endocrine control of calcium balance Flashcards
What are the roles of calcium in the body
Signalling - exocytosis of synaptic vesicles
Blood clotting - component of the clotting cascade
Apoptosis
Skeletal strength - the majority of calcium is in bone where it gives strength to the skeleton
Membrane excitability - calcium decreases sodium permability
What is the effect of hypocalcaemia on membrane excitability
Increases sodium permeability and leads to hyper excitation of neurons which can cause tetany
What is tetany
Involuntary contraction of muscles caused by low calcium levels
What is the effect of hypercalcaemia on membrane excitability
Decreases sodium permability which reduces excitability of neurons and depresses neuromuscular activities
What is the calcium in bone stored as and why is phosphateimportant
It is stored as hydroxyapatite (Ca10(PO4)6(OH)2) - contains phosphate in it
how much calcium is found in the plasma
2.2-2.6 mM
How much of the plasma calcium is bound to plasma proteins
40 % due to the high affinity due to charges
how much calcium is physiologically active (free and unbound)
1.2mM (50% of plasma calcium)
What 3 forms does plasma calcium exist as
40% protein bound
50% free
10% complexed
What is the effect of PH on the binding of proteins to calcium
Increased pH (alkaline) causes increased binding - proteins are deprotonated and the negative charge in the proteins allows calcium to bind
What are osteoblasts and what do they do
They are bone building cells - highly active cells which lay down a collagen extracellular matrix which then calcifies
What do differentiated osteoblasts form
Osteocytes - regulate the activity of osteoblasts and osteoclasts
What are osteoclasts and what do they do
Bone breaking cells - secrete protons to dissolve the calcium salts and provide proteolytic enzymes to digest the extracellular matrix
What are the two hormones which increase calcium in the plasma
PTH - parathyroid hormone
Calcitrol - active form of vitamin D produced by liver and kidney
What hormone decreases calcium in the plasma
Calcitonin - peptide hormone released from the clear cells of the thyroid cells
Where are the parathyroid glands located
usually 4 present posterior to the thyroid gland
What causes increased secretion of PTH
Decreasee in free plasma calcium
How does PTH increase plasma calcium levels
Stimulates osteoclasts to increase resorption of calcium from bone and inhibiting osteoblasts so that they can not deposit into the bone
It also increases resorption of calcium from the kidney tubules which decreases it’s excretion
PTH excretes more phosphate so that there is less deposits being put back into the bone
What is the effect of vitamin D on calcium
It causes calcium absorption
What in lactating in women can enhance production
prolactin
Where does Vitamin D become activated
Kidney and liver where it becomes calcitriol in the presence of PTH
What is the action of calcitriol on calcium absorption at the gut
Calcitriol controls the active transport system which moves calcium from the intestinal lumen to the blood
What are the actions of calcitriol
Increases absorption of calcium in the gut
Facilitates renal absorption calcium
Stimulates osteoclasts
What is the correlation between prolactin and calcitriol
Prolactin stimulates calcitriol synthesis with increased demands for calcium
What is hypocalcaemic tetany
voluntary twitching caused by hypocalcaemia
What inhibits PTH
increased plasma concentration of calcium
What is the effect of vitamin D3
Increase plasma calcium concentration and mineralization of bone
What happens in Vit D3 deficiency
The PTH works extra hard to control plasma and continually activates osteoclasts which results in easily fractured bones
What is the condition were bones become soft and easily fractured in adults
Osteomalacia
What is the condition where bones are soft and if still growing become bent in children
rickets
What is the effect of vitamin D defficiency
Intestinal malabsorption of calcium therefore decreased plasma calcium which activates PTH and that actives osteoclasts which aggravates calcium loss from bone
What population are at most risk of vitamin D3 defficiency
Asian and elderly populations
What hormone acts to decrease calcium levels
calcitonin and maybe cortisol
What produces calcitonin
thyroid gland
What stimulates secretion of calcitonin
Increased plasma calcium
What are the main functions of calcitonin
Bind to osteoclasts and inhibit bone resorption and increases calcium excretion - therefore reduces plasma calcium
What happens if there is excess calcitonin in the body
PTH overrides the effects
What is paget’s disease and what can help to treat it
Overactive osteoclasts breaking down bone - calcitonin can rarely be used to treat Paget’s disease
What is the effect of cortisol on calcium balance
Inhibits osteoblasts and increases renal excretion of calcium and phosphate excretion which decreases calcium in the plasma which stimulates bone resorption to increase the levels
What is the effect of insulin on calcium balance
Increases bone formation - diabetics can have slight bone loss due to low insulin and antagonises action of cortisol
What is effect of testosterone and oestrogen
Activate osteoblasts
What is the effect of growth hormone on calcium balance
Constant stimulus for bone formation
What is the effect of prolactin on calcium balance
Promotes calcium absorption from the the gut by stimulating calcitriol
