Parathyroid Gland and Calcium Homeostasis

Question 1

How many parathyroid glands are there ? Where are they located ?

Answer 1

4 glands at poles of thyroid

Question 2

Identify the main kinds of cells present in the parathyroid glands. State the function of each.

Answer 2

• Chief cells - parathyroid hormone (parathormone)

* Oxyphilic cells - unknown funcCon

Question 3

Identify the main regulators of Calcium plasma levels.

Answer 3

• PTH + Vit D3 increase plasma Calcium

* Calcitonin decreases plasma Calcium

Question 4

Where in the body is Calcium located ?

Answer 4

Body Ca2+ - 99% bone/teeth (remaining 1% mainly intracellular, and 0.1% present in plasma) (1g only is extracellular)

Plasma Calcium present in the concentration of 2.3-2.6 mM:
50% free; 45% bound to protein; 5% chelated to di-carboxylic acids (e.g. citrate / lactate)

Question 5

What is the main function of Parathormone ?

Answer 5

Maintenance of plasma Calcium

Question 6

Identify the main physiological functions of Calcium.

Answer 6

• Prosthetic group for many enzymes and
structural proteins (± calmodulin)
• Structure of the plasma membrane (essential
for structure of Na+ channel and permeability of plasma membrane to Na+)
• Excitation-contraction coupling in muscle
• Excitation-secretion coupling at axonal terminals and in endocrine and exocrine
glands
• Blood coagulation
• Major intracellular second messenger

Question 7

State the values of Calcium resulting in hypoCa. At which value is this lethal ?

Answer 7

Total plasma Ca < 8 mg/dL (<2 mmol/l)

– lethal if < 1.6 mmol/l

Question 8

State the values of Calcium resulting in hyperCa. At which value is this lethal ?

Answer 8

total plasma Ca > 10.6 mg/dl (>2.3 mmol/l) – lethal if > 3.8 mmol/l

Question 9

What are the symptoms of Hypocalcemia ?

Answer 9

• Muscle cramps/twitches (tetany, -Chvostek’s/Trousseau’s signs)
• Numbness in fingers/toes
• Brittle nails
• Irritability
• Reduced mental capacity

Question 10

What are the symptoms of Hypercalcemia ?

Answer 10

• Anorexia
• Various GI tract disturbances
• Lethargy
• Depression
• Confusion
• Various aches/pains

Question 11

Identify possible causes of hypocalcemia.

Answer 11

• Post-surgical hypoparathyroidism
• Inherited hypoparathyroidism (mutation of calcium-sensing receptor - higher affinity for Ca2+)
• Pseudo-hypoparathyroidism (maternally inherited, with associated hyperphosphatemia, both bone and kidney affected)
• Pseudo-pseudo-hypoparathyroidism (paternally inherited, PTH activity suppressed only in bone)
• Vitamin D-related (secondary hyperparathyroidism)– lack of dietary Vit D, lack of sufficient UV exposure, renal lesions

Question 12

Identify possible causes of hypercalcemia.

Answer 12

• Hyperparathyroidism/adenoma in the parathyroid gland, various ectopic malignant tumours raising PTH/ PTHrP levels or tumours causing direct bone resorption
• High vitamin D intake (v. rare)
• Familial hypocalciuric hypercalcemia (mutations in calcium-sensing receptor - lower affinity for Ca2+)
• Sarcoidosis/granulomas – lymphoid granulomas synthesising 1, 25 dihydroxy vitamin D

Question 13

How are Calcium and Phosphate linked ?

Answer 13

Calcium and phosphate are the principal components of hydroxyapatite crystals [Ca 10 (PO 4 ) 6 (OH) 2 ], which by far constitute the major portion of the mineral phase of bone.

Reaction:
CaPO4 Hydroxyapatite [Ca10(PO4)6(OH)2]

Question 14

What are the main organs involved in body Calcium homeostasis ? Describe this body Calcium homeostasis.

Answer 14

Guy, kidneys, bone

1) The typical daily dietary intake of calcium is ~800 to 1200 mg. Dairy products are the major dietary source of calcium. Although the intestines absorb approximately one half the dietary calcium (~500 mg/day), they also secrete calcium for removal from the body (~325 mg/day), and therefore, the net intestinal uptake of calcium is only ~175 mg/day.
2) Bone, in the steady state, deposits ~280 mg/day of calcium and resorbs an equal amount.
3) Kidney filters ~10 times the total extracellular pool of calcium per day, ~10,000 mg/day. The kidneys reabsorb ~99% of this Ca 2+ , so that the net renal excretion of Ca 2+ is ~1% of the filtered load. In a person in Ca 2+ balance, urinary excretion (~175 mg/day) matches net absorption by the GI tract.

Question 15

What is the concentration of total phosphate in adult plasma ? What form is it found in ?

