L7 Plasma Calcium Flashcards
What physiological processes does calcium play a role in?
- Muscle contraction
- Blood coagulation
- Enzyme activation
- Neurotransmitter and hormone release
- Structural component of teeth and bones
What is the normal plasma calcium concentration range?
2.2 - 2.6mM
What are the 3 forms calcium can be present in?
- Protein bound calcium: cannot diffuse through membranes, not usable by tissues
- Ionised/free calcium: physiologically active
- Complexed/chelated calcium: calcium bound to phosphate, bicarbonate, sulphate, citrate or lactate
What are the conesequences of ionised calcium levels being too high or low?
Too high = kidney stones
Too low = neuronal hyper-excitability, constant muscle contraction
What are the 3 methods of calcium regulation?
- Absoprtion via intestines
- Excretion via kidneys/urine
- Temporary storage via bones
What are the calcium reuqirements for people of different ages?
18 = 1300mg/day
19-50 = 1000mg/day
Over 51 = 1200mg/day
Pregancy/breastfeeding = 1300mg/day
Describe acute calcium control.
- Maintenance of constant free calcium concentration in plasma
- Mostly through rapid exchange between bone and ECF
Describe chronic calcium control.
- Maintenance of total level of calcium in the body long term
- Adjusts absorption of calcium from the GI tract and amount excreted through urine
Plasma calcium concentration is controlled by which hormones?
In order of importance:
- Parathyroid hormone
- Calcitriol (1,25-dihydroxycholecalciferol)
- Calcitonin
Which cell type has an essential role in the exchange of calcium between ECF and bone?
Osteocytes
What does the parathyroid gland monitor?
The concentration of calcium in the blood that perfuses the gland.
What is the major cell type of the paraythroid gland and its funciton?
Chief cells:
- Calcium sensing action
- Produce and secrete parathyroid hormone in response to hypocalcaemia
Describe parathyroid hormone.
- Peptide hormone
- Short half life (5 mins) so rapidly inactivated
- Continuously secreted at very low rate
- Exerts effects on bone, kidneys and gut (indirectly)
- Secretion stimulated by hypocalcaemia, inhibited by hypercalcaemia
Explain the principles effects of parathyroid hormone.
- Increases expression of enzyme 1α-hydroxylase, which activates vitamin D
- Prevents excretion of calcium from urine
- Increases osteoclast activity, causing bone resorption to increase calcium
What are the 2 forms of vitamin D?
- Vitamin D2: ergocalciferol found in food supplements
- Vitamin D3: cholecalciferol found in diet and synthesised in skin in presence of sunlight
Vitamin D is relatively inactive so is converted to what?
1,25-dihydroxycholecalciferol (calcitriol)
How is vitamin D converted to calcitriol?
- 7-dehydrocholesterol is converted to cholecalciferol in the skin (vitamin D3)
- Cholecalciferol is converted to 25-hydroxyvitamin D3 in the liver
- Converted to 1,25-hydroxyvitamin D in the kidney through the action of the enzyme 1α-hydroxylase (parathyroid hormone increases expression of this enzyme)
- Converted to active form of vitamin D (calcitriol)
Explain the actions of calcitriol (1,25-dihydroxycholecalciferol)
- Acts on cell of the GI tract to increase production of calcium transport proteins
- Causes increased calcium uptake from the GI tract
- Also receptors for calcitriol in the bone, which causes increased rate of bone resorption
- Minor effect on the kidneys in decreasing urinary loss of calcium
What type of hormone is calcitriol?
Steroid hormone
Where is calcitonin secreted from?
From the parafollicular cells of the thyroid gland.
What is the role of calcitonin?
Decreases plasma calcium.
- Little importance in adults but more important in children
- Inhibits osteoclast activity
- Increases excretion of calcium and phosphate by the kidneys
What other hormones have minor roles in plasma calcium regulation?
- Oestrogen and androgens
- Growth hormone
- Adrenal glucocorticoids
Describe hypercalcaemia.
- Caused by primary hyperparathyrodisim
- Autonomous secretion of parathyroid hormone
- 85% of cases caused by a parathyroid adenoma (benign tumour)
- Presents as kidney stones, digestive issues, brain fog, depression, muscle weakness
Describe hypocalcaemia.
Commonly due to vitamin D deficiency
- Caused by poor diet, malabsoprtion, decreased sunlight, liver or kidney disease
- Causes increased excitability of nervous tissue, pins and needles, muscle cramps and convulsions
- Can lead to osteomalacia (adults) or rickets (children)
What is osteomalacia?
Osteomalacia = normal bone mass but reduction in bone mineral density
What is the presentation of vitamin D deficiency?
- Increased PTH
- Decreased or normal plasma calcium due to compensatory rise in PTH
What is the presentation of chronic renal disease?
- Increased PTH
- Decreased or normal plasma calcium
What are the effects of hyperparathyroidism in dentistry?
- Calcium loss of maxilla and mandible
- Increased trabecular bone
- Decreased cortical bone
- Increased incidence of tori
