Calcium homeostasis is tightly regulated by several hormones and organs. Calcium levels are maintained within a narrow range through intestinal absorption, bone resorption and deposition, and renal excretion. Parathyroid hormone (PTH) increases calcium and phosphate levels by acting on bone, kidney, and intestine. Vitamin D facilitates intestinal calcium absorption and renal reabsorption. Calcitonin counters PTH by inhibiting bone resorption and reducing calcium and phosphate levels. Together these hormones work in a feedback loop to precisely control circulating calcium.

1. ©dharmdmd@gmail.com
CalciumCalciumCalciumCalcium HomeostasisHomeostasisHomeostasisHomeostasis
Dr Dharmendra
Dept of Physiology
AFMC

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Sequence of PresentationSequence of PresentationSequence of PresentationSequence of Presentation
Introduction
History & Comparative Physiology
Calcium: Metabolism & Role
Regulation of Ca++ Levels:
Parathyroid Hormone (PTH)
Vitamin D
Calcitonin
Applied Aspects
Conclusion

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IntroductionIntroductionIntroductionIntroduction
•Ca++ is a chemical element, an alkaline earth metal
•5th most abundant element
•An essential dietary element for human life
•As an electrolyte, Ca++ plays vital role in the
physiological and biological processes
•Both Intracellular and Extracellular functions

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•Ca++ outside cells are important for maintaining the
potential difference across excitable CM as well
proper bone formation.
•Regulation of Ca++ in blood occurs within narrow
limit
•Organ Ca homeostasis
•Target organs : bones, kidneys, intestines
•Endocrine Ca homeostasis
•Three hormones primarily regulate Ca++
metabolism

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•Primary process for release of Ca into circulation
• Ca GI absorption and bone resorption
•Primary process for removal of Ca from blood
• Ca Renal excretion and bone formation
•Phosphate metabolism is closely linked
•Multiple disorders

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Historical AspectsHistorical AspectsHistorical AspectsHistorical Aspects
• 1808- Humphry devy
• Isolated from oxide and named it
• 1883- Sydney Ringer:
• Ist to reveal importance of Ca++
1888- Role in cellular Adhesion
• 1888- Alexis Hartmann
• Ringer solution- modified to ‘Ringer Lactate Solution’ or
‘Hartmann's solution’
• 1927- Heilbrum- roll in clotting
1940 1970 1993
NT Ca channels CaSR

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Comparative PhysiologyComparative PhysiologyComparative PhysiologyComparative Physiology
•Calcitonin/ PTH/ Vitamin D:
• Fish, Reptiles, Birds and Mammals
• Same functions as in humans
• Fish rich source of Vit D
• Salmon Calcitonin

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Calcium:Calcium:Calcium:Calcium: MetabolismMetabolismMetabolismMetabolism
Distribution of Ca:
• Average human body – 1000 to 1100 gm
• 99% - skeleton
• 0.9% - intracellular
• 0.1%- ECF (one gram)
• Three forms in plasma:
• Normal value – 9.4 mg/dl (2.4 mmol/l)
• Total diffusible (ionic)- 5.3 mg/dl
• Total nondiffusible (protein-bound)- 4.6 mg/dl
• Ca estimation: Corrected Ca (mg/dl)= measured total Ca+.8 (4-Albumin)

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Calcium: MetabolismCalcium: MetabolismCalcium: MetabolismCalcium: Metabolism
• Overview of calcium exchange between different tissue
compartments
(90%)
Rapid exchange
Exchangable &
Stable form
(10%)
(98–99%)

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Calcium:Calcium:Calcium:Calcium: MetabolismMetabolismMetabolismMetabolism---- OrganOrganOrganOrgan CaCaCaCa homeostasishomeostasishomeostasishomeostasis
Target organs:
1. Intestine- Ca absorption
2. Kidney- Ca excretion
3. Bone- uptake & release of Ca
•Plasma Ca conc depends on net effect of these
organ physiology

