2. PARATHYROID HORMONE
• PTH is a peptide hormone which is synthesised
in the parathyroid glands.
• Its main function is to increase the
concentration of calcium in the blood plasma.
• Intact PTH (iPTH) is the biologically active form
and is secreted when the calcium level is low.
(c) Natalia Dachanidze
4. CALCITONIN
• Calcitonin (also known as thyrocalcitonin) is
produced in the thyroid gland and is the
antagonist of PTH.
• The production and secretion of calcitonin is
stimulated by an elevated calcium level and
results in a reduction in the calcium
concentration in the blood.
(c) Natalia Dachanidze
6. VITAMIN D
• Vitamin D exists in two forms: vitamin D2
which is acquired by consumption of plant foods
such as mushrooms and avocado, and vitamin
D3 which is obtained from animal food sources
or produced in skin exposed to ultraviolet B
radiation.
(c) Natalia Dachanidze
9. Introduction
• Total plasma [Ca++] = 2.5mmol/L
• Range is 2.0 to 2.5 mmol/L
• Very tightly controlled
Body
content
Bone intracellu
lar
extracellu
lar
Calcium 1300 gms 99% 1% 0.1%
(c) Natalia Dachanidze
13. Functions of Calcium
• 1. Nerve and muscle functions
• Depolarization and muscle contaction
(c) Natalia Dachanidze
14. • Increase in intracellular Ca2+ concentration →
Ca2+ binds to troponin C → conformational
change in the troponin complex → moves
tropomyosin out of the way → permitting the
binding of actin to the myosin heads leading
cross-bridge formation and the muscle contracts
as a whole
(c) Natalia Dachanidze
16. Functions of Calcium
• 2. Homeostasis
-activation of clotting enzyme is the plasma
(c) Natalia Dachanidze
17. Functions of Calcium
• 3. Preserving bone density
- construction, formation and maintenance of
bone and teeth. This function helps reduce the
occurrence of osteoporosis
(c) Natalia Dachanidze
18. Functions of Calcium
• 4. Neurotransmitter release
- Directly proportional to the calcium level
- Arrival of action potential to axonal terminal
→opening of voltage gated calcium channels
→calcium influx into the terminal → transmitter
vesicle fuse with the release sites → exocytosis-
release of transmitters into the cleft
(c) Natalia Dachanidze
19. Functions of Calcium
• 5. Calcium assists in maintaining all cells and
connective tissues in the body and regulating
mitotic transition and cell division.
• 6. Essential component in the production of
enzyme and hormones that regulate digestion,
energy, and fat metabolism.
(c) Natalia Dachanidze
20. Calcium Homeostasis
• Blood calcium is tightly regulated by:
1) Principle organ systems:
Intestine
Bone
Kidney
2) Hormones:
Parathyroid hormone (PTH)
Vitamin D
Calcitonin
(c) Natalia Dachanidze
22. Parathyroid Hormone (PTH)
• There are 4 parathyroids glands, located on the
dorsal side of the thyroid
• The blood supply to the parathyroid glands is
from the thyroid arteries.
(c) Natalia Dachanidze
24. Parathyroid Hormone (PTH)
• Mechanism
Increase in extracellular calcium concentration
→ Ca2+ binds to the receptor and activates
phospholipase C → increased levels of IP3/Ca2+
→ which inhibits PTH secretion.
When extracellular Ca2+ is decreased, there is
decreased Ca2+ binding to the receptor, which
stimulates PTH secretion.
(c) Natalia Dachanidze
25. Parathyroid Hormone (PTH)
• Actions of PTH on bone, kidney and small
intestine
1) Actions on bone
- Overall effect : promote bone resorption and
increase calcium concentration
(c) Natalia Dachanidze
27. Parathyroid Hormone (PTH)
a) Inhibits PO4 reabsorption (inhibits Na-PO4
cotransport in PCT) – phosphaturia – less
complexed Ca-PO4 – increase plasma calcium
b) Stimulates calcium reabsorption
(c) Natalia Dachanidze
28. Parathyroid Hormone (PTH)
• 3. Actions on small intestine (indirect)
- Stimulates Ca2+ reabsorption via activation of
vitamin D.
(c) Natalia Dachanidze
32. Calcitonin
• a straight-chain peptide with 32 amino acids.
• synthesized and secreted by the parafollicular
cells of the thyroid gland.
• major stimulus for calcitonin secretion is
increased plasma Ca2+ concentration
• The major action of calcitonin is to inhibit
osteoclastic bone resorption, which decreases
the plasma Ca2+ concentration.
(c) Natalia Dachanidze
