Calcium Homeostasis Flashcards
State the roles of calcium in the body
- Signalling
- Skeletal strength
- Blood clotting (Ca2+ is a cofactor in the coagulation cascade - Silverthorn)
- Apoptosis
- Membrane excitability (affected by Ca2+ plasma concentrations - Silverthorn)
Describe the effects of hypercalcaemia and hypocalcaemia on neuronal Na+ permeability
- Hypercalcaemia: Decreases Na+ permeability which will reduce excitability and depress neuromuscular activity
- Hypocalcaemia: Increases Na+ permeability which will increase excitability and promote neuromuscular activity
How is calcium distributed in the body?
- Bones: 99% (1kg stored in the calcified extracellular matrix of bone, mostly in the form of hydroxyapatite)
- Intracellular: 0.9%
- Extracellular fluid: 0.1%
The homeostasis of what element is important in determining calcium balance?
- Phosphate
(since calcium is stored in the form of hydroxyapatite - (Ca10(PO4)6(OH)2)
Why does calcium have a very high affinity for proteins?
- Small positive charge of the calcium is attracted to the large negative charge of the protein
In the body, calcium situated where is physiologically active?
- Calcium that is free in the plasma
In the plasma, what % of proteins is bound to plasma proteins?
40%
(the rest either bind to plasma anions or are free in solution)
How does the binding capacity of calcium for plasma proteins change with pH?
- Binding capacity is increased under alkalotic conditions
In terms of calcium balance, explain what hyperventilation leads to?
- Hypocalcaemic tetany
- This is becuase if you hyperventilate, plasma pH ↑ (alkalosis) (this is becuase as you breathe out more rapidly, you effectively drive out the CO2 which is acidic)
- As plasma pH ↑, then more plasma proteins bind with Ca2+
- Therefore ↓ Ca2+ plasma concentration
- Hypocalcaemia results leading to hypocalcaemic tetany (not in lecture but REMEMBER: ↓ Ca2 increases neuronal Na+ permeability. This leads to depolarisations and action potentials)
(opposite occurs with acidosis)
Look at the following diagram:
Does the Ca2+ which is stored in bone always stay in bone?
- No
- Can be added to or released as requried
(Ca2+ balance is a very important function of bone)
Define/give the function of the following type of cells in bone:
(a) Osteoblasts
(b) Osteoclasts
(c) Osteocytes
(a) Osteoblasts = Bone building cells that control the deposition of Ca2+ in bone
(b) Osteoclasts = Mobilises/absorbs bone tissue by secreting H+ (pH ≈ 4) ions to dissolve the calcium salts
(c) Osteocytes = Differentiated osteoblasts in established bone
State the 2 key hormones which act to increase the concentration Ca2+ in the plasma
- Parathyroid hormone (PTH) (polypeptide hormone produced by the parathyroid hormones)
- Calcitriol (steroid hormone produced from vitamin D by the liver ans kidneys - active form of Vitamin D3)
How many parathyroid glands are there?
4
State the function of parathyroid hormone (PTH) AND explain how it does this
Function: Acts to increase free [Ca2+]plasma due to ↓ in free [Ca2+]plasma
It does this by:
- Stimulating osteoclasts to ↑ resorption (release) of Ca2+ and phosphate in bone
- Inhibiting osteoblasts to ↓ Ca2+ deposition in bone
- Increasing reabsorption of Ca2+ from the kidney tubules, therefore decreasing its excretion in the urine
- Increasing renal excretion of phosphate. This elevates free [Ca2+] by preventing it from being deposited back into bone, a process that requires phosphate
- Stimulates the kidney to synthesise calcitriol from vitamin Dwhich promotes calcium absorption at the gut and kidney
State the function of calcitriol
- Complements the action of parathyroid hormone (PTH) by ↑ [Ca2+]plasma
How is calcitriol (active vitamin D3) formed?
- It is formed from cholesterol derivatives by the action of UV light on the skin
Look at the following diagram:
State the 3 actions of calcitriol
- Increases absorption of Ca2+ from the gut
- Facilitates renal absorption of Ca2+
- Mobilises calcium stores in bone by stimulating osteoclast activity
How does calcitriol increase the absorption of Ca2+ from the gut?
- Calcitriol controls an ACTIVE TRANSPORT SYSTEM which moves Ca2+ from the intestinal lumen to the blood
(REMEMBER: much of Ca2+ is excreted in the faeces)
Look at the following diagram:
How does the hormone prolactin relate to calcitriol?
- Prolactin increases the synthesis of calcitriol
What are the general levels of vitamin D3 in the following people:
(a) Healthy individual
(b) Vitamin D deficient
(c) Pregnancy, lactation, growth spurts
(a) Normal
(b) Low
(c) High (due to prolactin)
Define vitamin D deficency
Vitamin D levels fall to less than 20ng/ml
How is plasma [Ca2+] maintained in those people that are vitamin D deficient?
- Because vitamin D is low, parathyroid hormone (PTH) must work hard to maintain plasma [Ca2+]
- To do this, it needs to continually remove Ca2+ from bone resulting in bones which are soft and, if still growing, become bent
(Rickets in children, Osteomalacia in adults)
Which hormone acts to decrease [Ca2+]plasma?
- Calcitonin
Explain how calcitonin performs its function
The function of calcitonin is to decrease [Ca2+]plasma
It does this by:
- Binding to osteoclasts and inhibit bone resorption
- Increasing renal excretion so preventing a further increase in Ca2+
Other than PTH, calcitriol and calcitonin, what other hormones alter Ca2+ balance?
- Cortisol
- Oestrogen
- Prolactin
- Insulin
- Growth hormone
