Endocrine control of calcium metabolism Flashcards

1
Q

What is the most abundant metal in the body and the 5th most abundant element?

A

Calcium is the most abundant metal in the human body (fifth most abundant element).

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2
Q

List 7 roles of calcium in the body?

A
  • Neuromuscular excitability
  • Muscle contraction
  • Strength in bones
  • Intracellular second messenger
  • Intracellular co-enzyme
  • Hormone/neurotransmitter stimulus-secretion coupling
  • Blood coagulation (factor IV)
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3
Q

Where is most of the calcium in the body found and what form is it found in?

A
  • Most calcium is present in the body as calcium salts
  • It is mainly found in bone (99%, approx. 1kg) as complex hydrated calcium salt (hydroxyapatite crystals)
  • In blood, some is present as ionized calcium (Ca2+), some bound to protein and the tiny bit left as soluble salts
  • Only the free (unbound) Ca2+ is bioactive
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4
Q

What is the total amount of calcium in the blood?

A

approximately 2.5mM

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5
Q

What proportion of calcium in the blood in unbound (ionized), bound to plasma proteins and as diffusable salts?

A

Most of this calcium is present in the unbound ionised form (1.25 mM/L)

Calcium is present in the blood in DYNAMIC EQUILIBRIUM

The important component that needs to be controlled is the unbound ionised form (bioactive component)

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6
Q

What 4 parts of the body are involved in the calcium handling in the body and why?

A

GI tract - most calcium is absorbed in the GI tract

It is absorbed into the blood and some of it will get excreted in faece

Kidney- Once in the blood, the calcium can pass to the kidneys which regulates the content of the blood

A lot of the calcium passing into the kidneys will return to the blood - excretion and absorption of Ca2+into the blood

About 150 mg/24h is excreted from the kidneys per day - this maintains equilibrium

Bone- The hydroxyapatite crystals in the bone can be broken down to increase blood calcium levels - depository for calcium

Calcium is also needed for bone strength

Blood- Transport of calcium

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7
Q

Name two main hormones involved in increasing Ca2+ concentration?

Name the main hormone involved in decreasing calcium concentration?

A

Calcium Ion Regulation

TWO main hormones involved in RAISING blood calcium concentration:

PARATHYROID HORMONE (PTH)

1,25-dihydroxycholecalciferol (or CALCITRIOL)

NOTE: Calcitriol is a steroid and is also called 1,25-dihydroxy vitamin D3 Main

CALCITONIN

This doesn’t seem to have a major effect in the long run

No one really knows the importance of calcitonin

Calcitonin is NOT the main controlling influence on calcium ions

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8
Q

Draw a diagram showing wher PTH and Calcitonin is produced?

A

Parathyriod glands- Back of thyriod

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9
Q

How are calcium levels sensed in the body?

Where are these receptors primarily found?

A

The calcium-sensing receptor (CaSR) is a Class C G-protein coupled receptor which senses extracellular levels of calcium ion. It is primarily expressed in the parathyroid gland and the renal tubules of the kidney. In the parathyroid gland, the calcium-sensing receptor controls calcium homeostasis by regulating the release of parathyroid hormone (PTH).[5] In the kidney it has an inhibitory effect on the reabsorption of calcium, potassium, sodium, and water depending on which segment of the tubule is being activated.

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10
Q

Explain how parathormone (PTH), 1,25-dihydroxycholecaciferol (calcitriol) and calcitonin are synthesised.

A

Initially synthesised as protein pre-proPTH

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11
Q

What is PTH’s mechanism of action once it has binded onto a receptor?

A

Binds to transmembrane G-protein linked receptors

Binding of parathormone to the G-protein linked receptor leads to activation of adenylate cyclase (with phospholipase C acting as a second messenger)

PTH is a polypeptide with 84 amino acid

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12
Q

Draw a diagram showing the effects of PTH in the body?

