Calcium Homeostasis Flashcards
Calcitonin is produced in ‘C-cells’ and is involved in calcium homeostasis. In which of the following are ‘C-cells’ located?
Thyroid gland
There are three major hormones involved in calcium regulation. Which of the following hormones is produced in response to an elevated serum calcium concentration?
Calcitonin
What is the main reservoir of calcium in the body
Skeleton
Amount of calcium in skeleton
1200g
Amount of calcium in extracellular space
1g
Function of calcium in extracellular space
Muscle contractility
Nerve function
Normal blood clotting
How is calcium in circulation
Ionised
Protein-bound
Complexed
Total serum calcium
2.4 mmol/L
Ionised serum calcium
1.1 mmol/L
Ionised calcium
Metabolically active
Protein bound calcium
Not metabolically active
What is calcium complexed with
Citrate
Phosphate
Modulation of ionised calcium by pH
Alkalosis
Higher pH- albumin more negatively charged to albumin strongly binds to calcium
Total serum calcium at alkalosis
Total serum calcium unchanged
Ionised serum calcium at alkalosis
Decreased
What is low ionised calcium associated with
Contraction of the small muscles of the hands and feet
- tetany
There is depolarisation of the long nerves of the upper limb
Major sources of dietary calcium
Dairy products
-2/3
Minor sources of dietary calcium
Vegetables eg broccoli
Cereals eg white bread
Oily fish eg sardines
Recommended intake of calcium
700 mg/day
What percentage of dietary calcium do we absorb
30%
Where does active absorption of calcium occur
Duodenum
Jejunum
Where does passive absorption of calcium occur
Ileum
Colon
When there is low availability of calcium , higher fractional excretion by
More active transport
Mediated by calcitriol (active form of vitamin D)
What is the active form of vitamin D
Calcitriol
Release of calcium from bone
Can be released rapidly from exchangeable calcium on the bone surface
Can be released more slowly by osteoclasts during bone resorption
What does the amount of calcium filtered by the glomerulus depend on
Glomerular filtration rate
Ultrafiltrable calcium (ionised and complexed)
What percentage of filtered calcium in the kidney is reabsorbed
98%
What is reabsorption of calcium in the kidney increased by
Parathyroid hormone
What is the reabsorption of calcium in the kidney decreased by
If filtered sodium is high
Fractional excretion of calcium from the kidney
2%
Relationship between serum calcium and parathyroid hormone
Small changes in serum ionised calcium causes a big change in parathyroid hormone
Parathyroid hormone actions
Increases Ca2+ reabsorption
Increases 1 alpha-hydroxylation of 25-OH vit D
Increases bone remodelling (resorption>formation)
Increases Ca2+ absorption due to increased 1,25 (OH)2 vit D
Where is calcitonin produced
Thyroid C cells
What causes the secretion of calcitonin
Increased serum calcium
Effect of calcitonin
Lower bone resorption
Calcitriol structure
Hydroxylated at positions 1 and 25
Function of calcitriol
Stimulates intestinal calcium absorption
Passive absorption of calcium in kidney
65% in PCT
25% in loop of henle
Active absorption of calcium in kidney
8% in DCT
Calcium sensing receptor on parathyroid gland
Gq coupled receptor
Response to a low calcium diet
Lower serum calcium
Increased parathyroid hormone secretion
-exchangeable calcium released from surface of bone
-decreased excretion of calcium from kidney
Vitamin D overdose
Increased gut absorption of calcium
Higher serum calcium
Decreased PTH
-decrease renal absorption of calcium
-Decreased bone resorption
Substrate for vitamin D
Cholesterol
Primary production of vitamin D in skin
UV light on 7-dehydroxycholesterol
Where does primary production of vitamin D occur
Skin
Where is vitamin D activated
Kidney
Where is vitamin d first hydroxylated
Liver
- forms 25 -OHase
Main function of calcitriol
Active gut absorption of dietary calcium
Vitamin D synthesis
Can be ingested or synthesised from a cholesterol precursor: 7-dehydrocholesterol is converted into vitamin-D3 under the influence of UV radiation in the skin. In the liver, vitamin D3 is converted into 25-hydroxyvitamin D by 25-hydroxylase. In the kidney, 1-alpha-hydroxylase converts it into 1,25-dihydroxyvitamin D (calcitriol)
Calcitriol mechanism
stimulates intestinal epithelial cells to increase synthesis of calbindin-D proteins which facilitate the transport of calcium from the intestinal brush border to the basolateral membrane for absorption into the blood.