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OrganOrganOrganOrgan CaCaCaCa homeostasis: GI Absorptionhomeostasis: GI Absorptionhomeostasis: GI Absorptionhomeostasis: GI Absorption
• Mainly Small intestine- duodenum /proximal jejunum
• Absorption and fecal excretion
• Fractional absorption stimulated by- 1,25 DHCC
• Brush border : active, carrier mediated energy dependent
• Active transport transcellular route
• Passive & bulk flow paracellular route

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OrganOrganOrganOrgan CaCaCaCa homeostasis: GI Absorptionhomeostasis: GI Absorptionhomeostasis: GI Absorptionhomeostasis: GI Absorption
• TRPV5/6- ‘Transient Receptor Potential Vanilloid Type 5/6 ’
• 1,25-DHCC- increases the expression of all these proteins
• PTH- 1 α hydroxylase enzyme
/NCX1

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OrganOrganOrganOrgan CaCaCaCa homeostasis: Renal excretionhomeostasis: Renal excretionhomeostasis: Renal excretionhomeostasis: Renal excretion
(98–99%)

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Renal Ca reabsorption
PCT- Passive paracellular pathway
TAL- Both paracellular & transcellular
pathway

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OrganOrganOrganOrgan CaCaCaCa homeostasishomeostasishomeostasishomeostasis: Bone: Bone: Bone: Bone
• Accretion & resorption are in balance
• Bone remodeling can be modulated
• Chronic Ca & Pi dysregulation or H dysregulation
• Pathological changes
• Chemical Composition:
• Mineral salts - 35% (Ca++ & phosphates, Ratio: 1.3 to 2)
• Organic matrix - 20%
• Water – 45%
• Amorphous(CaHPo4) compounds –loosely bound-1% of
bone Ca++ - easily exchangeable – First line of Defense

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OrganOrganOrganOrgan CaCaCaCa homeostasis: Bonehomeostasis: Bonehomeostasis: Bonehomeostasis: Bone
Structure: Two types
• Compact or Cortical (75%)
• Outer layer
• Nutrients by Canaliculi/Haversian Canals
• Low surface to volume ratio
• Collagen arranged in circles -Osteons
• Trabecular or Spongy (25%)
• Lie inside the compact bone
• Made up of spicules
• High surface to volume ratio
• Nutrients diffuse through ECF

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OrganOrganOrganOrgan CaCaCaCa homeostasis: Bonehomeostasis: Bonehomeostasis: Bonehomeostasis: Bone
In adult- Bone remodeling involve:
1. Destruction of preformed bone with release of
Ca, Pi & hydrolysed fragments of protein metrix
(osteoid) into blood.
2. New systhesis of osteoid at the site of resorption
with subsequent calcification
Cell types: 02 major class
•Osteoblasts
•Osteoclasts

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OrganOrganOrganOrgan CaCaCaCa homeostasis: Bonehomeostasis: Bonehomeostasis: Bonehomeostasis: Bone
•Osteoblast regulation of osteoclast differentiation
and function.
After 2 wks

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•osteoid (eg. Osteocalcin & ALP)- promote
calcification
Calcium phosphate
Hydroxyappetite crystals
Lamellae
Haversian Lacunae
Haversian system (osteon)

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PTH
PTH/PTHrP
Sustained PTH- bone turnover & resorption
Intermittent low dose PTH- promote OB survival &
bone anabolic function
OB

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EndocrineEndocrineEndocrineEndocrine CaCaCaCa homeostasishomeostasishomeostasishomeostasis: PTH: PTH: PTH: PTH
•PTH & 1,25 DHCC = calciotropic Hormone
Synthesis:
•Two distinct cells:
• Chief Cells- prominent ER,
golgi appt – secrete PTH
• Oxyphil Cells
•Target organs
•Net effect- S. Ca, S. Pi

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EndocrineEndocrineEndocrineEndocrine CaCaCaCa homeostasis: PTHhomeostasis: PTHhomeostasis: PTHhomeostasis: PTH
Actions of PTH:
>Bone:
• Increased resorption of bone, Growth & development of cartilage
• Rapid and slow phase
>Kidney:
1. Ca resorption & Pi resorption
2. Hyperphosphaturia
3. Increased urinary excretion of Hydroxyproline
4. Increased conversion of 25-HCC to 1,25 DHCC
>Intestine: indirect action
>Other actions:
• Activation of Adenyl Cyclase in target tissues
• Brain , placenta , Teeth

