Calcium and Phosphate Metabolism: Control

Question 1

Describe the process of calcium and phosphate homeostasis in relation to its endocrinological control and the roles of bone, parathyroid glands and kidney
Recognise the signs and symptoms of hyper- and hypo-calcaemia
List the common causes of of hyper- and hypo-calcaemia and explain in overview how they cause abnormal calcium levels
Outline the investigative pathways for common causes of hypercalcaemia (hyperparathyroidism, hypercalcaemia of malignancy) and interpret basic blood tests in relation to calcium levels
Outline the treatment options for hypercalcemia including immediate and longer-term management
Outline the treatment of hypocalcaemia
Understand what is meant by primary, secondary and tertiary hyperparathyroidism

Question 2

Draw a diagram to show the distribution of calcium in the body

Answer 2

The bound part is not biologically active.
Ionised Ca2+ is free and can cross membranes. This is what is exchanged with cells to give electricity. This is also the part that the body can sense, and controls the level of.

Question 3

Draw a diagram to show the distribution of phosphate in the body

Answer 3

This time, phosphate is mainly intracellular, small part is extracellular
Again 50% split
Free amount is regulated by the kidney and PTH and Fibroblast Growth Factor 23

Question 4

What is the difference between the function of calcium and the function of phosphate?

Answer 4

Ca involved in bone growth, muscle contraction, blood clotting, etc
Is a co-enzyme. Also stabilises membrane potentials etc

Phosphate is an element in compunds like ATP and cAMP, as well as bone, cell membranes and DNA. Mostly phosphorylates/activates enzymes

Question 5

Describe the daily turnover of calcium and phosphate

Answer 5

Take in calcium in the gut.
Absorb it from the gut
Goes into bloodstream
Buffered into the bone
Exchange between the bone and bloodstream

Some Ca/phosphate is sent out in the faeces
But most of the regulation occurs through Ca and phosphate resorption OR excretion in the kidney.

Question 6

What are the key regulators for Ca and phosphate (+ what makes them)?

Answer 6

Key regulators for calcium are PTH and vitamin D

Key regulators for phosphate are PTH and FGF23

Chief cells from the parathyroid produce PTH

Osteocytes produce fibroblast growth factor 23
(FGF23)

Osteoblasts produce uncarboxylated osteocalcin
(uOCN)

Question 7

Other hormones govern bone turnover and bone density. Describe these

Answer 7

ANABOLIC (bone growth):
Osteoblasts become active = lay down bone
Oestrogen, androgen and growth hormone makes this happen.
Testosterone is more anabolic than oestrogen = men have thicker bone than women
Glucocorticoids STOP osteocytes from working. If on steroids, bones are thinner

CATABOLIC (bone decrease):
Osteoclasts break bone.
Vit A and Thyroxine increase the action of osteoclasts.
Can get bone thinning in thyrotoxicosis
Oestrogen and Calcitonin inhibits osteoclasts.

Question 8

What does this image show? Use it to explain what other factor contributes to bone growth

Answer 8

Cross section of the femur; bone NOT laid down in even pattern

Mechanical stress leads to lines of extra bone laid down.

Structure of the bone is hydroxyapatite crystal which produces piezoelectricy.

Strain on bones = piezoelectricity = stimulates OBs

Osteoblasts recognises more strain in this area = puts down more bone in that area.

Question 9

Bone must control its calcium and phosphate amounts by producing different factors. Describe the role of FGF23

Answer 9

When growing bone, you need to control the calcium and phosphate that go into the bone = an energy producing event. So you must generate the energy that’s needed to allow the bone to grow.

