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Unit II – Endocrine Section
Calcium Metabolism
Daylily S OoiMBBS, FRCPC (Med Biochemistry)
3973: Describe the function of parathyroid hormone 3974: Explain the physiological actions of PTH on bone kidneys and intestines 3975: Describe Vitamin D action on target tissues3976: Describe the regulation of 1, 25 di-OH vitamin D3977: Explain the regulation of serum calcium3978: Describe the physiological action of calcitonin
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You may only access and use this presentation for educational purposes. You may not post this presentation online or distribute it without the permission of the author.
Slides marked with ✪ are modified from Dr. D Liu’s lecture 2014.
Disclosure
Unit II – Calcium Metabolism – DS Ooi
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Objectives Additional topics added to this lecture are in blue
Unit II – Calcium Metabolism – DS Ooi
Distribution of calcium, phosphate and magnesium in the body3977: Explain the regulation of serum calcium• Organs involved• Hormonal and other regulators
Parathyroid hormone• Production and regulation• 3973: Describe the function of parathyroid hormone • 3974: Explain the physiological actions of PTH on bone, kidneys and intestines
Vitamin D• Forms• Production• 3976: Describe the regulation of 1, 25 di-OH vitamin D• 3975: Describe Vitamin D action on target tissues
Calcitonin• Production• 3978: Describe the physiological action of calcitonin• Clinical uses
Disturbances of calcium homeostasis – causes, symptoms, management• Hypercalcemia• Hypocalcemia• Secondary hyperparathyroidism
Disturbances of Magnesium and Phosphate
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Distribution of Calcium
Total body calcium ~ 1kg (25.5 moles)• 99% in Bone (25 moles).
With phosphorus, constitutes 65% of bone by weight.
• Soft tissue - intracellular (25 mmoles)– Very little as cytosolic free calcium (100 nmoles)– 99% is within cellular compartments – bound to inner plasma
membrane of mitochondria, or endoplasmic reticulum
• Extracellular fluid (23 mmoles)
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Distribution of calcium in blood
• 45% protein bound (80% albumin, 20% globulins)
• 10% complexed (citrate, lactate, phosphate, bicarbonate)
• 45% free ionized form (physiologically active)Varies with pH
alkaline pH binding free ionized form acid pH binding free ionized Ca
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Calcium
Function:
Extracellular• Excitation-contraction in muscles
• Synaptic transmission
• Platelet aggregation and coagulation
Intracellular• Secretion of hormones and other regulators by
exocytosis
• Secondary messenger in cell division, cell motility
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Magnesium
Distribution:Total body magnesium 1 mole (Total body Ca = 25 moles)
– Bone predominantly– Cells– Serum - 30% protein bound (Serum Ca 40% bound)
Function: Neuromuscular conductionParathyroid hormone secretion
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Phosphate
Distribution:Total body phosphate 700g (24 moles)• Bone 83% - hydroxyapatite (calcium phospate)
• Cells 16% - organic phosphates (nucleic acid, ATP, phospholipids)
• Extracellular 1% - inorganic phosphates Fluid (H2PO4
- : HPO4= 1:4)
Function:• High energy phosphate bonds• Buffer
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Hormones:
Organs involved:• Intestine• Kidneys• Bone
Regulation of calcium metabolism:
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Calcium balance over 24 hours
PTH
14 mmoles/d
Vit D
Soft Tissue25 mmoles(1000 mg)
Bone 25 moles
(1000 mg)
Extracellular Fluid23 mmoles(920 mg)
GI Tract
Kidney
20 mmoles (800 mg)
8 mmoles (320 mg)
16 mmoles (640 mg)
4 mmoles (160 mg)
4 mmoles
Vit DGlomerular filtrate270 mmoles PTH
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Calcium
Intestine
Absorption process: • Regulated saturable transcellular absorption• Nonsaturable paracellular absorption (dependent on
mineral concentration in lumen)
Main sites:Duodenum and jejunum
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Factors affecting intestinal calcium absorption
Unit II – Calcium Metabolism – DS Ooi
GIT: Gastro-intestinal tract; diOH: dihydroxy
