Endocrine Control of Calcium Balance Flashcards by Peter Savage

What are the rolls of calcium in the body?

Signalling
Blood clotting
Apoptosis
Skeletal strength
Membrane excitability

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What is the signalling role of Ca2+?

Exocytosis
Contraction of muscle fibres
Altering enzyme function

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What is the blood clotting role of Ca2+?

Part of clotting cascade

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What is the membrane excitability role of Ca2+?

Ca2+ decreases Na+ permeability

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What effect does hypocalcaemia have?

Increased neuronal Na+ permeability causing hyper-excitations
- Tetany

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What effect does hypercalcaemia have?

Decreased neuronal Na+ permeability
Reduced excitability
Depressed neuromuscular activity
Arrhythmias!!

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What is the distribution of calcium in the body?

Bones 99%
Intracellular 0.9%
Extracellular fluid 0.1%

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Where is intracellular calcium stored?

Inside mitochondria, sarcoplasmic reticulum

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How is calcium stored in the extracellular fluid?

~50% bound

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How much calcium in the body is free in solution?

0.05%

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How much calcium is stored in bones? How is it stored?

99% - 1kg

Calcified extracellular matrix in hydroxyapatite with PO4

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What other factor is also important in determining calcium balance?

Phosphate homeostasis

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Calcium is stored with what in the bones?

Ca10(PO4)6(OH)2

Hydroxyapatite

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What is the normal range of plasma Ca2+?

2.2-2.6mM in plasma

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Calcium has a high affinity for what? Why?

Proteins

they have a large -ve charge

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Which part of [Ca2+]plasma is physiologically active?

~50%, unbound, 1.2mM

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What happens to the 10% of calcium ions not bound to plasma protein, nor free in solution?

Bound to plasma anions

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What proteins, and to what proportion, is calcium bound?

Albumin - 80%

Globulin - 20%

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Which anions is calcium typically bound to?

HCO3
H2PO4
HPO4
Sulphate
Citrate
Lactate

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What is the calcium distribution in blood?

40% protein bound
10% complexed
50% free

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The binding capacity of Ca2+ is dependent on what?

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When is binding capacity increased?

Alkalotic conditions

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What is the effect on plasma pH of hyperventilation?

Increased pH

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How can hyperventilation precipitate hypocalcaemic tetany?

Decreased CO2
Increased pH
Increased bound [Ca2+]
Hypocalcaemia

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What is the effect of acidosis on protein bound calcium?

Decreased pH increases plasma [H+] which displaces bound Ca2+, increasing free [Ca2+]

Total body calcium = ?

Calcium in - Calcium out

What is the most important role of bone?

Calcium balance maintenance

What are osteoblasts?

Bone building cells

How do osteoblasts work/

Highly active Lay down collagen extracellular matrix which they then calcify Differentiate into osteocytes in established bone

What do osteocytes do?

Regulate osteoblast/osteoclast activity

What is the function of osteoclasts?

Secrete H+ (pH 4) to dissolve calcium saltes | Provide proteolytic enzymes to digest extracellular matrix

Which hormones increase [Ca2+]plasma?

Parathyroid hormone | Calcitriol

What is parathyroid hormone?

Polypeptide hormone produced by the parathyroid gland

What is calcitrol?

Active form of Vitamin D | Steroid hormone produced by liver and kidneys

Where are the parathyroid glands located?

Lying on the posterior surface of the thyroid gland

When is parathyroid hormone secreted?

Response to ↓[Ca2+]plasma

How does parathyroid hormone increase [Ca2+]?

1. Stimulate osteoclasts to resorb Ca and Phosphate 2. Inhibit osteoblasts to reduce Ca deposition 3. Increase reabsorption of Ca2 from kidney tubules 4. Increase renal phosphate excretion 5. Stimulate kidney calcitrol synthesis promoting calcium absorption

How does increasing renal excretion of phosphate increased [Ca2+]?

Prevents Ca2+ from being deposited back into bone

What is calcitriol?