Osteocytes produce fibroblast growth factor 23 (FGF23)

OBs produce uncarboxylated osteocalcin (uOCN)

FGF23 acts on kidney to decrease active vit D synthesis and to increase inorganic phosphate (Pi) excretion

Question 10

Bone must control its calcium and phosphate amounts by producing different factors. Describe the role of uOCN

Answer 10

OBs produce uncarboxylated osteocalcin (uOCN)
uOCN acts on pancreatic β-cells to increase insulin
production + secretion on adipocytes to increase adiponectin, and on muscle to increase insulin sensitivity and glucose uptake. (insulin allows glucose to go into the bone cells so they will grow).
uOCN also increases testosterones
Basically it traps energy of the body into the bones to allow them to grow.

Question 11

Describe parathryoid development ??? Go over this w someone bc what the heck

Answer 11

In embryology, the thyroid gland starts at the root of the tongue and descends to the final position in the chest
As it descends, it brings branchial arches with it.
Parathyroid glands come from the 3rd and 4th branchial arch.
Thymus comes from the 3rd branchial arch. In development, the thymus descends from the neck and goes down to the chest. Drags the parathyroid glands with it to come down.
The inferior parathyroid glands are derived from the third branchial arch and the superior glands are derived from the fourth.

Question 12

Label this

Answer 12

Question 13

Synthesis of parathyroid hormone???

Answer 13

Sandwich assay (2 antibodies) detect whole hormone not fragments. If you used one antibody, it would measure four different cuts of the parathyroid hormone (PTH).
If use two, then 1 Ab would sandwich one end of the active version of the PTH, and the other would sandwich the other end of the same PTH. Meaning you can detect just the right one.
Ratio of fragments to full length PTH increases when plasma Ca2+ is high
T1/2 2-4 minutes bc enzymes need to break the peptide, longer for fragments.
Only 20% of circulating PTH is the full length PTH

Question 14

Why do we need to correct the calcium measured in the plasma?

Answer 14

Because only the free and ionised calcium level is biologically active - we may have to correct the measured calcium in the plasma.
Acidosis affects the amount of free, ionised calcium that we measure.
When you take blood, if you use a tourniquet= lack of oxygen–> increased acid levels in the arm. H+ displace calcium from plasma protein binding sites. So you have a falsely raised ionised calcium.

Question 15

To correct calcium, we use which formula?

Answer 15

Question 16

PTH is controlled by chief cell through Calcium Sensing Receptor. Describe the calcium sensing receptor

Answer 16

Circulating calcium levels sensed by calcium sensing receptor (CaSR)

High Ca levels:
Activates Gαi = inhibits AC = cAMP and PKA reduced = PTH secretion reduced = Ca in blood reduced

Gαq increases IP3 pathway = intracellular Ca2+ conc rises, PKA falls, PTH secretion is inhibited.

Low Ca levels:
Decrease IP3, increase PKA and PTH secretion

Question 17

What are the other regulators of PTH?

Answer 17

Another regulator of PTH is Vit D. When Vit D levels high = INHIBITS PTH TRANSCRIPTION (negative feedback)
3rd way of PTH regulation is by having protective factors that protect the break down of the mRNA.
When Ca low = protective factors higher = mRNA level is there for longer = more PTH

Question 18

What are the actions of parathyroid hormone?

Answer 18

Stimulate osteoblasts to produce M-CSF and RANK ligand–>
increased bone resorption + Ca release from bone

Increase Ca2+ reabsorption in the DCT
Increase phosphate excretion
Increases 1-α hydroxylase in the proximal tubule

Note these 3 actions increase Ca and decrease phosphate
Also causes activation of Vit D
Vit D is activated by 1-alpha hydroxylase in the PCT. That activates Vit D to cause resorption of calcium from the gut

Question 19

Actions of parathyroid hormone
on bone?

Answer 19

Stimulates osteoblasts
Production of M-CSF and RANKL
Osteoclast differentiation
Bone resorption, increased calcium and phosphate
Release of growth factors to stimulate OB maturation and new bone formation

Question 20

Actions of parathyroid hormone
in the kidney?