Answer 15

The concentration of total phosphate in adult plasma—predominantly inorganic phosphate in the form of (H2PO4)- and (H2PO4)2- ranges from 0.8 to 1.5 mM. It is ~50% higher in children.

Between 85% and 90% of the circulating inorganic phosphate is filterable by the kidneys, either ionized (50%) or complexed to Na + , Ca 2+ , or Mg 2+ (40%); only a small proportion (10% to 15%) is protein bound.

Question 16

Where in the body is phosphate located ?

Answer 16

• Most total-body phosphate is present in bone, which contains ~0.6 kg of elemental phosphorus.
• A smaller amount of phosphorus (0.1 kg) resides in the soft tissues, mainly as organic phosphates, such as phospholipids, phosphoproteins, nucleic acids, and nucleotides.
• An even smaller amount (~500 mg) is present in the extracellular fluid (ECF) as inorganic phosphate.

Question 17

What are the main organs involved in body Phosphate homeostasis ? Describe this body Phosphate homeostasis.

Answer 17

Guy, kidneys, bone

1) The daily dietary intake of phosphorus is typically 1400 mg, mostly as inorganic phosphate. Again, dairy products are the major source. The net absorption of phosphate by the intestines is ~900 mg/day.
2) In the steady state, bone has relatively small phosphate turnover, ~210 mg/day.
3) The kidneys filter ~14 times the total extracellular pool of phosphate per day (~7000 mg/day) and reabsorb ~6100 mg/day. Hence, the net renal excretion of phosphorus is ~900 mg/day, the same as the net absorption by the GI tract.

Question 18

Describe regulation of PTH secretion.

Answer 18

The major stimulus for PTH secretion is a decline in the concentration of Ca 2+ in the blood (hypocalcemia) and ECF. Hypocalcemia also stimulates synthesis of new PTH, which is necessary because the parathyroid gland contains only enough PTH to maintain a stimulated secretory response for several hours.

Ca2+ binds Ca 2+ -sensing receptor (CaSR) in the plasma membrane of the parathyroid cell in a saturable manner. CaSR is a member of the G protein–coupled receptor (GPCR) family. Coupling of this Ca 2+ receptor to Gα q activates PLPC, which generates IP 3 and DAG and results in the release of Ca 2+ from internal stores and the activation of protein kinase C. Unlike most endocrine tissues, in which activation of these signaling systems promotes a secretory response, in the parathyroid the rise in [Ca 2+ ] i and activation of PKC inhibit hormone secretion.

Thus, increasing levels of plasma [Ca 2+ ] decrease PTH secretion.

Question 19

Describe synthesis, and processing of PTH.

Answer 19

• The PTH gene possesses upstream regulatory elements in the 5′ region, including response elements for both vitamins D and A.
• The vitamin D response element binds a vitamin D receptor (VDR) when the receptor is occupied by a vitamin D metabolite, usually 1,25-dihydroxyvitamin D.
• VDR is a nuclear receptor and forms a heterodimer with the retinoid X receptor (RXR) and acts as a transcription factor.
• The receptor has a very high affinity for the 1,25-dihydroxylated form of vitamin D, less affinity for the 25-hydroxy form, and little affinity for the parent vitamin (either D 2 or D 3)
• Binding of the vitamin D–VDR complex to the VDR response element decreases the rate of PTH transcription.
• After transport of the mature PTH messenger RNA (mRNA) to the cytosol, PTH is synthesized on ribosomes of the rough endoplasmic reticulum (RER) and begins its journey through the secretory pathway
• PTH is transcribed as a prepro-PTH
• The “pre” fragment targets PTH for transport into the lumen of the RER. This signal sequence appears to be cleaved as PTH enters the RER.
• During transit through the secretory pathway, the pro-PTH is further processed to the mature, active, PTH. This cleavage appears quite efficient, and no pro-PTH appears in the storage granules. Conversely, the breakdown of 1-84 PTH or “intact” PTH into its N- and C-terminal fragments already starts in the secretory granules.

Question 20

Identify the main forms of vitamin D.

Answer 20

Vitamin D exists in the body in two forms, vitamin D 3 and vitamin D2

Question 21

How are vitamins D2 and 3 obtained ?

Answer 21

Vitamin D 3 can be synthesized from the 7-dehydrocholesterol that is present in the skin, provided sufficient ultraviolet light is absorbed. Vitamin D 3 is also available from several natural sources, including cod and halibut liver, eggs, and fortified milk.

Vitamin D2 is obtained only from the diet, largely from vegetables. Vitamin D3 and vitamin D2.

Question 22

How do vitamins D2 and 3 differ structurally ?

