rest is stored in bones

3. 85% of body's phosphate is stored in bones
14-15% in cells,  1% in ECF
concentration is not nearly as well regulated as calcium
same factors that regulate calcium

4. Calcium in the Plasma and Interstitial Fluid (IF)
present in three forms :
41% - combined with plasma proteins
9% - combined with anionic substances (citrate and phosphate)
50% - ionized
ionic calcium is the form that is important for most functions of calcium in the body

6. Inorganic Phosphate in the ECF
in the plasma is mainly in two forms :
HPO mmol/L
H2PO mmol/L
concentration depends on pH of plasma
total quantity of phosphate is expressed in terms of phosphorus per liter of blood
1.3 mmol/L (depending on age)

7. Nonbone Physiologic Effects of Altered Calcium and Phosphate Concentrations
hypocalcemia causes nervous system excitement ( permeability to Na+)
tetany occurs when calcium decreases to 1.5 mmol/L, and death when it is 1 mmol/L
hypocalcemia : dilatation of heart, changes in enzyme activities, increased membrane permeability of some cells, impaired blood clotting – nonphysiological decrease

9. Hypercalcemia nervous system becomes depressed
lack of appetite and constipation
decreases the QT interval of the heart
depressive effects begin to appear when calcium rises above 3.0 mmol/L, they can become marked above 3.8 mmol/L

10. Absorption and Excretion of Calcium and Phosphate
daily intake is about 1000 mg/day each for calcium and phosphorus (25 mmol) – 1L of milk
90% of daily intake of calcium is excreted in feces
almost all the dietary phosphate is absorbed into the blood from the gut and later excreted in the urine

12. kidney filers calcium (ionized and combined with anions)
renal tubules reabsorb 99 percent of the filtered calcium
renal phosphate excretion is controlled by an overflow mechanism (critical value of about 1 mmol/L – no phosphate is lost in the urine)

14. Bone and Its Relation to ECF Calcium and Phosphate
bone = organic matrix + calcium salts
compact bone: 30% matrix, 70% salts
organic matrix :
90-95 % collagen fibers (tensile strength)
5-10 % ground substance (ECF + proteoglycans, chondroitin sulfate and hyaluronic acid)
bone salts: calcium and phosphate

15. Bone Salts
Ca10(PO4)6(OH)2
hydroxyapatite – shaped like long, flat plate
relative ratio of Ca/P
ions Mg2+, Na+, K+ & HCO3-
many ions of many types of ions (transuranic elements and heavy metals)
osteogenic sarcoma (bone cancer)

16. Tensile and Compressional Strength of Bone
compact bone is composed of repeating periodic segments of collagen
hydroxyapatite crystals bound tightly to it – prevents "shear" in the bone
collagen fibers – tensile strength
calcium salts – compressional strength

17. Precipitation and Absorption of Calcium and Phosphate in Bone
concentrations of calcium and phosphate ions in ECF are considerably greater than those required to cause precipitation of hydroxyapatite
pyrophosphate – inhibitor which prevent precipitation

18. Bone Calcification
osteoblasts produce osteoid (collagen molecules + ground substance)
calcium salts begin to precipitate on the surfaces of the collagen fibers, forming minute nidi
mixture of amorphous compounds (noncrystalline) salts of Ca & P, converted into the hydroxyapatite crystals over a period of weeks or months
few percent may remain permanently in the amorphous form (important role)
mechanism is not fully understood

19. Precipitation of Calcium in Nonosseous Tissues
arteriosclerosis
degenerating tissues
old blood clots

20. Calcium Exchange Between Bone and ECF
fast correction of plasma Ca concentration
exchangeable calcium – in equilibrium with the calcium ions in the ECF – rapid buffering mechanism
in all tissue cells (liver and gastrointestinal tract) & in the bone (amorphous calcium salts)

21. Deposition and Absorption of Bone
osteoblasts (on outer surfaces of bones and in bone cavities):
active on 4% of all surfaces in an adult
osteoclasts :
large, multinucleated cells (50 nuclei)
derivatives of monocytes, phagocytic
active on less than 1% bone surfaces
PTH controls bone absorption by osteoclasts

