19. Calcium Flashcards
Where is the majority of calcium stored in the body?
In bone - about 99% of body calcium
How is calcium carried in the blood?
Mostly, calcium is bound to albumin in the blood - 50%
40% travels in the blood as ionised calcium Ca2+
10% is carried in complexes with phosphate/citrate
How are calcium levels altered during a panic attack?
During a panic attack, will start hyperventilating
SO will be blowing of CO2 and so the H+ will enter the blood stream and leave from where it is bound to albumin
SO there will be more binding sites on the albumin now available for calcium to bind to and so ionised serum calcium levels will fall - hypocalcaemia
NB. hypocalcaemia results in a tingling sensation of e.g. the lips
What is the main role of calcium within a cell?
Mainly involved in cell signalling
What controls the levels of calcium in the blood?
PTH (parathyroid hormone)
How does the level of PTH alter according to the levels of calcium?
If the calcium level falls - the PTH level increases and brings the calcium levels back up to normal via standard negative feedback
How many parathyroid glands do we have?
Most people have four but can have more
Which cells make up parathyroid glands and which ones are responsible for the release of PTH?
Oxyphilic cells and chief cells
Chief cells secrete PTH
What are the three types of hormones in the body and which one is PTH?
Peptide hormone
Steroid hormone
Amine hormone
PTH is a peptide hormone
Very very briefly, how is PTH produced?
Same way as insulin
PreproPTH to proPTH to PTH
Specifically, how do low calcium levels result in increased PTH levels?
Low calcium levels will result in the release of stored PRH, increased production of PTH and a hyperplasia of the chief cells in the parathyroid gland meaning that there is an increased production of the PTH
How do the PTH levels change when the levels of calcium are raised?
When serum ionised calcium levels rise, the serum PTH falls BUT should note that the PTH production is never fully suppressed
Just is consistently produced at a low level
Describe the calcium sensing receptor and the role of this when calcium binds to it
This is a G-protein coupled receptor
SO if calcium is binding to this then that means that there are high levels of serum ionised calcium and so you want a reduced level of PTH:
Reduces the level of PTH secretion
Increases the breakdown of the stored PTH
Suppresses the transcription of the PTH gene
Where is the calcium sensing receptor located?
These are present everywhere but are most important at the Parathyroid gland and the kidney
What are the actions of PTH?
SO the main action of PTH is to increase calcium levels:
Bone - increase the calcium resorption from here
Intestine - Increase the absorption of calcium in the intestine
Kidney - decrease the level of calcium excretion
How does the level of calcium excretion/absorption link to the level of phosphate excretion/absorption?
In the bone, calcium and phosphate are bound together SO an increased resorption of calcium will increase the level of phosphate that is also reabsorbed - too high a phosphate level
SO to compensate for this, as the PTH reduces the level of calcium excretion from the kidneys, it will also act to increase the level of phosphate excretion to balance the phosphate levels
Where in the kidney does PTH have a role on calcium excretion/reabsorption?
Has no effect in the proximal convoluted tubule or the loop of Henle
has an effect in the distal tubule
How does PTH have an effect on the distal convoluted tubule?
Upregulates calcium channels in the the tubule and so there is an increased reabsorption of calcium
What other actions can PTH have in the kidney?
Downregulation of sodium absorption
Activation of vitamin D
Gluconeogenesis in the proximal tubule
What type of hormone is vitamin D?
Steroid hormone
What is the role of PTH on vitamin D?
PTH activates vitamin D
Describe the vitamin D receptor
This is a nuclear receptor and so has an effect on gene transcription - genomic effects - which take a while to occur (most of the effects of vitamin D)
BUT can also find vitamin D receptors on the membrane and here, it has non-genomic and rapid effects
What is the normal effect of vitamin D on it’s receptor?
The presence of vitamin D will inhibit the production of the vitamin D receptor
What is the role of vitamin D on calcium absorption and why?
Activated vitamin D acts to increase the levels of calcium absorption
This is because increased levels of PTH (stimulated by decreased levels of Ca) lead to an increase in activated vitamin D
SO what is the effect of increased PTH on activated vitamin D?
Increased PTH levels cause there to be an increased activated vitamin D level and hence increases calcium reabsorption in the kidney and increased calcium release in e.g. the bone
What is the role of calcitonin?
Calcitonin plays a small role in the calcium regulation but is not massively important
It is produced in C cells in the thyroid and there will be elevated levels in medullary thyroid cancer
What is the difference between vitamin D2 and D3?
These are the same vitamin and are interchangable with one another
BUT D2 is from vegetables and D3 is from meat
What is hyperparathyroidism?
This is where there is an abnormally high concentration of PTH in the blood - leads to a loss of calcium (hypercalcemia) and hence a weakening of bones
What are the different types of hyperparathyroidism?
Primary
Secondary
Tertiary
What are the different clinical effects of hypercalcemia?
Polyuria and polydipsia
Kidney stones
Osetoporosis
Mood disorder
What are the effects of hypoparathyroidism?
SO leads to hypocalcemia - convulsions, arrhythmia, tetany, paraesthesia