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Kidney: Calcium handling
Filtered Ca270 mmol/24h
Prox Convoluted
Tubule(passive)
Distal Convoluted Tubule(active)
Collecting Duct
Thick Asc
Loop of Henle
70%
20%
8%
(GF:180LConc: 1.5 mmol/L)
98% reabsorbed~ 5 mmol excreted in 24h
<5%
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Bones:
PTH
14 mmoles/d
Vit D
Soft Tissue25 mmoles(1000 mg)
Bone 25 moles
(1000 mg)
Extracellular Fluid23 mmoles(920 mg)
GI Tract
Kidney
20 mmoles (800 mg)
8 mmoles (320 mg)
16 mmoles (640 mg)
4 mmoles (160 mg)
4 mmoles
Vit DGlomerular filtrate270 mmoles PTH
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Regulation of calcium:
Hormones involved
Parathyroid hormone (PTH, parathyrin) Bone Kidneys Intestines
Vitamin D Intestines Bone Kidneys
Calcitonin Bone Kidneys
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Parathyroid hormone (PTH)
Synthesized as preprohoromone that undergoes cleavage before secretion
Continuously synthesized, minimal storage in parathyroid glands
Metabolized by liver & kidneys, plasma t1/2 2 min Laboratory assays measure:
C terminal (long t½), N-terminal (short t½ ) Mid-terminal (long half-life)
Intact PTH – most intact assays, also measure a 7-84aa fragment which accumulates in renal failure
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Regulators of Plasma PTH
Plasma calcium Hypocalcemia PTH biosynthesis
Ca-sensing receptor (CaSR) on parathyroid cellsCalcium binding results in PTH synthesis & secretion degradation of stored PTH
Calcitriol 1,25(OH)2D3 PTH gene transcription Hypocalcaemia overrides calcitriol effect on PTH production
Less important regulators: Catecholamines Magnesium - low Mg can cause hypocalcaemia Prostaglandins
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PTH secretion vs. Plasma calcium
From Williams Textbook of Endocrinology
Unit II – Calcium Metabolism – DS Ooi
Secretion rate of PTH varies inversely with plasma calcium concentation, in a sigmoid fashion
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PTH
Actions:
Maintains ionized calcium concentration within narrow range
Bones: Initiates osteoclastic bone resorption– release of calcium from bones
At high concentrations (e.g. 1o or 2o hyperparathyroidism)
– bone resorption >> bone formation
– cortical bone mass
At lower concentrations, especially if episodic release– bone formation >> bone resorption
– trabecular bone mass
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PTH
Actions on kidneys:
Kidneys: – calcium reabsorption in distal convoluted tubules
Note: 90% of filtered Ca reabsorbed in proximal tubule and loop of Henle independent of PTH, mostly via passive paracellular route
– phosphate reabsorption in proximal and distal convoluted tubules
– Stimulates 1 a-hydroxylase (calcidiol to calcitriol)
Intestines: effect through vitamin D
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Vitamin D
Unit II – Calcium Metabolism – DS Ooi
Action:• Binds to nuclear Vitamin D Receptor (VDR) – resulting in regulation of
DNA transcription• Calcitriol has highest affinity
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Vitamin D
7-dehydrocholesterol Cholecalciferol
25-OH vitamin D
Calcitriol
PTH
24,25 - D
IntestinesCa & PO4
absorption
Parathyroids¯ PTH Secretion¯ Cell proliferation¯ Gene transcription
BoneMultiple effects
Muscle(deficiency associated with
myopathy)
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Hormonal Regulation of Blood Calcium
CalciumAbsorption
GastrointestinalTract
Ca excretion P excretion
Blood Ca
mineralization
Bone loss
Bone resorption
PTH
Parathyroid Glands
Blood calcium
Unit II – Calcium Metabolism – DS Ooi
1,25 diOH Vit D
Cholecalciferol (Vit D3)
7-Dehydrocholesterol
Effect of UV on Skin
25-OHcholecalciferol
25 hydroxylase
1 hydroxylase
Legend:
Stimulate
Inhibit
Delayed effects (Stimulation)
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Regulation of Blood CalciumWhen blood calcium falls:
Bone resorption
PTH
Blood Ca ++
Parathyroid Glands
Ca excretion Serum Ca
P excretion Serum P
T½ 10 mMetabolised in liver to n-terminal (active, t½ 1-2m) c-terminal fragments (t½1-2h)which are cleared by kidney
Ca P
Serum Ca Serum P