Include kidneys, bone and small intestine

A

Remeber calcuim is in a dynamic equilibrium with calcium phosphate

PTH stimulates the kidneys to excrete more phosphates- in urine

People used to think that as PTH causes the loss of free phosphates from the kidneys, the equation is no longer in equilibrium so the phosphate salt will dissociate to make amends for the phosphate that has been excreted and so calcium concentration increases secondarily to the replenishment of phosphate levels

We have since found out that PTH actually has direct effects on the calcium in the kidneys

The OVERALL effect is an increase in calcium reabsorption

PTH has another important effect in the kidneys: stimulates the synthesis of 1a HYDROXYLASE

1a Hydroxylase is involved in the synthesis of calcitriol

Calcitriol has an important effect on the small intestines: controls the absorption of calcium and phosphate (increases absorption)

PTH also has an effect on bone:

Stimulates osteoclasts - causes resorption of the bone matrix and release of calcium from hydroxyapatite crystals into the gut

Inhibits osteoblasts

THIS ALL LEADS TO AN INCREASE IN BLOOD CALCIUM CONCENTRATION

In the intestine, via kidney, PTH enhances the absorption of calcium in the intestine by increasing the production of activated vitamin D. Vitamin D activation occurs in the kidney. PTH up-regulates 25-hydroxyvitamin D3 1-alpha-hydroxylase, the enzyme responsible for 1-alpha hydroxylation of 25-hydroxy vitamin D, converting vitamin D to its active form (1,25-dihydroxy vitamin D). This activated form of vitamin D increases the absorption of calcium (as Ca2+ ions) by the intestine via calbindin.

However, PTH enhances the uptake of phosphate from the intestine and bones into the blood. In the bone, slightly more calcium than phosphate is released from the breakdown of bone. In the intestines, absorption of both calcium and phosphate is mediated by an increase in activated vitamin D. The absorption of phosphate is not as dependent on vitamin D as is that of calcium. The end result of PTH release is a small net drop in the serum concentration of phosphate.

PTH increases the activity of 1-α-hydroxylase enzyme, which converts 25-hydroxycholecalciferol, the major circulating form of inactive vitamin D, into 1,25-dihydroxycholecalciferol, the active form of vitamin D, in the kidney.

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13
Q

Draw a diagram showing the effect of PTH on blood [Ca2+]?

Include in the diagram kidneys, bone and small intestine

A
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14
Q

What are the effects of PTH on bone?

A

Osteoclasts are activated by PTH but NOT directly

PTH works on osteoblasts and inhibits various activities

However, PTH stimulates osteoblasts to produce various OSTEOCLAST ACTIVATING FACTORS (OAFs)

OAFs move to the osteoclasts and stimulates the breakdown of bone matrix to RELEASE CALCIUM

One of the OAFs is called RANKL which links PTH via the osteoblasts to the main target which are the osteoclasts

REMEMBER: PTH binds directly to osteoblasts but has an INDIRECT effect on OSTEOCLASTS

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15
Q

What does PTH cause Osteoblasts to do?

A

However, PTH stimulates osteoblasts to produce various OSTEOCLAST ACTIVATING FACTORS (OAFs)

OAFs move to the osteoclasts and stimulates the breakdown of bone matrix to RELEASE CALCIUM

One of the OAFs is called RANKL which links PTH via the osteoblasts to the main target which are the osteoclasts

REMEMBER: PTH binds directly to osteoblasts but has an INDIRECT effect on OSTEOCLASTS

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16
Q

How do OSTEOCLAST ACTIVATING FACTORS (OAFs) work?

A

Osteoclasts are activated by PTH but NOT directly

PTH works on osteoblasts and inhibits various activities

However, PTH stimulates osteoblasts to produce various OSTEOCLAST ACTIVATING FACTORS (OAFs)

OAFs move to the osteoclasts and stimulates the breakdown of bone matrix to RELEASE CALCIUM

One of the OAFs is called RANKL which links PTH via the osteoblasts to the main target which are the osteoclasts

REMEMBER: PTH binds directly to osteoblasts but has an INDIRECT effect on OSTEOCLASTS

An osteoclast (from the Greek words for “bone” (ὀστέον), and “broken” (κλαστός)) is a type of bone cell that breaks down bone tissue.

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17
Q

Draw a diagram showing how PTH is regulated?

A

Overall effect of PTH: raise calcium ion concentration

PTH stimulates an enzyme that leads to the synthesis of calcitriol

Increased synthesis of calcitriol also leads to increased plasma calcium concentration

The cells in the parathyroid gland which produce PTH respond to changes in plasma calcium concentration - they have calcium ion receptors

These receptors are activated whenever there is a fall plasma calcium concentration

Calcitriol also has a negative feedback effect on PTH

There are Beta Receptors on the cells that produce PTH so they can be stimulated by catecholamines to secrete PTH

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18
Q

What effect does catecholamines have on PTHs secretion?

What type of receptors are found in the parathyroid glands?