Answer 20

1. PTH stimulates cAMP pathway.
2. This inserts epithelial Ca2+ channels in the luminal DCT membrane.
3. Entry driven by the steep electrochem gradient between the filtrate and the cytoplasm
4. Calcium is bound and transported to the basolateral surface by calbindin
5. PTH also stimulates the Na/Ca exchanger and the
   CaATPase
   calcium goes back into the body

Question 21

PTH is part of a peptide family, there are 3 peptides in this family. One of them is PTHRP. Describe this

Answer 21

In fetal life, we do not produce PTH, but produce PTHrp. PTHrp production stops after birth. Functions:
Ca transport in the kidney, placenta and mammary gland to give Ca from mother to baby
Lactation: takes Ca from bone so it can go into breast milk
PTHrP is important in development.
– Skeletal and dental
– Haematopoietic
– Mammodevelopment
– Embryonic gene deletion is lethal in mammals.

Question 22

What is Tip 39?

Answer 22

Tuberoinfindibular peptide binds to the PTH2 receptors produced in the thalamus
Involved in
– pain perception
– fear response
– anti-anxiolytic antidepressive
– Promotes maternal behaviours
– thermoregulation
Causes release of: CRH, Somatostatin, AVP, may mediate stress responses

Question 23

What is the link between PTHRP and cancer or hypercalcaemia?

Answer 23

PTH RP is important in cancer. Embryonic form of PTH
PTHrp production stops after birth
In tumours, cells are often immature we can sometimes see levels of PTHrp.
PTHrp increases bone resorption and hypercalcemia.

Question 24

Vitamin D is made in the skin. How does this happen?
How does kidney disease affect vitamin D production?

Answer 24

When UV falls on the skin hay conversion of 7-dehydrocholesterol–> cholecalciferol. Otra vit D source is from the diet. Vit D2 and D3 then pass into the liver.
In the liver, vit D is hydroxylated to 25-hydroxy vit D.
In the kidney 1α hydroxylase (aka CYP 27B1) adds a hydroxyl group at position 1 (regulatory step). Turns it into the active form
This is why kidney disease pts get severe Vit D deficiency - not because of a lack of dietary uptake - but because they can’t activate it

Question 25

What type of receptor does vitamin D act on?

Answer 25

Acts at nuclear recptor because made of fat. Can cross the cell membrane more easily.
Vit D forms a heterodimer with a retinoic acid receptor - this is a type two receptor.
The retinoic acid receptor is expressed ubiquitously (in all cells).
Vit D binds to the RNA and then causes gene expression

Question 26

Actions of Vitamin D in Calcium
homeostasis?

Answer 26

Increases Ca2+ absorption in the gut
Synergises with PTH on bone
Inhibits PTH synthesis
Inhibits 1a-hydroxylase as a way to regulate itself

Question 27

Describe transport of calcium across the
epithelial cells of the intestine.

Answer 27

Paracellular transport –diffusion through tight junctions dependent on conc gradient; passive
Transcellular – at apical region Ca enters cell through a selective Ca transporter (TRPV) - which binds to calbindin - then that is actively transported out to the sodium calcium transporter into the body
This is the major action of Vit D - to increase expression of the transporters and to increase the action of the co-transporter into the cell.

Question 28

What are the actions of vitamin D on bone?

Answer 28

When hypocalcaemic, you must release Ca from your bones .
1,25 Vit D increases activity of PTH
Recruitment of osteoblasts, osteoblasts change into osteocytes
Bc more OBs change into OCs, hay mas OC action- so they release calcium

When vit D levels are normal, Vitamin D is anabolic for bone. It increases the density of your bones (puts calcium into your bones, your bones get stronger).

Increase of osteoblasts.

Question 29

Vitamin D actions beyond the
gut, bone, kidney axis?

Answer 29

Vit D receptors in> 30 diff cell types e.g keratinocytes, lymphocytes, macrophages, adipocytes, pancreatic β cells.