23. Absorption of Bone
osteoclasts send out villus-like projections toward the bone (ruffled border) & secrete:
proteolytic enzymes – organic matrix
acids (citric, lactic acid) – bone salts
PTH stimulates osteoclast activity and bone resorption, but through indirect mechanism

24. PTH binds to receptors on adjacent osteoblasts
osteoblasts release cytokines: osteoprotegerin ligand (OPGL or RANKL)
OPGL activates receptors on preosteoclast cells  mature multinucleated osteoclasts  development of ruffled border (release of enzymes and acids)

26. osteoblasts also produce osteoprotegerin (OPG, also called OCIF)
OPG inhibits bone resorption
OPG acts as a "decoy" receptor binding to OPGL and preventing its actions
vitamin D and PTH inhibit OPG production and stimulate OPGL formation
estrogen stimulates OPG production
used for treatment

27. bone deposition and absorption are normally in equilibrium – total mass of bone remains constant.
concentrated mass of osteoclasts  tunnel in diameter 0,2-1 mm and several mm long  tunnel is invaded by osteoblasts  new bone begins to develop

28. new bone being laid down in successive layers of concentric circles (lamellae)
deposition of new bone ceases when the bone begins to encroach on the blood vessels supplying the area
haversian canal, is all that remains of the original cavity
osteon

30. Value of Continual Bone Remodeling
bone strength in proportion to bone stress
shape of the bone can be rearranged
replacement of old organic matrix
bone is deposited in proportion to the compressional load (athletes & cast)
repair of fracture activates osteoblasts
callus, rings

31. Gavril Ilizarov

32. Vitamin D
potent effect to increase calcium absorption from the intestinal tract
vitamin D itself is not the active substance
must first be converted into:
1,25-dihydroxycholecalciferol
7-dehydrocholesterol + ultraviolet rays (in the skin) – cholecalciferol (vitamin D)

33. a) precise control
b) spearing vitamins

35. Formation of 1,25-Dihydroxycholecalciferol
proximal tubules of the kidneys
the most active form of vitamin D
in the absence of the kidneys, vitamin D loses almost all its effectiveness
requires PTH

36. a) precise control
b) spearing vitamins

38. Actions of Vitamin D intestines, kidneys, bones
intestines : hormone,  absorption of Ca by  formation of calbindin, a calcium-binding protein
+ Ca ATP-aza & alkaline phosphatase in the brush border
 absorption of P by the intestines, direct effect and secondarily from this hormone's action on calcium absorption

39. kidneys:  reabsorption of Ca & P by the epithelial cells of the renal tubules
bone:
extreme quantities of vitamin D – absorption of bone ( Ca transport)
smaller quantities of vitamin D – promotes bone calcification (increase absorption from the intestines;  Ca transport in opposite direction)

40. Parathyroid Hormone
powerful mechanism for controlling ECF Ca & P concentrations
regulates intestinal reabsorption and renal excretion
regulates exchange between the ECF and bone of these ions

41. increased secretion causes rapid absorption of calcium salts from the bones – hypercalcemia
hypofunction of the parathyroid glands – hypocalcemia (tetany)

42. normally – 4 parathyroid glands in humans
located immediately behind thyroid gland
6 x 3 x 2 mm, appearance of dark brown fat
danger of removal during thyroid operations (2(3) out of 4 – OK)
chief cells (majority) and oxyphil cells (function unknown, absent in many animals and in young humans)

44. Chemistry of PTH PTH has been isolated in a pure form
preprohormone (110 AA)  prohormone (90 AA)  PTH itself (84 AA)
smaller compounds have also been isolated that exhibit full PTH activity
kidneys rapidly remove big PTH within minutes but fail to remove many of the fragments for hours

45. Effect of PTH on Ca & P Concentrations in ECF

47. Effect of PTH on Ca & P Concentrations in Plasma
phosphate concentration falls more rapidly than the calcium rises
both reach a plateau in about 4 hours
increase Ca & P absorption from bone
decrease excretion of Ca by kidneys
increase renal P excretion

48. PTH Increases Ca & P Absorption from Bone
rapid phase: activation of the already existing osteocita (begins in minutes)
much slower phase: proliferation of the osteoclasts (requiring several days or even weeks)