A

Increase Secretion

There are Beta Receptors on the cells that produce PTH so they can be stimulated by catecholamines to secrete PTH

19
Q

Draw a diagram showing the synthesis of 1,25 (OH)2 VITAMIN D3 (DIHYDROXY- CHOLECALCIFEROL, or CALCITRIOL)

A

Calcitriol Synthesis

Calcitriol is the other hormone, other than PTH, which causes an increase in plasma calcium concentration

The precursor is CHOLECALCIFEROL

Cholecalciferol has TWO main sources:

Diet

Different types of vitamin D come from the diet (Vitamin D2 = plants and fungi/ Vitamin D3 = meat/milk)

Sunlight

UV B works on the skin

UV B converts 7-dehydrocholesterol to cholecalciferol

REMEMBER THESE TWO IMPORTANT SOURCES OF CHOLECALCIFEROL: DIET and SUNLIGHT

Cholecalciferol is Vitamin D3

Vitamin D3 is a steroid which circulates around the body and is taken up by the liver

The liver has an enzyme called 25-hydroxylase which converts cholecalciferol to 25-hydroxy-cholecalciferol which is then stored in the liver

25-hydroxy-cholecalciferol is stored in the liver but then circulates from the liver and reaches the kidneys where you find 1a hydroxylase (which is stimulated by PTH)

1a hydroxylase is responsible for the conversion of 25-hydroxy-cholecalciferol to 1,25-dihydroxy-cholecalciferol (calcitriol)

20
Q

What are the actions of Calcitriol?

A

Main action is on the SMALL INTESTINE: stimulates calcium and phosphate absorption (through separate pathways)

It has minor effect on bone - it stimulates osteoblast activity (secondary effect of raising the plasma calcium concentration - by raising the calcium level in the blood, the calcium is there to be stored (e.g. in the bone))

Kidneys - calcitriol increases calcium reabsorption and phosphate reabsorption

21
Q

State how PTH effects phosphate reabsorption in kidney cells?

State how Calcitriol effects phosphate reabsorption in kidney cells?

Draw a diagram

A

On the apical membrane there are transporters for the phosphate ions

PTH inhibits this transporter so phosphate will NOT be reabsorbed so it will be excreted in the urine

Calcitriol also has an effect on phosphate reabsorption

Through the FGF23 (fibroblast growth factor 23 (from osteocytes)) molecule, calcitriol can block the phosphate transporter

So PTH and Calcitriol (via FGF23) inhibit phosphate reabsorption in the kidneys

22
Q

Fibroblast growth factor 23 or FGF23 is a protein that in humans is encoded by the FGF23gene.[5] FGF23 is a member of the fibroblast growth factor (FGF) family which is responsible for phosphate and vitamin D metabolism

Fibroblast growth factor 23 (FGF-23) is a phosphaturic hormone produced in bone

A
23
Q

What type of receptor does calcitonin act on and what happens when it binds to that receptor?

A

Synthesised as pre-procalcitonin

Calcitonin is a 32 amino acid polypeptide

Calcitonin acts via transmembrane G-protein linked receptors

Activation of adenyl cyclase or phospholipase C as second messenger systems

24
Q

State the effects of calcitonin?

State how it effects bone and kidney

What substance causes calcitonin levels to increase?

A

Calcitonin works on bone and inhibits osteoclast activity - thus decreasing the release of calcium from bone into the blood

Calcitonin also affects the kidneys: increases the excretion of sodium ions which, in turn, an increase in the urinary excretion of calcium and phosphate ions

The effects of calcitonin are relatively small and short-lived

Physiological benefit:

Plasma calcium levels are raised in PREGNANCY when you need more calcium ions (e.g. in milk)

Calcitonin protects the bone from being broken down to provide calcium

Calcitonin is stimulated by an increase in plasma calcium concentration and its overall effect is to decrease plasma calcium ion concentration

25
Q

Draw a diagram showing the regulation and effects of calcitonin?

A

Gastrin is a peptide hormone that stimulates secretion of gastric acid (HCl) by the parietal cells of the stomach and aids in gastric motility.

26
Q

list 3 ENDOCRINE CAUSES OF HYPOCALCAEMIA?

A

Hypoparathyroidism (insufficient PTH)

Pseudohypoparathyroidism (associated with resistance to PTH)

Vitamin D Deficiency

27
Q

What are the two signs of hypocalcaemia?

State how you would cause the patients to show these signs?