Several tissues also locally synthesise 1,25(OH)2D from circulating 25(OH)D bc tienen 1α-hydroxylase (CYP27B1). Tissues inc macrophages, monocytes, keratinocytes, breat tissue, parathyroid, colon, placenta etc.

Question 30

Actions of 1,25-
dihydroxyvitamin D?

Answer 30

Question 31

What is the FGF23 receptor?

Answer 31

Klotho, a single-pass transmembrane protein
Expressed pre- dominantly in DCT
Also expressed in epithelium of the choroid plexus in the brain and the parathyroid glands

It serves as an obligate co-receptor, enabling
FGF23 to interact with its receptor
Klotho is the modifier dictating which tissues will to FGF23

Question 32

What are the overall actions of FGF23 in calcium
and phosphate homeostasis?

Answer 32

High Vit D and high phosphate increase FGF 23 secretion.
Actions:
increases phosphate excretion in PCT
inhibits 1a-hydroxylase (CYP27B1) production
Inhibits PTH Secretion

Question 33

How does FGF23 control serum phosphate levels?

Answer 33

So phosphate from diet goes into bone
If phosphate levels high, responds by increasing FGF23 -> causes excretion of phosphate in the kidney
So FGF23 reduces serum phosphate concentrations

Question 34

What is hypercalaemia and calcitonin?

Answer 34

In hypercalcaemia, PTH production inhibited
Less calcium resorption from the kidney
Less calcium released from the bone
Inhibition of 1-alpha hydroxylase activation = Vit D levels go down = less action of Vit D which is mainly on the gut

Calcium also stimuates calcitonin. Calcitonin is the only hormone that decreases Ca. It is produced by the C cells in the thyroid. They can become tumorous=medullary carcinoma of the thyroid.
Use calcitonin therapeutically as a drug to reduce calcium

Question 35

What are the causes of hypercalcaemia?

Answer 35

In primary hyperthyroidism, parathyroid glands produce too much PTH. This excess PTH causes Ca release from the bones and into the bloodstream, resulting in hypercalcemia. Some cancers can also do this
If you have hypercalcaemia with Low PTH, then causes inc:
Cancers: can invade the bone, release cancer into the circulation. Myeloma can directly activate OBs and OCs to release calcium from the bone.
Hypervitamosis D: excessive vit D supplements or sun exposure can lead to increased calcium absorption
Granulomas which produce 1a hydroxylase
Increased bone turnover due to acromegaly (excess IGF 1 causes faster bone turnover and Ca release), or thyrotoxicosis (activates OCs)

Question 36

Investigations into someone with hypercalcaemia?

Answer 36

CXR to check for cancer or sarcoidosis
Mg is a cofactor which causes PTH release
Myeloma screen- bone scans are negative

Question 37

Hypocalcaemia? What is the body’s response to this?

Answer 37

When Ca is low the PTH increases → acts on bone causes bone resoprtion to put more Ca in the blood
Activation of Vit D to 1,25 dihydroxy Vit D = acts on the gut = resorp calcium.
1,25-di Vit D also activates FGF-23 = excrete phosphate.
PTH also acts on the kidney again to increase calcium in the plasma and increase phosphate excretion.
Net result = high calcium and reduced phosphate

Question 38

Causes of Hypocalcaemia?

Answer 38

LOW PTH:
Hypothyroidism, this has several causes:
Born without parathyroid glands, associated w immunodeficiencies
Surgery to the parathyroid glands, radiotherapy to the neck. Both damages the glands
Autoimmune -glands are attacked by autoantibodies

HIGH PTH:
Vit D deficiency
Renal and liver disease = cannot perform 1,25 dihydroxylase reaction
End stage renal failure = no 1,25 enzyme = renalosteodystrophy = often get severe osteoporosis in their bones cause not enough Vit D
Chelation - chemicals that are given into the bloodstream that bind the calcium to the circulation and drop it. That’s how blood transfusion works.

Question 39

How would you investigate hypocalcaemia?

Answer 39