49. Rapid Phase – Osteolysis
osteocytic membrane system – separates the bone itself from ECF
bone fluid – small amount between osteocytic membrane and bone
pumping of Ca ions from bone fluid into ECF – osteocytic pump

50. Rapid Phase – Osteolysis
bone fluid calcium concentration falls even lower  absorbed of Ca & P from the bone – osteolysis
PTH strongly activates calcium pump ( Ca permeability of the bone fluid side of the osteocytic membrane)

51. Slow Phase of Bone Absorption
PTH activates osteoblasts and osteocytes which send secondary "signals" to the osteoclasts (OPGL)
immediate activation of the osteoclasts that are already formed
formation of new osteoclasts
osteoclastic resorption of bone can lead to weakened bones and secondary stimulation of the osteoblasts

52. PTH Decreases Ca Excretion and Increases P Excretion by Kidneys
increased P excretion is based on diminish proximal tubular reabsorption of P ions
simultaneous increase in renal tubular reabsorption of Ca
PTH increases reabsorption of Mg & H, and diminish reabsorption of Na, K, AA

53. Control of PTH Secretion by Ca Ion Concentration
PTH increases reabsorption of Ca & P from the intestines
cAMP mediates the effects of PTH
Ca concentration controls secretion of PTH
parathyroid glands become greatly enlarged in rickets (5x) in pregnancy, and during lactation

54. Decrease in PTH Secretion
excess quantities of Ca in the diet
increased vitamin D in the diet
bone absorption caused by factors other than PTH (immobilization)

55. changes in ECF Ca ion concentration are detected by a calcium-sensing receptor (CaSR) in parathyroid cell membranes
CaSR is a G protein-coupled receptor
stimulated by calcium ions, activates phospholipase C ( in IP3 and DAG)
stimulation of Ca release from intracellular stores  decreases PTH secretion

57. Calcitonin
synthesis and secretion of calcitonin occur in the parafollicular cells, or C cells of thyroid gland (0,1% of the human thyroid gland, remnants of ultimobranchial glands)
effects opposite to those of PTH
32-amino acid peptide
 ECF Ca ion concentration – primary stimulus for calcitonin secretion

58. Mechanism of Calcitonin Action
 absorptive activities of the osteoclasts
 osteolytic effect of osteocytic membrane
 formation of new osteoclasts
minor effects on calcium handling in the kidney tubules and the intestines
calcitonin has bigger effect in children and certain diseases (Paget) than in adult human

59. Summary of Control of Ca Ions
first line of defense – buffer function of the exchangeable calcium in bones – amorphous compounds (Ca HPO4)
+ mitochondria of the liver and intestine
second line of defense – hormonal control

61. Pathophysiology Hypoparathyroidism ( PTH secretion)
 osteocytic resorption of Ca + osteoclasts inactivity   Ca  tetany (laryngeal muscles)
extremely large quantities of vitamin D or giving PTH (expensive, short-acting, antibodies)
Hyperparathyroidism (primary & secondary)
ordinarily tumor (in women because pregnancy)
extreme osteoclastic activity   Ca &  P
fractures, decalcification, muscle weakness, metastatic calcification, kidney stones

62. Rickets
 Ca (slightly) & P (greatly) in ECF, usually caused by lack of vitamin D
occur in spring months after depletion of stores
compensatory increase in PTH secretion causes extreme osteoclastic absorption of the bone
tetany occurs in later stages – death
treatment : vitamin D + Ca & P in the food
steatorrhea – osteomalacia (adult rickets)
renal rickets – failure of the damaged kidneys to form 1,25-dihydroxycholecalciferol

63. Osteoporosis most common of all bone diseases in adults
especially in old age
decreased bone matrix
osteoblastic activity is usually less than normal – osteoid deposition is depressed
lack of physical stress on the bones because of inactivity, malnutrition, lack of vitamin C, postmenopausal lack of estrogen secretion, old age ( hGH), Cushing's syndrome ( glucocorticoids)

64. antiresorptive drugs:
treatment:
antiresorptive drugs:
bisphosphonates (Fosamax, Actonel & Boniva)
estrogen substitution therapy
SERMs (raloxifene, Evista) – osteoclasts
calcitonin
inhibitors of RANKL (monoclonal antibody, Denosumab)
anabolic drugs:
teriparatide (Forteo) – recombinant PTH
calcium salts (carbonates, citrates, lactates)
sodium-fluoride

