1. Calcium Homeostasis
Dr. Vivek Pant
MD Clinical Biochemistry

2. Outline of presentation
• Calcium distribution in body
• Function of Calcium
• Calcium Homeostasis
Parathormone
Calcitonin
Vit. D
• Calcium Homeostasis: Molecular Mechanism
• Disorder of Calcium Homeostasis

3. Distribution of Calcium in Body
99% in bone 1% in blood and body fluids
Blood Calcium (10mg/dl)
Non diffusible 35 %
Albumin bound
80 %
Globulin bound
20 %
Diffusible 65 %
Ionized 80 % Complexed 20 %
Bicarbonate
Citrate
Phosphate
Total body calcium- 1kg

4. Biochemical parameters affecting Calcium level
 Normal total serum calcium is approximately 8.9–10.1 mg/dl (2.2–2.5 mmol/l)
Calcium can be bound to albumin and globulins.
 For each 1.0g/dl decrease in serum albumin, total serum calcium decreases by
0.8 mg/dl.
 For each 1.0g/dl decrease in serum globulin, total serum calcium decreases by
0.12 mg/dl.
 Since both hydrogen ions and calcium are bound to serum albumin, in the
presence of metabolic alkalosis, bound hydrogen ions dissociate from albumin,
freeing up the albumin to bind with more calcium and thereby decreasing the
freely ionized portion of the total serum calcium.
 For every 0.1 change in pH, ionized calcium changes by 0.12 mg/dl

5. ROLE OF CALCIUM
• Excitability of cell membranes
• Neuromuscular transmission and muscle contraction
• Releasing of transmitters from synapses
• Stimulates secretory activity of exocrine glands and releasing of hormones
• Contractility of myocardium
• Blood coagulation
• Second messenger: such as cAMP and IP3 and thus mediates the cellular
response to epinephrine, glucagon, ADH, secretin, cholecystokinin.

6. Regulation of Calcium Homeostasis
Three principal hormones are involved in calcium homeostasis
• Vitamin D,
• Parathormone and
• Calcitonin
Acting at three target organs,
• Intestine,
• Bone and
• Kidneys

7. Parathormone
MOA
 Increases Calcium reabsorption in early distal tubule.
 Increase bone resorption of calcium.
 Increases the synthesis of 1-ᾳ-hydroxylase in the proximal renal tubule
which increase the synthesis of 1,25-(OH)2D which in turn increases
intestinal reabsorption of calcium

8. Clinical Correlation
Teriparatide (Recombinant PTH)
Used in treatment of primary hypoparathyroidism

9. Calcitonin (CT)
 Secreted from the parafollicular C cells in the thyroid
 32 aa
 t₁⁄₂ 10 minutes
Secretion of calcitonin is stimulated by:
an increase in serum [Ca2+], gastrin and pentagastrin
 MOA
 Direct inhibition of osteoclasts
 Promotes deposition of Ca++ into bone
 Lowers Ca++ in blood

10. Mechanism of action of calcitonin
Calcitonin induces contraction and inhibition of osteoclast motility
within 1 min.
This is followed by a more gradual retraction of the osteoclasts.
Both cAMP and intracellular calcium (Ca2+) are second messengers
for these effects, and both are G-protein mediated.

11. Clinical Correlation
Salmon calcitonin (synthetic) is used for the treatment of:
o Postmenopausal osteoporosis
o Paget's disease
o Bone metastases

12. 1,25-Dihydroxyvitamin D
MOA
 Stimulates GI calcium and phosphate absorption.
 Promotes renal calcium and phosphate re-absorption.
 Together with PTH it mobilises calcium from skeletal stores.

13. Calcium homeostasis: At Intestine
For calcium absorption two mechanisms have been proposed-
• Simple passive diffusion
• Active transport- involving energy and calcium pump.
o Vitamin D is important for the active process.
o Active calcium transport depends on the presence in the intestinal cell of
calbindin protein , the biosynthesis of which is totally dependent on vitamin D.

