Calcium Balance

Question 1

Describe the distribution of calcium in the body

Answer 1

1. 99% in bones and teeth
2. Extracellular (blood plasma): Mostly ionized, biologically active
3. Intracellular

Question 2

Describe the biological functions and importance of calcium

Answer 2

Biological functions: Excitation-contraction coupling, second messenger, mineralisation of bone, cofactor for the clotting and complement cascades, involved in cell shape and mobility changes

Question 3

Describe the role of PTH

Answer 3

PTH: Secreted from the chief cells of the parathyroid gland in response to hypocalcaemia
PTH acts to increase resorption of calcium from the bone and in the kidneys. It also acts to increase conversion of calcifediol to calcitriol (active Vitamin D). Vitamin D then acts to increase the absorption of calcium within the intestines.
All this leads to an increase in plasma calcium levels.

Question 4

Describe the role of Calcitonin

Answer 4

Calcitonin: Released from parafollicular cells of the thyroid gland. Release is stimulated by elevated plasma calcium levels.
Calcitonin acts to decrease resorption of calcium from the bone and by the kidneys (increases calcium excretion in the urine).

Question 5

Describe the role of calcitriol

Answer 5

Calcitriol (active vitamin D) acts to increase intestinal absorption of calcium, in response to elevated PTH levels.

Question 6

Describe the biochemical metabolism of vitamin D

Answer 6

Dietary or synthesised through UV mediated reactions.

Cholecalciferol –liver–> calcifediol –kidneys–> calcitriol

Question 7

Other substances which modulate calcium balance

Answer 7

Oestrogen: Regulates the production of osteoprotegerin (OPG). Binds to RANKL on osteoblasts, decreasing activity/RANKR binding (on osteoclasts)
Corticosteroids: Decrease absoprtion of calcium by the GI tract

Question 8

Describe the regulation of phosphate balance

Answer 8

Reduced by PTH

Question 9

Tumour peptide - PTH related protein (PTH-rP)

Answer 9

Secreted by squamous cell carcinomas.
Its structural homology to PTH allows binding to PTH receptors on osteoblasts. Stimulates upregulation of RANKL and binding to naïve osteoclasts through RANKR - activating the cells.
Causes malignancy associated hypercalcaemia