65. Physiology of the Teeth
teeth cut, grind, and mix the food eaten
forces: N & N
occlusion – fitting of upper set of teeth with the lower
major functional parts: enamel, dentin, cementum and pulp
another division: crown, neck and root

67. Enamel outer surface of the tooth; ameloblasts
formed before eruption of the tooth
composed of large and dense crystals of hydroxyapatite with adsorbed Mg, Na, K + insoluble protein fibers (similar to keratin)
extremely hard and resistant to acids, enzymes, and other corrosive agents

68. Dentin main body of tooth, strong, bony structure
hydroxyapatite crystals much denser than in bone, embedded in a strong meshwork of collagen fibers
does not contain: osteoblasts, osteocytes, osteoclasts, spaces for vessels or nerves
odontoblasts: formation and nurishment
calcium salts: compressional forces
collagen: toughness and tensional forces

69. Cementum
bony substance secreted by cells of the periodontal membrane – lines tooth socket
collagen fibers pass directly from jaw bone through periodontal membrane into cementum
increases in thickness and strength with age

70. Pulp filling pulp (tooth) cavity
composed of connective tissue, nerve fibers, blood vessels, and lymphatics
odontoblasts – cells lining the surface of the pulp cavity
during the formative years lay down dentin
send projections into small dentinal tubules that penetrate all the way through the dentin

71. Dentition
humans and most other mammals develop two sets of teeth during a lifetime
deciduous or milk teeth (20)
erupt between 7th month and 2nd year
last until 6 th and 13 th year
permanent teeth (32)

72. Formation of the Teeth
invagination of the oral epithelium into the dental lamina and development of a tooth-producing organ
epithelial cells above form ameloblasts (enamel)
epithelial cells below invaginate upward into the middle of the tooth to form the pulp cavity and the odontoblasts (dentin)

74. Eruption of Teeth cause of "eruption" is unknown
most likely theory is that growth of the tooth root and the bone underneath the tooth progressively shoves the tooth forward

75. Development of Permanent Teeth
when each permanent tooth becomes fully formed, it pushes outward through the bone
erodes the root of the deciduous tooth and eventually causes it to loosen and fall out

76. Metabolic Factors thyroid and growth hormones
availability of Ca & P in the diet
amount of vitamin D
rate of PTH secretion

77. Mineral Exchange in Teeth
salts composed of hydroxyapatite with adsorbed carbonates and various cations
new salts are constantly being deposited while old salts are being reabsorbed
deposition and reabsorbtion – in dentin and cementum, limited extent in enamel (saliva)
rate of exchange in dentin 3x slower than in bone

78. Dental Abnormalities caries (erosion of the teeth)
malocclusion (failure of the projections of the upper and lower teeth to interdigitate properly)

79. 1) Caries action of bacteria on the teeth (Streptococcus mutans)
deposit of plaque (film of saliva and food)
dependance on carbohydrates (form acids (lactic) and proteolytic enzymes)
acids – slow dissolvement of calcium salts
enamel of the tooth is primary and most important barrier to development of caries

80. small amounts of fluorine develop enamel that is more resistant to caries
fluorine does not make the enamel harder
fluorine ions replace many of hydroxyl ions in hydroxyapatite crystals, which in turn makes enamel several times less soluble
fluorine may also be toxic to the bacteria
fluorine promote deposition of calcium phosphate to "heal" the enamel surface

81. 2) Malocclusion hereditary abnormality
teeth of one jaw grow to abnormal positions
teeth do not interdigitate properly and therefore cannot perform their normal grinding or cutting action adequately
pain in mandibular joint and deterioration of the teeth
treatment: continuous force on teeth

83. FIGURE 1. Phosphorus homeostasis in normal humans
Berndt, T. et al. Physiology 24: ;
doi: /physiol
Copyright ©2009 American Physiological Society

84. FIGURE 4. Adaptations to changes in dietary phosphate
Berndt, T. et al. Physiology 24: ;
doi: /physiol
Copyright ©2009 American Physiological Society