Regulation of calcium and phosphate Flashcards
What is the most abundant metal in the body?
Calcium
What meets all requirements for calcium?
Diet
What percentage of calcium is found in skeleton and teeth?
99%
What form is calcium in in bone?
Calcium hydroxyapatite crystals
Is extracellular calcium tightly regulated?
Yes
What is the biologically active component of calcium?
‘Unbound’ ionised calcium
What percentage of calcium is intracellular?
1%
What percentage of calcium is extracellular?
0.1%
What hormones increase serum calcium?
Parathyroid hormone
Vitamin D
What hormones decrease serum calcium?
Calcitonin- can reduce calcium if injected, but generally in body has not overall effect
What are the names of the two types of vitamin D?
Ergocalciferol-
Cholecalciferol-
Where is Ergocalciferol derived from?
UV irradiation of plants
Where is Cholecalciferol derived from?
UV irradiation of skin (animals and humans) and certain foods (oily fish, egg yolks)
What is calcitriol?
The active form of vitamin D
What is the major role of calcitriol?
↑ Ca2+ and
PO43- reabsorption from the GUT
What cells secrete PTH?
Chief cells in parathyroid gland
What is PTH secreted as?
large precursor (pre-pro-PTH) & cleaved to PTH
What does the G-protein coupled calcium sensing receptor on chief cells do?
detects change in circulating calcium concentration
What is the relationship between PTH secretion and serum calcium?
Inversely proportional
Why does low extracellular calcium cause release of PTH?
Less calcium binds to receptors on parathyroid cells
Does PTH increase or decrease bone re-absorption?
Increase
How does PTH lead to bone resoprtion?
Binds to PTH receptor on osteoblasts, activating osteoclast activating factors (OAFs)
How does cacitriol affect bone when serum calcium is low?
calcitriol works with PTH to increase calcium reabsorption from bone
osteoclasts > osteoblasts
How does cacitriol affect bone when serum calcium is normal?
calcitriol promotes bone formation
osteoblasts > osteoclasts
What is calcitonin?
A hormone
Where is calcitonin secreted from?
parafollicular (C) cells of the thyroid gland
What does calcitonin do?
Reduces serum calcium
Does removal of the thyroid gland affect serum calcium?
No
What are the effects of calcitonin?
- Decreased osteoclast activity
- Increased calcium excretion in kidneys
- ↓ plasma Ca2+
Describe the affects of hypercalcaemia on membrane excitability
Ca2+ blocks Na+ influx, so LESS membrane excitability
Describe the affects of hypocalcaemia on membrane excitability
enables GREATER Na+ influx, so MORE membrane excitability
What are the symptoms of hypocalcaemia?
1.Paraesthesia (hands, mouth, feet , lips)
2. Convulsions
3. Arrhythmias
4. Tetany
Mnemonic - [CATs go numb]
What is Chvostek’s sign?
Tap facial nerve just below zygomatic arch
Positive response = twitching of facial muscles
Indicates neuromuscular irritability due to hypocalcaemia
What is Trousseau’s sign?
Inflation of BP cuff for several minutes induces carpopedal spasm = neuromuscular irritability due to hypocalcaemia
What are the two causes of hypocalcaemia?
Low PTH levels = hypoparathyroidism
Vitamin D deficiency
What are the causes of hypoparathyroidism?
Surgical – neck surgery
Auto-immune
Magnesium deficiency
Congenital (agenesis, rare)
What are the causes of vitamin D deficiency?
- Inadequate sun exposure
- Malabsorption or dietary insufficiency leads to lack of ergocalciferol
- Liver disease
- Renal disease
- Vit D receptor defects
What are the consequences of vitamin D deficiency?
Lack of bone mineralisation = ‘soft’ bones
In children – rickets (bowing of bones)
In adults – osteomalacia (fractures, proximal myopathy)
What are the symptoms of hypercalcaemia?
‘Stones, abdominal moans and psychic groans’
Reduced neuronal excitability – atonal muscles
Stones – renal effects
Nephrocalcinosis – kidney stones, renal colic
Abdominal moans - GI effects
Anorexia, nausea, dyspepsia, constipation, pancreatitis
Psychic groans - CNS effects
Fatigue, depression, impaired concentration, altered mentation, coma (usually >3mmol/L)
What are the 3 causes of hypercalcaemia?
Primary hyperparathyroidism
Malignancy
Vitamin D excess (rare)
What are the causes of Primary hyperparathyroidism
Too much PTH
Usually due to a parathyroid gland adenoma
No negative feedback - high PTH, but high calcium
How does malignancy cause hypercalcaemia?
Bony metastases produce local factors to activate osteoclasts, increasing calcium reabsorption from bone
