Endocrine Control of Calcium Balance

Question 1

What is the role of calcium in the body?

Answer 1

Signalling
Blood clotting
Apoptosis
Skeletal strength
Membrane excitability

Question 2

How does calcium effect membrane excitability?

Answer 2

Ca decreases Na permeability

Question 3

Describe hypocalcaemia and membrane permeability

Answer 3

Increases neuronal Na permeability leading to hyperexcitation of neurons
Can cause muscle tetany
Asphyxiation - if spreads to larynx and resp. muscles

Question 4

Describe hypercalcaemia and membrane permeability

Answer 4

Decreases neuronal Na permeability which reduces excitability and depresses neuromuscular activity and trigger cardiac arrhythmias

Question 5

Describe calcium distribution in the body

Answer 5

99% in bones
0.9% intracellular - inside mitochondria and SR. Free Ca is low
0.1% extracellular fluid - half bound to proteins

Question 6

What percentage of calcium is physiologically active?

Answer 6

0.05%

Question 7

How is calcium stored in bone?

Answer 7

In calcified extracellular matrix of bone
In form of hydroxyapatite so phosphate homeostasis is important in determining calcium balance

Question 8

How much calcium is in plasma?

Answer 8

2.2-2.6mM

Question 9

How much calcium is bound to proteins and why?

Answer 9

Has a high affinity for protein as small positive charge attracted to large negative
40% bound

Question 10

What does calcium bind to if not protein or free?

Answer 10

Plasma anions

Question 11

When is binding capacity increased?

Answer 11

Under alkalotic conditions
Ex. hyperventilation - plasma pH rises so more Ca bind to proteins - giving lower Ca in plasma (hypocalcaemia tetany)

Question 12

What happens if decrease in pH and Ca conc. in plasma?

Answer 12

Increased Ca conc. in plasma
As hydrogen ions increase and compete/ displace Ca from protein

Question 13

How is total body calcium determined by?

Answer 13

Ca from diet
Ca excretion from kidneys and faeces
But bone and ECF is crucial

Question 14

What is the main role of bone?

Answer 14

Role in maintaining Ca balance over mechanical support for the body

Question 15

What is the role of osteoblasts?

Answer 15

Bone building cells
Highly active cells and lay collagen extracellular matrix which they then calcify

Question 16

What is the role of osteocytes?

Answer 16

Osteoblasts differentiate to form these
Regulate activity of osteoblasts and osteoclasts

Question 17

What is the role of osteoclasts?

Answer 17

Responsible for mobilising bone
Secrete H+ ions to dissolve calcium slats and provide proteolytic enzymes to digest extracellular matrix

Question 18

What 2 key hormones act to increase Ca conc. in plasma?

Answer 18

Parathyroid hormone
Calcitriol

Question 19

What hormone acts to decrease Ca conc. in plasma?

Answer 19

Calcitonin

Question 20

What is parathyroid hormone?

Answer 20

Polypeptide hormone produced by parathyroid glands - 4 glands laying on surface of thyroid gland

Question 21

What is calcitriol?

Answer 21

Active form of Vit D
Steroid hormone produced from Vit D by liver and kidneys

Question 22

What is calcitonin?

Answer 22

Peptide hormone released from parafollicular cells in thyroid gland

Question 23

When is PTH secreted?

Answer 23

When decreased free Ca conc. in plasma

Question 24

How does PTH act to increase free Ca conc. in plasma?

Answer 24

Stimulates osteoclasts to increase Ca and phosphate in bone
Inhibits osteoblasts so reduce Ca deposition
Increase reabsorption of Ca from kidney tubules so decrease excretion
Increase renal excretion of phosphate
Stimulates kidneys to synthesis calcitriol from Vit D

Question 25

Why does increasing renal excretion of phosphate help increase free Ca conc. in plasma?

Answer 25

As it stops Ca being deposited back into bone which is a process which requires phosphate

Question 26

What is the negative feedback of PTH?

Answer 26

Increased Ca conc. in plasma has negative feedback on parathyroid cell

Question 27

What is the role of calcitriol?

Answer 27

Increases Ca conc. in plasma

Question 28

What type of hormone is calcitriol?

Answer 28

Steroid hormone derived from cholesterol Produced in 2 step process - liver then kidneys It is enhanced by hormone prolactin in lactating women

Question 29

Describe the action of calcitriol

Answer 29

Binds to nuclear receptors in target tissues - intestine, bone and kidney Increase absorption of Ca from gut Facilitates renal absorption of Ca Mobilises Ca stores of bone by osteoclasts

Question 30

How does calcitriol increase absorption of Ca from gut?

Answer 30

Active transport system which moves Ca from intestinal lumen to blood is under calcitriol control Increased PTH then increases calcitriol which increases absorption

Question 31

What is the collective action of calcitriol?

Answer 31

Complement actions of PTH Increase Ca conc. in plasma

Question 32

Why does prolactin stimulate calcitriol synthesis?

Answer 32

As increased demand for Ca for milk production in lactation

Question 33

Where does prolactin stimulate?

Answer 33

Stimulates kidneys to produce calcitriol

Question 34

What controls proportion of dietary calcium absorbed from intestine?

Answer 34

Circulating level of 1,25(OH)2D3 - active vit D (calcitriol)

Question 35

What is classed as Vitamin D deficiency?

Answer 35

Circulating levels fall below 20ng/ml

Question 36

What happens in Vitamin D deficiency?

Answer 36

PTH works hard to maintain Ca conc. in plasma by continually removing Ca from bone making bone soft and easily fracture Osteomalacia and rickets

Question 37

What happens to Ca and phosphate in vitamin D deficiency?

Answer 37

Intestinal malabsorption of Ca decreases Ca conc. in blood so PTH increases which promotes phosphate deficiency which aggravates Ca loss from bone

Question 38

What condition is vitamin D implicated in?

Answer 38

MS, cancer, arthritis and CVD

Question 39

How does calcitonin decrease Ca conc. in plasma?

Answer 39

Stimulated by increase in Ca conc. Binds to osteoclast to inhibit bone reabsorption and increases renal excretion which prevents increase

Question 40

What is XS calcitonin overridden by?

Answer 40

PTH

Question 41

What are other hormones altering Ca balance?

Answer 41

Cortisol Insulin - increases bone formation and antagonises action of cortisol Oestrogen/ Testosterone GH Prolactin

Question 42

How does cortisol affect Ca balance?

Answer 42

Inhibits osteoblasts and increases renal excretion of Ca Also reduces intestinal absorption of Ca so decreased Ca conc. in plasma Increases PTH so increases bone resorption Over time get osteoporosis