Answer 22

Differ only in the side chains of ring D. The side chain in vitamin D 3 (cholecalciferol) is characteristic of cholesterol, whereas that of vitamin D 2 (ergocalciferol) is characteristic of plant sterols.

Question 23

Describe synthesis and processing of Calcitonin.

Answer 23

A common primary RNA transcript gives rise to both calcitonin and CGRP.

1) In the thyroid gland, C cells produce a mature mRNA that they translate to procalcitonin.
They then process this precursor to produce an N-terminal peptide, calcitonin (a 32–amino-acid peptide), and calcitonin C-terminal peptide (CCP).

2) In the brain, neurons produce a different mature mRNA and a different “pro” hormone. They process the peptide to produce an N-terminal peptide, CGRP, and a C-terminal peptide.

Question 24

Identify between the main features of cortical.

Answer 24

• The fundamental unit of cortical bone is the osteon, a tube-like structure that consists of a haversian canal surrounded by ring-like lamellae.
• The superficial lining cells surround the osteoblasts, which secrete osteoid, a matrix of proteins that are the organic part of bone.
• The lining cells are formed from osteoblasts that become quiescent. Osteocytes are osteoblasts that have become surrounded by matrix.
• Canaliculi allow the cellular processes of osteocytes to communicate, via gap junctions, with each other and with osteoblasts on the surface.

Question 25

What proportion of the total bone mass does cortical bone represent ? Trabecular ?

Answer 25

Cortical: 80%
Trabecular: 20%

Question 26

Identify the main features of trabecular bone.

Answer 26

Trabecular bone has both osteoblasts and osteoclasts on its surface; this is where most bone remodeling takes place.

Question 27

Describe synthesis of vitamin D3.

Answer 27

VITAMIN D3

-Ultraviolet light triggers the cleavage in the skin of the B ring of 7-dehydroxycholesterol, creating an unstable intermediate that—over a period of about 2 days—rearranges to form cholecalciferol (vitamin D3).
(Vitamin D 3 can also come from animal sources in the diet)

-However, vitamin D 3 is not active as such. In the liver, a P-450 enzyme hydroxylates vitamin D3 at the 25 position, creating 25-hydroxyvitamin D3. Then in the proximal-tubule cells of the kidney, another P-450 enzyme hydroxylates 25-hydroxyvitamin D 3 at position 1, forming 1,25-dihydroxyvitamin D3 , the active form of vitamin D 3 .

Question 28

Describe synthesis of vitamin D2.

Answer 28

(Like vitamin D3), vitamin D 2 undergoes 25-hydroxylation in the liver and 1-hydroxylation in the kidney. Also like vitamin D3 , the 1,25-dihydroxylated metabolite of vitamin D 2 is about 1000-fold more active than the 25-monohydroxylated form.

Question 29

Identify another name of vitamin D2, and D3.

Answer 29

D2: Ergocalciferol
D3: Cholecalciferol

Question 30

Describe intestinal absorption of Ca+2.

Answer 30

-Calcium moves from the intestinal lumen to the blood by both paracellular and transcellular routes

1) PARACELLULAR ROUTE
- Occurs throughout the small intestine
- Ca 2+ moves passively from the lumen to the blood; 1,25-dihydroxyvitamin D does not regulate this pathway.

TRANSCELLULAR ROUTE

• Occurs only in the duodenum, involves three steps.
  1) First, Ca 2+ enters the cell across the apical membrane via TRPV6 Ca 2+ channels
  2) The entering Ca 2+ binds to several high-affinity binding proteins, particularly calbindin. These proteins, together with the exchangeable Ca 2+ pools in the RER and mitochondria, effectively buffer the cytosolic Ca 2+ and maintain a favorable gradient for Ca 2+ entry across the apical membrane of the enterocyte. Thus, the intestinal cell solves the problem of absorbing relatively large amounts of Ca 2+ while keeping its free cytosolic [Ca2+] low.
  3) Third, the enterocyte extrudes Ca 2+ across the basolateral membrane by means of both a Ca pump and an Na-Ca exchanger.

Question 31

Identify factors which regulate intestinal absorption of Calcium.

Answer 31

VITAMIN D
-Promotes intestinal Ca 2+ absorption primarily by genomic effects that involve induction of the synthesis of epithelial Ca 2+ channels and pumps and Ca 2+ -binding proteins, as well as other proteins (e.g., alkaline phosphatase)

PTH
-Stimulate intestinal Ca 2+ absorp­tion, by mechanism which is thought to be entirely indirect and mediated by PTH’s action to increase the renal formation of 1,25-dihydroxyvitamin D, which then enhances intestinal Ca 2+ absorption.

Question 32

Describe intestinal absorption of Phosphate.