14. How Vit D induces calcium absorption in intestine???
Uptake of calcium into enterocyte occurs by TRPV6 (membrane calcium channel
transient receptor potential cation channel, subfamily V, member 6) followed by
Intracellular binding of calcium to calcium binding protein CaBP-9K
Then energy dependent transport of calcium across basolateral membrane via
Ca++-ATPase
Vit. D increases gene expression of TRPV6 and CaBP-9K

15. Factors affecting calcium absorption
A) Factors favoring calcium absorption
• An acidic pH
• High protein diet- Lysine and Arginine cause maximal absorption
• Presence of vitamin D : 1,25(OH)2D interacts with its specific nuclear
vitamin D receptor (VDR) in the small intestine to enhance the efficiency
of intestinal calcium absorption.
• Ca : P ratio- A ratio of dietary Ca: P not more than 2:1 is adequate for
optimal absorption, ratio of less than 1:2 reduces absorption

16. Factors affecting calcium absorption
B) Factors inhibiting absorption of calcium
• Alkaline pH
• High fat diet- Fatty acids form calcium soaps that can not be absorbed
• Presence of Phytates and oxalates- Insoluble calcium salts are formed
• Dietary fiber in excess inhibits absorption
• Advancing age and intestinal inflammatory disorders inhibit absorption of
calcium

17. Calcium homeostasis: At Bone
• Bone resorption is the normal destruction of bone by osteoclasts, which
are indirectly stimulated by PTH.
• Stimulation is indirect since osteoclasts do not have a receptor for PTH;
rather, PTH binds to osteoblasts.
• Binding stimulates osteoblasts to increase their expression of RANKL and
inhibits their secretion of Osteoprotegerin (OPG).
• Free OPG competitively binds to RANKL , preventing RANKL from
interacting with RANK, a receptor for RANKL

18. Molecular Mechanism

19. Calcium Homeostasis: At Kidney
Renal tubule Percentage of Ca++
reabsorption
Mechanism
PCT 60-70 Parallels Na+ and H20 absorption via
passive diffusion
Thin segment of LoH 0 No reabsorption
Thick segment of LoH 20 Transcellularly via transtubular
electrochemical driving force i.e.
Na+K+-2Cl- co transporter
DCT and CT 10-15 Transcellularly via active transport and
effect mediated by PTH

20. Increase Calcium Absorption
Hyperparathyroidism
Calcitriol
Hypocalcemia
Metabolic alkalosis
Thiazides diuretics
Factors that alter renal regulation of calcium
Decrease Calcium Absorption
Hypoparathyroidism
Low calcitriol levels
Hypercalcemia
Metabolic acidosis
Loop diuretics
Vit D regulates epithelial channel TRPV5 (membrane calcium channel transient
receptor potential cation channel, subfamily V, member 5) which helps passage
of calcium across luminal membrane

21. Calcium homeostasis : Molecular level
Calcium-sensing receptor (CASR)
The CASR plays an essential role in maintaining calcium ion homeostasis.
•This receptor is expressed in all tissues related to calcium control, i.e.
parathyroid glands, thyroid C-cells, kidneys, intestines and bones.
•It has the ability to sense small changes in plasma calcium concentration.
•This information is conveyed to intracellular signaling pathways that modify
PTH secretion or renal calcium handling.

22. Calcium-sensing receptor (CASR): At Parathyroid
Changes in concentrations of serum Ca2 are sensed by chief cells
through a cell-surface, seven-transmembrane, G protein–coupled
receptor, the CaSR.
When extracellular Ca binds to the CaSR, it elicits conformational
changes within the receptor.
G protein subunits, Gq and Gi, are recruited to the receptor and alter
the amounts or activity of several intracellular mediators within the
chief cell.
Intracellular Ca2 is altered as a result of activation of phospholipase C
(PLC) by the Gq subunit.
This increased intracellular concentration of calcium, inhibits vesicle
fusion and exocytosis of parathyroid hormone

23. The CaSR also interacts with Gi to inhibit adenylate cyclase activity that
reduces intracellular cyclic AMP.
These changes within chief cells rapidly enhance the release of preformed
PTH from the parathyroid gland

24. Calcium homeostasis : Molecular level
Roles of 1,25-Dihydroxyvitamin D
Independent of changes in intestinal calcium absorption and serum calcium,
1 ,25-dihydroxyvitamin D also represses the transcription of PTH.
Vit.D associate with the vitamin D receptor, which heterodimerizes with retinoic
acid X receptors to bind vitamin D-response elements within the PTH gene.
1 ,25-Dihydroxyvitamin D additionally regulates the expression of calcium-
sensing receptors to indirectly alter PTH secretion.

