Calcium,vitamin D,parathyroid And Calcitonin

Question 1

Daily calcium requirements

Answer 1

• Daily calcium requirements are about 1g per day, however, net dietary calcium absorption is only about 150 mg / day:
~ renal calcium losses are also about 150mg/day

Question 2

What are normal blood calcium levels

Answer 2

150mg/day

Question 3

What hormones regulate blood calcium levels

Answer 3

~ 1,25-dIhydroxyvitamin D3
~ parathyroid hormone (PTH)
~ calcitonin

Question 4

Role of calcium

Answer 4

~ neurotransmitter / hormone secretion
~ muscle contraction (actin / myosin interactions)

Question 5

What type of calcium forms bones

Answer 5

• Calcium hydroxyapatite forms bone ~ Ca10(PO4)6(OH)2

Question 6

Hypercalcemia and the relationship between blood ca2+ PTH and calcitonin

Answer 6

Hypercalcamia is associated with high levels of circulating concentration of calcitonin

Question 7

Hypocalcemia and the relationship between blood ca2+ pth and calcitonin

Answer 7

Hypocalcaemia is associated with low levels of pth

Question 8

Synthesis of 1,25 dihydroxyvitamin d3

Answer 8

• 1,25-dihydroxyvitamin D3
(calcitriol) is formed from
skin cholesterol and Skin dietary vitamin D
• U-V light is required to form Vitamin D
Skin
• Vitamin D forms
25-hydroxyvitamin D
in liver
Liver
• Parathyroid hormone stimulates renal proximal tubule cells to form 1,25-dihydroxyvitamin D3
Kidney

Question 9

Sources of vitamin d

Answer 9

-diet eg oily fish eggs
-mainly formed In skin during uv light exposure

Question 10

Functions of vitamin d

Answer 10

• 1,25-dihydroxyvitamin D3 (calcitriol) is then synthesised from 25-hydroxyvitamin D3 by proximal tubule cells in the kidney
• The main actions of 1,25-dihydroxyvitamin D3 include:
~ absorption of Ca2+ by small intestine
mucosa cells via calbindin
~ acts like a steroid hormone binding to specific nuclear receptors on DNA
that produce calcium transport calbindin
~ also increases phosphate uptake
~ increased calcification of bone via osteoblasts & osteoclasts

Question 11

What is calcium required for

Answer 11

Bone remodelling and Growth

Question 12

Role of parathyroid hormone

Answer 12

Promotes Ca release from bone when serum Ca is low (bone breakdown)

Question 13

Role of vitamin d

Answer 13

Required for Ca absorption into bone

Question 14

Role of calcitonin

Answer 14

Opposes PTH- Reduced blood Ca

Question 15

Where’s re parathyroid glands located

Answer 15

4 pairs above the thyroid gland

Question 16

Where is calcitonin released from

Answer 16

C cells

Question 17

Actions of para thyroid hormone

Answer 17

? Watch lecture

Question 18

Parathyroid chief cells

Answer 18

• Low blood Ca2+ stimulate PTH secretion by Chief cells
• PTH raises blood Ca2+ via:
-1,25-dihydroxyvitamin D3 in gut
-renal tubular reabsorption
-osteoblast stimulation osteoclast bone resorption

Question 19

Calcitonin

Answer 19

• Calcitonin (thyrocalcitonin) is secreted by parafollicular Chief- cells
• Calcitonin lowers blood Ca2+ levels in two ways:
~ major effect: Inhibits osteoclast activity in bones
~ minor effect: Inhibits renal reabsorption of Ca2+ and
phosphate, allowing them to be excreted in the urine
• Calcitonin receptors expressed by bone osteoclasts, kidney cells and parts of the brain:
~ calcitonin can be used therapeutically in post-menopausal osteoporosis in women

Question 20

Main actions of calcitonin

Answer 20

When plasma levels are high this stimulates secretion of calcitonin from parafollciular cells this then either causes the inhibition of osteoblasts activity or increased excretion of ca2+ and phosphate by kidneys

Question 21

How is Hypocalcemia caused

Answer 21

~ dietary calcium and vitamin D deficiency
~ vitamin D deficiency due to low UV-light exposure
~ hypoparathyroidism (may be due to thyroid surgery)

Question 22

Effects of hypocalcaemias if severe

Answer 22

-CATs go numb
~ Convulsions; Arrythmias; Tetany and numbness in the
hands and feet and around the mouth
~ mainly due to increased nerve and muscle excitability

Question 23

Results of Hypocalcemia combined with vitamin deficiency

Answer 23

-poor bone formation combined and mineralisation resulting in

~ rickets in children
~ osteomalacia
(bone softening) in adults

Question 24

Causes of hypervalcemia

Answer 24

~ parathyroid tumours that secrete excess PTH
~ malignancy due to PTH-like peptide secretion
~ very large doses of Vitamin D (Vitamin D intoxication)
~ extra-renal 1,25-dihydroxyvitamin D3 synthesis by macrophages, particularly in sarcoidosis

Question 25

Effects of hypercalcaemia if severe and rapid in onset

Answer 25

~ abdominal pain, nausea, vomiting and dehydration
~ demineralisation of bone if PTH is raised
~ calcification of soft tissues and blood vessels, kidney stones
~ increased blood pressure due to impaired renal function
~ anxiety, confusion; depression; weakness; coma

Question 26

Main actions of 1,25 dihydroxyvitamin d3

Answer 26

-absorption of ca2+ by small intestine mucosa cells via calbindin
-acts like a steroid hormone binding to steroid bonding to specific nuclear receptors on dna that produce calcium transport calbindin
-increase phosphate uptake c
-increased classification of bone via osteoblasts and osteoclasts

Question 27

Actions of PTH when plasma calcium levels are low

Answer 27

When plasma levels are low, calcium receptors on chief cells of parathyroid gland increase pth secretion
1)pth acts via osteoblasts which active osteocytes development causing increased bone reabsorption which increases plasma calcium
2)pth acts via distal tubules and collecting ducts causing increased calcium re absorption and decreased phosphate re absorption by kidneys causing increased plasma calcium
3)pth acts on proximal tubules causing increased synthesis of 1,25-dihydroxycholecalciferol by kidneys which increases calcium absorption by the gut and increases plasma calcium