24 Parathyroid Gland and Calcium for Students
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Got Calcium? Ca2+
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Calcium body distribution
Plasma calcium regulation
Action control of calciumSynthesis and actions of hormones
regulating calcium ions
DiseasesLaboratory methods and interpretations
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Plasma calcium totals 2.4 mM (9.4 mg/dl)
o Free calcium is 1.2 mM
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Free calcium is tightly regulated (5%)
o Too low = neuronal hyper-excitability
o Too high = neuronal depressionControl points for calcium
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Vitamin D3
Parathyroid Hormone
CalcitoninSkeletal Loading
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Vitamin D3 (Cholecalciferol)
o25-Hydroxycholecalciferol
o1,25-Dihydroxycholecalciferol
http://en.wikipedia.org/wiki/Image:Cholecalciferol.png7/30/2019 24 Parathyroid Gland and Calcium for Students
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Synthesis of
Vitamin D
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Humans acquire vitamin Dfrom two sources
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Vitamin D is not a classichormone because it is not
produce and secreted by anendocrine gland.
Nor is it a true vitaminsince it can be synthesizedde novo.
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Vitamin D is a true hormonethat acts on distant target cells
to evoke responses afterbinding to high affinityreceptors
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A lipid soluble hormonethat binds
to a typical nuclear receptor,
analogous to steroid hormones.
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Promotes intestinal absorption
of calcium
Causes synthesis of calcium-binding protein and related
facilitated transport
Takes a couple of days to fully
develop response
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Works with PTHto cause
calcium absorption from
bone
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Osteoblasts, but not osteoclasts havevitamin D receptors.
1,25-(OH)2-D acts on osteoblasts
which produce a paracrine signal thatactivates osteoclasts to resorb Ca++from the bone matrix.
1,25-(OH)2-D also stimulatesosteocytic osteolysis.
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Secreted by parathyroid glands
Polypeptide
Peptide fragments can be active for periods
measured in hours
Operates in tissues via cAMP second
messenger
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Parathyroid Hormone
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G-ProteinCoupled Receptor
ActivatesPhospholipase Co Leads to increased
Diacylglycerol (DG) and Inositoltrisphosphate (IP3) as
secondary messengerso Inhibits adenylate cyclase to
reduce cAMP
Present in C cells of ThyroidGlando Regulates calcitonin secretion
Calcium Signal Transduction in Parathyroid Gland
600 AA external
domain
200 AA internal
domain
Phosphorylation
sites
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Hyperparathyroidism
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Calcitonin acts to decrease plasmaCa++ levels.
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The major stimulus of calcitonin
secretion is a rise in plasma Ca++
levelsCalcitonin is a physiological
antagonist to PTH with regard to
Ca++ homeostasis
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The target cell for calcitonin is theosteoclast.
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Serum Calcitonin
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Pentagastrin (Pg) Stimulation Test
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Calcium Stimulation Test
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Rickets
oAbnormal mineralization of boneresulting from vitamin D deficiency
(growing bone in children)
Osteomalacia
oAdults
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Osteoporosis
oPorous bones
oDual Energy X-ray Absorptiometry(DXA)
oBone Mineral Densitometry
oCT or MRI
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