Calcium

Question 1

what are the 2 main biological roles of calcium?

Answer 1

mineralisation of bone/teeth (hydroxyapatite)
calcium ions in intracellular and extracellular fluids are essential to many biochemical processes

Question 2

name biochemical processes where calcium is important

Answer 2

neuromuscular excitability
membrane potential maintenance
blood clotting cascade
hormone secretion
enzymatic regulation
cell signalling

Question 3

what 2 roles does calcium have in bone?

Answer 3

bone resorption by osteoclasts with calcium release
bone formation by osteoblasts - laying down matrix and remineralisation

Question 4

explain the role of calcium in heart muscle excitation

Answer 4

depolarisation from action potential opens Ca channels in surface membrane and T-tubules in cardiomyocytes
induces further Ca release from sarcoplasmic reticulum (Ca binds to troponin, sliding of thick/thin filaments, cell shortening, pressure development within ventricle inducing blood ejection)

Question 5

explain the body calcium distribution not including bone

Answer 5

50% non bound as ionised or free dialysable
10% bound to anions (PO4, HCO3)
40% blood calcium, protein bound (albumin = 70-80%)

Question 6

what are the general figures for plasma calcium levels and what conditions are associated with either extreme?

Answer 6

> 2.6 - hypercalcaemia
2.2-2.6 - normal
<2.2 - hypocalcaemia
(measurement - mmol/L)

Question 7

name examples of calcium presence in metabolic control

Answer 7

parathyroid hormone (PTH)
1,25 dihydroxyvitamin D

Question 8

what organs are involved in calcium metabolic control?

Answer 8

parathyroid glands (calcium sensing receptors)
kidney
gut
bone

Question 9

explain the calcium sensing receptor (CaR)

Answer 9

on parathyroid cells - minute to minute responses to ionised Ca
main physiological ligand in Ca
serves as a ‘calciostat’ for calcium homeostasis

Question 10

describe the composition, secretion and measurement of PTH

Answer 10

an 84 amino acid polypeptide produced by parathyroid glands
secretion regulated by free/ionised Ca sensed by CaR
measurement is by immunoassay

Question 11

explain the metabolic pathway of vitamin D synthesis

Answer 11

originates from diet + sunlight as cholecalciferol
cholecalciferol turned into 25 (OH) choleclciferol in liver
PTH turns 25 cholecalciferol into 1,25 (OH)2 cholecalciferol in kidney

Question 12

explain the general process of calcium homeostasis

Answer 12

in response to decreases in ionised Ca, PTH:
- stimulates Ca reabsorption in renal tubule
- stimulates 1,25DHCC in kidney which enhances Ca absorption from gut/bone resorption
- promotes bone resorption