Active vitamin D3

What type of hormone is calcitriol?

Steroid hormone

What also stimulates formation of calcitriol?

Prolactin (lactating women)

How does sunlight form vitamin D3?

Action of UV light on the skin causing cholesterol --> vitamin D3

What are the dietary sources of vitamin D3?

Fatty fish Fish liver oils Egg yolks

How is calcitrol produced?

Vitamin D Processed by liver, then kidneys OR Steroid precursors activated by UV light

Which actions of calcitrol complement PTH?

Increased Ca2+ absorption in gut Facilitates renal absorption of Ca2+ Mobilises calcium stores in bone

How does calcitrol mobilise calcium stores in bone?

Stimulating osteoclast activity

How does calcitriol control intestinal intake of Ca2+?

Controlling active transport system

What is the effect of low plasma [Ca2+]?

↑ in PTH --> ↑calcitriol --> ↑intestinal Ca2+ absorption

What is the effect of prolactin in relation to Calcium?

Stimulation of calcitriol synthesis by kidneys to supply increased demand

What is the amount of dietary Ca2+ absorbed in a healthy individual?

30%

What is the amount of dietary Ca2+ absorbed in a vitamin D deficient individual?

10-15%

What is the amount of dietary Ca2+ absorbed in a pregnant/lactating/growing individual?

45-55%

What is the amount of calcitriol in a vitamin D deficient individual?

Low

What is the amount of calcitriol in a pregnant/lactating/growing individual?

High

When is Vitamin D considered 'deficient'?

Circulating vitamin D <20ng/ml

What % of scots are vitamin D insufficient?

Summer - 75% | Winter - 92%

When does Vitamin D deficiency become more prevalent?

>65 years due to reduce absorption

How does vitamin D deficiency present?

Children - Rickets | Adults - Osteomalacia

If calcitriol causes Ca release from bone, how does it make bones stronger?

Greater effect on gut absorption/kidney reabsorption, which increases bone mineralization

Vitamin D3 deficiency leads to what?

Ca2+ and Phosphate deficiency

How does vitamin D3 deficiency lead to phosphate deficiency?

Intestinal malabsorption means low [Ca2+], this increases PTH Increased PTH aggravated Ca loss from bone .'. loss of phosphate

Why are asians at risk of Ca deficiency?

Chapatti flour contains phytate Phytate binds dietary Ca2+ Pigmented skin less able to respond to UV light

Vitamin D deficiency is implicated in what?

MS Cancer Arthritis CVD

Which hormone decreases [Ca2+]

Calcitonin

What type of hormone is calcitonin?

Peptide hormone

What stimulates calcitonin secretion?

↑ [Ca2+]

Where is calcitonin produced?

Thyroid gland

What are the main actions of calcitonin?

Bind to osteoclasts and inhibit bone resorption | Increase Ca2+ excretion by kidneys

When is calcitonin used as a treatment?

Paget's Disease | overactive osteoclasts

What is the effect of Cortisol on Ca2+ balance?

Inhibit osteoblasts Increase Ca2+ and phosphate Reduce intestinal absorption Increase PTH .'. ↑bone resorption

What can increased cortisol lead to over time?

Osteoporosis

What is the effect of insulin on Ca2+ balance?

Inhibit cortisol | Increase bone formation

What is the effect of oestrogen on Ca2+ balance?

Promote bone formation via oestrogen receptors on osteoblasts

What is the effect of growth hormone on Ca2+ balance?

Constant stimulus for bone formation

What is the effect of prolactin on Ca2+ balance?

Promotes calcium absorption by the gut, stimulating synthesis of calcitriol

When are osteoclasts stimulated?

When calcium is low

Where is Calcitriol activated?

Kidneys

What causes the inhibition of PTH?

Increased plasma [Ca2+]

Calcitriol binds to what?

Target tissue nuclear receptors Intestine Bone Kidney

What is the primary action of vitamin D?

Get calcium out of the gut

Endocrine Control of Calcium Balance Flashcards

(80 cards)