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Regulation of Blood Calcium concentrationsIntermediate/Long term
PTH
25-OH D 1,25 diOH Vit D
Cholecalciferol (Vit D3)
CalciumAbsorption
Parathyroid Glands
7-Dehydrocholesterol
Effect of UV on Skin
mineralization
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Calcitonin
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Calcitonin
Production: 32-amino acid peptide hormone Produced by thyroid parafollicular C-cells
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Calcitonin
Action: Inhibits osteoclast-mediated bone resorption
(counteracts action of PTH) Renal (at higher concentrations):
Inhibits P reabsorption P excretion Some natriuretic effect mildly Ca excretion
Non-essential & less important than PTHTotal thyroidectomy does not result in hypercalcaemia
High calcitonin in medullary thyroid cancer does not result in hypocalcaemia
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Clinical uses of calcitonin
Tumour marker for medullary thyroid carcinoma Therapeutic applications:
Hypercalcemia - administration quickly lowers plasma Ca through reduced osteoclast activity
Osteoporosis - reduces fracture risk & pain associated with fractures (no longer used due to increased risk of malignancy)
Paget disease of bone (bisphosphonates preferred)
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Other hormones affect bone
Growth hormone & IGF-1
• bone remodeling
Glucocorticoids• Ca absorption• Long term administration bone
formation
Hyperthyroidism• skeletal growth in children• bone resorption in adults
Insulin • Required for normal growth
Gonadal hormones
• Critical for skeletal development & maintenance
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IGF – Insulin-like Growth Factor
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Local bone regulators
Cytokines• e.g., interleukins, TNF-, TNF-• bone resorption, formation
TGF- & EGF• Produced by neoplasms• bone resorption
Prostaglandins• Synthesized by many skeletal cells• Affects bone resorption & formation
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TNF: Tissue Necrosis Factor, TGF: Transforming Growth Factor, EGF: Epidermal Growth Factor
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Hormonal Regulation of Blood Calcium
Cholecalciferol (Vit D3)
7-Dehydrocholesterol
Effect of UV on Skin
25-OHCholecalciferol
(Calcidiol)
25 a hydroxylase
1,25 diOH Vit D(calcitriol)
1 a hydroxylase
PTH
Parathyroid Glands
Parafollicular C cells
CalcitoninOsteoclast inhibition bone remodelling
Diet
Ergocalciferol (D2)
25-OHErgocalciferol
1, 25diOH D2
Ca release from bone (permissive)Activates remodelling Ca and phosphate reabsorption Intestinal Ca and P transportInhibition of PTH synthesis
Ca release from bones Bone remodelling (RANKL) DCT Ca reabsorptionPCT and DCT P reabsorptionRenal 1 a hydroxylase activation
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Disorders of Calcium homeostasis
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Mechanisms for hypercalcemia
• Bone resorption
• Gastrointestinal absorption of calcium
• Renal excretion of calcium
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PTH Mediated
• Primary hyperparathyroidism– Sporadic– Inherited variants
• Multiple endocrine neoplasia (MEN1, 2a)• Familial isolated hyperparathyroidism
– Hyperparathyroid-jaw tumour syndrome
• Familial hypocalciuric hypercalcemiaCaSR mutation (AD inheritance)
• Tertiary hyperparathyroidismFollowing prolonged stimulation, part of the parathyroid gland escapes feedback control
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Primary Hyperparathyroidism
• F > M, up to 0.4% of F>60y may be affected• Pathology:
– Solitary adenoma 80%– Hyperplasia 15%– Parathyroid carcinoma 1-2%
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Non-PTH Mediated
• Malignancy– PTH-related peptide (PTHrP)– Osteolytic bone metastasis and local cytokines– Activation of extrarenal 1 -a hydroxylase
• Vitamin D– Chronic granulomatous disorders (sarcoidosis, TB) 1 -a hydroxylase– Exogenous vitamin D intake
• Drugs:– Milk-alkali syndrome ( Ca absorption, alkalosis renal Ca excretion)– Lithium (renal Ca excretion, ? block Ca feedback on parathyroids)– Thiazides ( Renal Ca excretion)– Vitamin A toxicity– Theophylline toxicity