26. Etiologies of Hypercalcemia
Increased GI
Absorption
Elevated calcitriol
Excessive dietary intake
Increased Loss
From Bone
Decreased Urinary
Excretion
Thiazide diuretics
Elevated calcitriol
Elevated PTH
Hyperparathyroidism
Malignancy
Metastasis
Pagets disease
hyperthyroidism
Disorders of calcium homeostasis

27. Clinical Features of Hypercalcemia
Acute Chronic
Gastro-
intestinal
Anorexia, nausea, vomiting Dyspepsia, constipation,
pancreatitis
Renal Polyuria, polydipsia Nephrolithiasis, nephrocalcinosis
Neuro-
muscular
Depression, confusion,
stupor, coma
Proximal muscle weakness,
atrophy of muscle fibers,
hyperreflexia, gait disturbance
Cardiac Bradycardia, first degree
atrio-ventricular block, ↓QTc
interval on ECG
Hypertension

28. Hypercalcemia : Approach
PTH dependent PTH independent
PTH or normal
Hyperparathyroidism
ALP
24 hr Ur Ca >300mg
Vit D in primary ( Serum Cl-:P ratio > 33)
Vit D in secondary
Familial hypocalciuric hypercalcemia
ALP
24 hr Ur Ca very less
Ur Ca/Cr <0.01
Hypercalcemia at birth
MEN
Type I : peptic ulcer, headache
Type 2A : goiter, Hypertension

29. Hypercalcemia
PTH independent
Malignancy
ALP
24 hr Ur Ca >300mg Sarcoidosis : Vit D , ACE in BAL
SCC, RCC, Pheochromocytoma, Breast Ca Milk-alkali Syndrome : Cl
BUN, Cr
Multiple Myeloma
ALP Thiazide diuretic
24 hr Ur Ca >300mg Hypocalciuric hypercalcemia
ESR Lithium
Hyperproteinemia, Renal failure
Vit D intoxication
Others

30. Management of Hypercalcemia
Hydration and Loop diuretics ( Fall in Ca by 2-3 mg/dl in 24 hrs)
Bisphosphonates along with steroid in cases of malignancy which will
inhibit osteoclastic activity.
Calcitonin reduces release of calcium from bones within minutes of
infusion.
Calcimimetics ( CINACALCET) – blocks PTH release

31. Etiologies of Hypocalcemia
Decreased GI Absorption Decreased Bone Resorption/
Increased Mineralization
 Hypoparathyroidism
 Pseudohypoparathyroidism
 Vitamin D deficiency
 Osteoblastic metastases
• Poor dietary intake of calcium
• Impaired absorption of calcium
• Vitamin D deficiency
• Malabsorption syndromes
• Liver failue
• Low PTH

32. Immediate post-operative period,
following removal of the thyroid or
parathyroid glands.
Rapid administration of citrated
blood or large volumes of albumin.
Mg depletion (can cause relative
PTH deficiency and end-organ
resistance to PTH action)
Acute pancreatitis (when lipolytic
products released from the
inflamed pancreas chelate Ca)
Hungry bone syndrome-(persistent
hypocalcaemia and hypophosphatemia
occurring after surgical or medical
correction of moderate to severe
hyperparathyroidism in patients in whom
serum Ca levels had been supported by
high bone turnover induced by greatly
elevated parathyroid hormone)
Septic shock (due to suppression of PTH
release and decreased conversion of
25(OH)D to 1,25(OH)2D)
Drugs including anticonvulsants
(e.g., phenytoin , phenobarbital, rifampin
which alter vitamin D metabolism)
Other causes of Hypocalcaemia

33. Clinical Presentation of Hypocalcemia
Tetany
Paresthesias
Muscle cramps
Seizures
Fatigue
Anxiety
Laryngeal bronchial spasms
Major clinical manifestations of hypocalcemia
are due to disturbances in cellular
membrane potential, resulting in
neuromuscular irritability.