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EndocrineEndocrineEndocrineEndocrine CaCaCaCa homeostasis: PTHhomeostasis: PTHhomeostasis: PTHhomeostasis: PTH
Mechanism of Action :
• 3 different receptors
• PTH1 (PTHrP receptor)
• PTH2: brain,placenta, kidney
• CPTH
• CPTH – acts at carboxyl terminal
• PTHrP:
• Produced by many tissues
• Marked homology bw PTH &
PTHrP
• Similar receptors
• On Osteoblast & PCT, DCT- for
PTH

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EndocrineEndocrineEndocrineEndocrine CaCaCaCa homeostasis: PTHhomeostasis: PTHhomeostasis: PTHhomeostasis: PTH
Regulation;
Major regulator:
• Ca++ ion
concentration
Minor regulator:
• Vit D
•Decreased
Ca++/Increased PTH
•Increased
Ca++/Decreased PTH
CaSR- PLC-IP3/DAG- IC Ca
CaSR

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EndocrineEndocrineEndocrineEndocrine CaCaCaCa homeostasis:homeostasis:homeostasis:homeostasis: VitVitVitVit DDDD
Synthesis:
•Vit D3- by UV irradiation of 7-dehydrocholesterol in
skin
•Indirect inhibition by Ca++
•Target organ: intestine, bone, kidney
parathyroid gland

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Vitamin DVitamin DVitamin DVitamin D
metabolismmetabolismmetabolismmetabolism
PCT
PCT
Ca

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Effect of plasma calciumEffect of plasma calciumEffect of plasma calciumEffect of plasma calcium concconcconcconc onononon
1,25 DHCC1,25 DHCC1,25 DHCC1,25 DHCC
Effect ofEffect ofEffect ofEffect of VitVitVitVit D3 intake on 1,25 DHCCD3 intake on 1,25 DHCCD3 intake on 1,25 DHCCD3 intake on 1,25 DHCC

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EndocrineEndocrineEndocrineEndocrine CaCaCaCa homeostasis:homeostasis:homeostasis:homeostasis: VitVitVitVit DDDD
Mechanism of Action:
Intestine:
1. 1,25 DHCC affects Ca++ absorption and transport
2. Act via Calbindin- D proteins
3. Intestinal epithelium – Calbindin D 9k
4. Also increases number of Ca++-ATPase/TRPV6
Kidney: Facilitates Ca++ resorption in kidneys via TRPV5
• Ca++ acts as transport mediator for phosphate
Bone:
• Increases synthetic activity of Osteoblasts
• Stimulate termination of OC differentiation
• Necessary for normal calcification of matrix

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GI AbsorptionGI AbsorptionGI AbsorptionGI Absorption
• TRPV5/6- ‘Transient Receptor Potential Vanilloid Type 5/6 ’
• 1,25-DHCC- increases the expression of all these proteins
• PTH- 1 α hydroxylase enzyme
/NCX1

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EndocrineEndocrineEndocrineEndocrine CaCaCaCa homeostasis:homeostasis:homeostasis:homeostasis: VitVitVitVit DDDD
Regulation:
•PTH
•Feedback - Ca++ and PO4
3+ levels
•High levels Ca-
• inhibit 1,25 DHCC production
• More 24,25 DHCC produced (Inactive form)
•Promotes mineral precipitation on collagen fibrils
•Low levels Ca-
• stimulates more PTH – more 1α hydroxylase

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• Regulation:
• A new protein called – α Klotho :
• Important role in calcium and phosphate homeostasis
• Stabilizes membrane proteins TRPV5/NaK ATPase.
• Enhances activity of, Fibroblast Growth Factor 23
(FGF23)
• FGF23 decreases renal NaPi-IIa and NaPi-IIc expression
• Inhibits the production of 1 -hydroxylase, reducing 1,25-
DHCC