Answer 32

Inorganic phosphate (Pi) enters the enterocyte across the apical membrane via an Na/P i cotransporter (NaPi). This step is rate limiting for transepithelial transport and subsequent delivery of phosphate to the circulation. Once inside the cell, the P i is extruded across the basolateral membrane.

Thus, the net effect is P i absorption.

Question 33

Identify any factors which regulate intestinal absorption of Phosphate.

Answer 33

VITAMIN D

• Stimulates phosphate absorption by the small intestine.
• 1,25-Dihydroxyvitamin D stimulates the synthesis of Na/Pi cotransporter and thus promotes phosphate entry into the mucosal cell.

Question 34

Describe the process of bone formation, including the roles of Calcium, Phosphate, PTH and vitamin D.

Answer 34

Osteoblasts have receptors for PTH, and also respond to vitamin D analogues.

• “Osteoblasts promote mineralization by exporting Ca 2+ and from intracellular vesicles that have accumulated these minerals”
• Low concentrations of PTH, and presence of vitamin D, causes osteoblasts to secrete osteoid. In the osteoid, there are various proteins that help nucleate these Calcium Phosphate salts to form hydroxyapatite.

Question 35

Describe the process resorption, including the roles of Calcium, Phosphate, PTH and vitamin D.

Answer 35

• If the levels of PTH increase substantially (i.e. higher levels than those necessary for bone formation by osteoblasts), obsteoblasts start releasing cytokines including Interleukin 6 and RANK ligand.
• RANK is a receptor which activates nuclear factor NF kappa B, and stimulates M-CSF.
  -M-CSF (release from osteoblast when high concentrations of PTH) binds to circulating stem cells, and causes them to differentiate into osteoclast precursors. These osteoblast precursors start synthesizing receptors for IL-6, and the RANK itself.
  -These cytokines stimulate development of the osteoclast precursors into mononuclear osteoclasts (AKA pre-osteoclasts)
  -Number of these pre-osteoclasts fuse together to form mature, multi nucleated, large cell osteoclast that attaches to the bone. It binds to the bone through sealing zone, where integrins in the osteoclast membrane bind to proteins (Vitronectin) in the bone
  -Although PTH stimulates osteoblasts to lay down new bone, at higher concentrations, they start stimulating
  activation of osteoclasts, which cause bone resorption
  -Osteoclast reabsorbs bone by:

1) Secreting H + and acid proteases into the lacuna. The acid secretion is mediated by a V-type H pump and ClC7 Cl − channel at the ruffled border membrane facing the lacuna. Carbonic anhydrase (CA) in the cytosol supplies the H + to the H pump and also produces HCO3- as a byproduct. Cl-HCO 3 exchangers—located on the membrane opposite the ruffled border—remove this.
2) Simultaneously, exocytosis of number of proteolytic enzymes from cytosol through lysosomes fusing with the apical surface. These lysosomal enzymes have proteases, including TRAP that breaks up the hydroxyapatite. These are all stimulated by the cytokines binding to the receptors.

-Uptakes systems across the apical surface then take calcium and phosphate back into the cell and they are released by basolateral surface into circulation

Question 36

Describe feedback loops controlling plasma Calcium.

Answer 36

•Intestinal Calcium asorption is stimulated by active form of vitamin D (1, 25 dihydroxy vitamin D), so plasma Calcium will rise, and PTH release will be suppressed

•PTH itself will be released if drop in plasma Calcium. Its actions are:
1) Stimulates Calcium reabsorption in renal tubules: much like vitamin D does this in intestine by promoting Calcium reabsorption from tubular fluid back into the blood.
2) Stimulates Calcium reabsorption from bone: stimulates release of cytokines which stimulate the production of more osteoclasts which start producing net breakdown of bone (hence, net increase in blood calcium)
3) Inhibits Phosphate reabsorption in renal tubules: because if want to raise calcium don’t want to raise phosphate too much as well otherwise will get calcium phosphate calcium precipitation
4) Stimulates synthesis of active form of vitamin D:
stimulates enzyme 1 hydroxylase which converts 25-hydroxyvitamin into 11, 25 dihydroxy vitamin D

•When the active form of vitamin D is synthesised, it will have its own effects:

1) Stimulate calcium reabsorption in intestine
2) Stimulates calcium reabsorption in the kidney
3) (at higher levels) stimulates osteoclast activity (i.e. bone resorption > bone formation)
4) Stimulate phosphate reabsorption
5) Inhibit their own secretion so (inhibit 1 hydroxylase enzyme)

• A drop in PTH levels will result in:
1) Inhibition of osteoclasts, while the osteoblasts continue laying down Calcium, resulting in bone formation (and decreased plasma Calcium

Question 37

Identify the role of calcitonin therapeutically.

Answer 37

If hyperCa, can give patient some calcitonin and it will reduce their plasma calcium by inhibiting osteoclasts
(normal physiological role under question)