A

Hypocalcaemia

Hypocalcaemia can be shown in TWO ways:

Trousseau’s Sign (main d’accoucheur)

Put slight pressure on the arm and the hand can go into contraction

Chvostek’s Sign

Tap the facial nerve at the angle of the jaw you get the muscles to contract

These are both forms of TETANY

Tetany is the bad news associated with hypocalcaemia

28
Q

Sate 3 CAUSES OF HYPOPARATHYROIDISM?

A

Idiopathic

Hypomagnesaemia

Suppression by raised plasma calcium concentration- suppress PTH levels

29
Q

What is PSEUDOHYPOPARATHYROIDISM

What is psuefohypoparathyroidism also known as?

A

•Also known as

ALLBRIGHT HEREDITARY OSTEODYSTROPHY

target organ resistance to PTH (multiple underlying causes). Believed due to defective Gs protein (needed to increase cAMP intracellularly in response to PTH receptor activation

30
Q

List 4 features of psuedohypoparathyroidism?

A

•Features include

– particular physical appearance (short stature, round face)

– low IQ

– subcutaneous calcification and various bone abnormalities (e.g. shortening of metacarpals)

– associated endocrine disorders (e.g. hypothyroidism, hypogonadism).

31
Q

Pseudohypoparathyroidism

A

There is a very abnormal 4th metacarpal

If you give someone PTH, you get an increase in urinary excretion of cAMP - this is what you’d expect in a normal person

In people with idiopathic hypoparathyroidism and surgical hypoparathyroidism you get normal results because their target cells are still sensitive to PTH

Pseudohypoparathyroidism you get no change because of target resistance to PTH

32
Q

What is Vitamen D deficiency known as in children and what is it known as in adults?

A
  • Rickets in children
  • Osteomalacia in adults
33
Q

What are the clinical features of Vitamen D deficiency?

A

•Clinical feature is the decreased calcification of bone matrix resulting in softening of bone

® bowing of bones in children

® fractures in adults

34
Q

How would you differentiate between

  • HYPOPARATHYROIDISM
  • PSEUDO- HYPOPARATHYROIDISM
  • VITAMIN D DEFICIENCY

Hint: PTH, Plasma Ca2+, Plasma P04

A

In the kidneys, PTH increases the excretion of phosphate thereby reducing the blood plasma concentration so in hypoparathyroidism you get an increase in plasma phosphate concentration because PTH isn’t stimulating the excretion of phosphate from the kidneys

Pseudohypoparathyroidism - the parathyroid glands are producing sufficient PTH so PTH levels are normal but the target organs are resistant to PTH and hence plasma calcium ion concentration is low

Vitamin D Deficiency - phosphate levels are low, plasma calcium concentration is normal and PTH is normal

35
Q

List 3 ENDOCRINE CAUSES OF HYPERCALCAEMIA

A

Primary Hyperparathyroidism

Tertiary Hyperparathyroidism

Vitamin D Toxicosis

36
Q

What is Primary Hyperparathyroidism?

State what conditio can cause this?

A

Primary Hyperparathyroidism - a tumour in the parathyroid causes a large increase in PTH secretion

Because it is a tumour it is unlikely to be regulated by the normal negative feedback - it will continue to produce large amounts of PTH leading to an increased plasma calcium ion concentration

37
Q

What is secondary hyperparathyroidism?

State what condition can cause this?

A

Secondary Hyperparathyroidism

Some people have low plasma calcium concentration for a variety of other reasons such as renal failure

Renail failure leads to loss of calcium in the urine which will stimulate the parathyroid to release PTH which will do its best to maintain the plasma calcium ion level

38
Q

What is tertiary hyperparathyroidism?

A

Tertiary Hyperparathyroidism

Initial chronic low plasma calcium ion concentration

The parathyroid gland is being massively stimulated for a long time

Eventually, the PTH becomes autonomous and it stops responding to the negative feedback

This is similar to primary hyperparathyroidism as it causes an increased plasma calcium ion level

PRIMARY and TERTIARY hyperparathyroidism are associated with HYPERCALCAEMIA

39
Q

Sate what causes primary, secondary and tertairy hyperparathyroidism?

A
40
Q

How does hyperparathyroidism effect your fingers?

A
41
Q

What effect does excess parathyroidehormone have on kidneys, GI tract and Bone?

A
42
Q
A
43
Q

These 4 conditions are all associated with high circulating parathyroidhormone levels.

Which are also associated with low circulating calcium levels?

Primary hyperparathyroidism

Seconday hyperparathyroidism

Tertiary hyperparathyroidism

Vitamen D deficiency

A

Secondary hyperparathyroidism and Vitamen D deficiency