Metabolizam kalcija i fosfata, vitamin D, paratireoidni hormon
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Transcript Metabolizam kalcija i fosfata, vitamin D, paratireoidni hormon
Parathyroid Hormone,
Calcitonin, Calcium and
Phosphate Metabolism
Prof. dr. Zoran Valić
Department of Physiology
University of Split School of Medicine
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
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
Calcium in the Plasma and
Interstitial Fluid (IF)
present
a)
b)
c)
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
Inorganic Phosphate in the ECF
in the plasma is mainly in two forms :
HPO42- - 1.05 mmol/L
b) H2PO4- - 0.26 mmol/L
a)
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)
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
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
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
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)
Bone and Its Relation to ECF
Calcium and Phosphate
bone = organic matrix + calcium salts
compact bone: 30% matrix, 70% salts
organic matrix :
a)
b)
90-95 % collagen fibers (tensile strength)
5-10 % ground substance (ECF +
proteoglycans, chondroitin sulfate and
hyaluronic acid)
bone salts: calcium and phosphate
Bone Salts
Ca10(PO4)6(OH)2
– shaped like long, flat plate
relative ratio of Ca/P 1.3-2.0
ions Mg2+, Na+, K+ & HCO3 many ions of many types of ions
(transuranic elements and heavy metals)
osteogenic sarcoma (bone cancer)
hydroxyapatite
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
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
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
Precipitation of Calcium in
Nonosseous Tissues
arteriosclerosis
degenerating
tissues
old blood clots
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)
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
Absorption of Bone
osteoclasts send out villus-like projections
toward the bone (ruffled border) & secrete:
a)
proteolytic enzymes – organic matrix
b)
acids (citric, lactic acid) – bone salts
PTH stimulates osteoclast activity and bone
resorption, but through indirect mechanism
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)
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
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
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
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
Gavril Ilizarov
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)
a) precise control
b) spearing vitamins
Formation of 1,25Dihydroxycholecalciferol
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
a) precise control
b) spearing vitamins
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
reabsorption of Ca & P by the
epithelial cells of the renal tubules
kidneys:
bone:
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)
extreme
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
increased secretion causes rapid absorption
of calcium salts from the bones –
hypercalcemia
hypofunction of the parathyroid glands –
hypocalcemia (tetany)
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)
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
Effect of PTH on Ca & P
Concentrations in ECF
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
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)
Rapid Phase – Osteolysis
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
osteocytic
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)
Slow Phase of Bone Absorption
PTH activates osteoblasts and osteocytes
which send secondary "signals" to the
osteoclasts (OPGL)
1)
immediate activation of the osteoclasts
that are already formed
2)
formation of new osteoclasts
osteoclastic resorption of bone can lead to
weakened bones and secondary stimulation
of the osteoblasts
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
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
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)
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
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
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
Summary of Control of Ca Ions
line of defense – buffer function of the
exchangeable calcium in bones –
amorphous compounds (Ca HPO4)
+ mitochondria of the liver and intestine
first
second
line of defense – hormonal control
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,
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
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)
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
Physiology of the Teeth
teeth
cut, grind, and mix the food eaten
forces: 250-450N & 650-900N
occlusion – fitting of upper set of teeth with
the lower
major
functional parts: enamel, dentin,
cementum and pulp
another division: crown, neck and root
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
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
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
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
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)
Formation of the Teeth
invagination
of the oral epithelium into the
dental lamina and development of a toothproducing 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)
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
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
Metabolic Factors
thyroid
and growth hormones
availability of Ca & P in the diet
amount of vitamin D
rate of PTH secretion
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
Dental Abnormalities
1)
2)
caries (erosion of the teeth)
malocclusion (failure of the projections of
the upper and lower teeth to interdigitate
properly)
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
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
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
FIGURE 1. Phosphorus homeostasis in normal humans
Berndt, T. et al. Physiology 24: 17-25 2009;
doi:10.1152/physiol.00034.2008
Copyright ©2009 American Physiological Society
FIGURE 4. Adaptations to changes in dietary phosphate
Berndt, T. et al. Physiology 24: 17-25 2009;
doi:10.1152/physiol.00034.2008
Copyright ©2009 American Physiological Society