34. Signs of hypocalcemia
Hypocalcemia causes partial
depolarization of nerves and
muscle which lowers the threshold
potential.
Threshold potential comes closer to
the resting membrane potential.
Thus an action potential can be
induced by smaller stimulus
e.g. carpopedal spasm, facial twitch

35. Hypocalcemia: Approach
PTH decreased PTH increased
Serum Mg++
Decreased : Hypoparathyroidism secondary to hypomagnesemia
Normal : Primary hypoparathyroidism Diarrhea
Alcohol
Aminoglycoside
Loop diuretic
Urinary Fractional excretion of calcium
Normal : Primary hypoparathyroidism
Increased : Gain of function mutation of CASR

36. Hypocalcemia: Approach
PTH increased
Serum phosphate
Decreased
VitD Decreased : Vit D deficiency– Pancreatitis, Crohns disease, Malnutrition
Increased : Vit D resistance
Increased
Pseudohypoparathyroidism : Normal 25(OH)D ,decrease 1,25(OH)D
Urinary cAMP level absent on PTH injection
Renal Failure
Excessive phosphate intake
Tumor lysis Syndrome

37. Type
1a
(Albright's hereditary osteodystrophy), short fourth and
fifth metacarpals and a rounded facies. Autosomal dominant . Associated
with TSH resistance.
Type
1b
Lacks the physical appearance of type 1a, but is biochemically similar.
associated with a methylation defect.
Type
2
Also lacks the physical appearance of type 1a.Since the genetic defect in
type 2 is further down the signaling pathway there is a
normal cAMP response to PTH stimulation.
Urinary cAMP levels (Ellsworth- Howard test)
Urinary cAMP response to infusion of synthetic PTH (1-34)
Differentiate Type 1 and 2 pseudo hypoparathyroidism.
The parathyroid hormone works on Gs G protein which then leads to increased levels of
cAMP (as second messenger).
Since pseudo hypoparathyroidism is caused by a fault in the G protein receptor signaling
pathway, so it has decreased urinary cAMP levels

38. Treatment of Hypocalcaemia
• IV Ca Gluconate for tetany and arrhythmia
• Oral Ca for postoperative hypoparathyroidism
• Oral Ca and vitamin D for chronic hypocalcemia
• Rebreathing expired air in a paper bag controls tetany in cases of
hypocalcemia due to hyperventilation.

39. Measurement of Calcium : Patient preparation
• Because citrate , oxalate and EDTA bind calcium and reduce it ; heparin is
the only anticoagulant of choice for calcium determination.
• Errors of 0.5-1.0 mg/dl in total calcium may result when tourniquet is
applied due to increase in protein bound calcium caused by efflux of water
from vascular compartment during stasis.
• Fist clenching should be avoided before phlebotomy as it decrease pH
(Lactic acid production) and an increase in free calcium.
• Hyperventilation decreases the free calcium concentration.

40. Measurement of Calcium
• Total calcium is measured by (10.1-10.5 mg/dl or 2.52-2.62 mmol/L)
 Photometric method
 Atomic Absorption Spectrometry method
 Isotope dilution-mass spectrometry
• Ionized calcium is measured by (4.6-5.3 mg/dl or 1.15-1.33 mmol/L)
Calcium ISE that contains inner reference solution of calcium chloride/ silver
chloride and physiological concentration of NaCl and KCl and internal
reference electrode

41. References
• Harrison's Principles of Internal Medicine 19th edition
• Teitz clinical biochemistry and molecular diagnosis 5th edition
• Medscape update

42. Thank you