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EndocrineEndocrineEndocrineEndocrine CaCaCaCa homeostasis:homeostasis:homeostasis:homeostasis: CalcitoninCalcitoninCalcitoninCalcitonin
Synthesis:
• Release in response of increase Ca
Action:
• Inhibits resorption of bones- Ca, Pi
• Reduces levels of Calcium and Phosphates:
• Inhibits activity of Osteoclasts
• Increasing efflux of calcium from kidneys
• Increased mitochondrial uptake of calcium
• Overall Minor Role:
• Post Thyroidectomy – Normal levels of Ca++ and bone density
• Medullary Ca Thyroid-Very high calcitonin – No effect on Ca++

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•In renal failure:
• calcitonin
•Regulation:
Bone-
• Ca
• Same by CaSR
• Similar PTH/PTHrP receptors
• ECF Ca--- calcitonin
• Act on Osteoclast
•Kidney- inhibit reabsorption

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Interplay ofInterplay ofInterplay ofInterplay of CaCaCaCa++++++++, PTH & Calcitonin, PTH & Calcitonin, PTH & Calcitonin, PTH & Calcitonin

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Effect of other Hormones onEffect of other Hormones onEffect of other Hormones onEffect of other Hormones on CaCaCaCa++++++++
• Glucocorticoids :
Bone:
• inhibiting osteoclast formation and activity.
• Osteoporosis - inhibits protein synthesis in osteoblasts.
Intestine:
• decrease the absorption of Ca2+ and PO4 3–
Kidney:
• Decrease reabsorption
• Growth hormone:
• increases intestinal absorption of Ca2+
• IGF-I stimulates protein synthesis in bone
• Estrogens:
• prevent osteoporosis - inhibits stimulation of osteoclasts
• Increase Ca absorption & reabsorption
• Insulin: increases bone formation - bone loss in untreated diabetes.

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Role of CalciumRole of CalciumRole of CalciumRole of Calcium
• Main component of Bony Skeleton (99%)
• Excitation-Contraction coupling
• Stabilization of Cell Membrane
• Release of Neurotransmitters at synaptic vesicle
• Secretion from Endocrine/Exocrine glands
• Secondary massenger
• Synthesis of Nucleic Acids
• Activation/Regulation of enzymes
• Blood Clotting
• Teeth

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Applied AspectsApplied AspectsApplied AspectsApplied Aspects
1. Hypercalcemia/Hyperpatathyroidism
• GI absorption/Renal reabsorption/Bone resorption
• Calcification in soft tissue
• Bone demineralization
• Osteoporosis
• FBHH- CaSR mutation
2. Hypocalsemia/Hypoparathyroidism
• Bone /kidney
• Alkalosis
• No bone demineralization
• Teteny/carpopedal spasm/trousseau’s sign/chvostic sign
• Laryngospasm
• First degree HB

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Integrated response to a hypocalcemic challenge.

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Applied AspectsApplied AspectsApplied AspectsApplied Aspects
3. Vitamin D deficiency:
• Defective bone mineralization
• Ca– PTH
• Child- Rickets
• Adult- Osteomalasia
4. Osteoporosis:
• Mutation OPG….. Highly overactive OC
5. Osteopetrosis:
• Mutation in RANK/RANKL gene

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Applied AspectsApplied AspectsApplied AspectsApplied Aspects
6. Hypervantilation:
• Decrease PTH
7. Paget disease:
• Increase bone resorption followed by new bone
formation
• Excessive bone turnover by increase OC actvity
• Bone deformity
• Raised ALP & Osteocalcin

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Applied AspectsApplied AspectsApplied AspectsApplied Aspects
8. Renal osteodystrophy
9. Neuromuscular junction
10. Pseudohyperparathyroidism
11. Significant tissue damage
12. Drugs: loop diuretics/thiazides/CaSR modified &
blocker
13. ECG changes
14. Effect of microgravity

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ConclusionConclusionConclusionConclusion
• Calcium plays a very IMPORTANT role in metabolic
processes of almost all body cells
• Necessary for normal Skeletal Bony Growth
• Levels are maintained in a narrow range
• Right conc. depends upon interplay of Ca absorption from
intestine, bone uptake/ release and renal excretion
• All the above are regulated in concert by PTH, Vit D &
Calcitonin
• Many disorders are linked with dysfunction

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