• Miscellaneous:– Hyperthyroidism– Acromegaly– Adrenal insufficiency– Immobilization
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Malignancy-associated hypercalcemia
• The most common cause of hypercalcemia in hospitalized patients
• Incidence: 15 cases/100,000/yr• Common cancers
– squamous cell cancer of lung, breast – renal cell carcinoma– myeloma, lymphoma
Rare in colon, gastric and thyroid cancers
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Hypercalcemia – Symptoms/Signs
Unit II – Calcium Metabolism – DS Ooi
Bones
Stones
Groans
Moans
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Management of Hypercalcemia:
• IV fluids• Loop diuretics (furosemide)• Calcitonin• Steroids• Bisphosphonates• Dialysis• Calcium sensor receptor agonist (Cinacalcet) – for
primary hyperparathyroidism
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Hypocalcemia
Causes:
1. Insufficient PTH activity– Hypoparathyroidism (post thyroid surgery)– Hypomagnesemia (Mg required for PTH release)– Pseudoparathyroidism (PTH resistance)
2. Insufficient Vitamin D action– Insufficient Dietary/Exposure to UV rays– 1a-hydroxylase
• Chronic renal failure• Vitamin D dependent rickets
– Vitamin D resistant rickets
3. Sequestration of calcium– Acute pancreatitis
4. Drugs – calcitonin, furosemide
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Hypocalcemia – Symptoms/Signs
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Management of hypocalcemia
Acute: Replace calcium
Calcium gluconate IV Oral calcium
Treat hypomagnesemia, if present May require vitamin D Correct underlying cause
Long-term High dose vitamin D (D2 50,000 IU daily, calcitriol – up to 2 ug daily)
Adequate calcium intake
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Secondary hyperparathyroidism
PTH caused by other conditions• Vitamin D disorders
– Deficiency or malabsorption– Rickets
• Phosphate disorders– Chronic kidney disease– Phosphate depletion– Malabsorption– Aluminium toxicity
• Calcium deficiency
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Chronic kidney disease
Parathyroid glands
¯ Ca absorption(passive P absorption)
PTH
PN or Ca2+
1,25 D
+
PTH
+
+
Ca, PP
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Hypermagnesemia
Causes:• Chronic renal failure• Intravenous MgSO4 - as antihypertensive, sedative during
parturitionEffects:Usually does not rise to critical concentrations, and not
clinically important.• Sedation• Neuromuscular activity
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Hypomagnesemia
Causes:1. Reduce intake
– malabsorption– intake (alcoholics)
2. renal loss– diuretics– alcohol– Renal tubular defects– drugs - gentamicin, amphotericin B
Effects: PTH release (hypocalcemia)
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Hyperphosphatemia
Cause: Most often seen in chronic renal failure
Effect: serum calcium
Secondary hyperparathyroidism
Management:• Oral Phosphate binders• Dialysis for CRF patients
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Hypophosphatemia
Cause:
1. intake - starvation, malabsorption, Al(OH)3
2. loss• renal tubular leaks• hyperparathyroidism• Vit D resistant rickets (impaired tubular Phosphate
transport)
Effect:1. Loss of RBC membrane integrity (hemolysis)2. Muscle weakness
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Key points
Blood calcium is tightly regulated, primarily by PTH & vitamin D. Calcitonin plays a far less important role.
PTH acts on kidneys and bones; Vit D on bones, intestines, kidneys Common clinical conditions:
Hypercalcemia Hypercalcemia in malignancy Primary hyperparathyroidism
Hypocalcemia Post thyroid, parathyroid surgery
Secondary hyperparathyroidism Vitamin D deficiency Renal failure
Remember! Always adjust serum total calcium for albumin concentration (0.2 mmol Ca for every 10 g of albumin)
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Appendix: Parathyroid hormone
From Endocrinology: An Integrated Approach, 2001
Unit II – Calcium Metabolism – DS Ooi
84 amino acid peptide hormone
Synthesis:
7-84aaBlocks PTH activityAccumulates in CKDMeasured by most intact PTH assays
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Vitamin D: Chemical structures
D2 D3
25-OH D31,25(OH)2D3
From www.chm.bris.ac.uk
✪ Unit II – Calcium Metabolism – DS Ooi