Page 124 - Clinical Biochemistry

• CT monitors the level of circulating Ca+2.
• A rise in Ca+2 level stimulates the cells to release calcitonin.
• Both bone cells & kidney cells have surface receptors for calcitonin.
Calcitonin reduces Ca+2 levels in the blood by two main mechanisms:
1- It inhibits the activity of osteoclasts, which are the cells responsible for breaking down bone.
• When bone is broken down, the calcium contained in the bone is released into the
bloodstream.
• Therefore, osteoclasts inhibition by calcitonin directly reduces Ca+2 released into the blood.
However, this inhibition has been shown to be short-lived.
• Bone cells respond by removing Ca+2 from the blood and storing it in the bone.
2- Kidney cells respond by increasing the excretion of Ca+2
Treatment for osteoporosis
Calcitonin has been examined as a possible treatment for osteoporosis, being a polypeptide,
calcitonin cannot be given by mouth (it would be digested) and giving by injection is not
appealing. However, inhaling calcitonin appears to be an effective way to get therapeutic levels
of the hormone into the blood.
4- The parathyroid Glands
• The parathyroid glands are 4 tiny structures embedded in the surface of the thyroid gland.
• Secretes parathyroid hormone (PTH), which absolute control over calcium levels
throughout the body.
• PTH is a polypeptide of 84 amino acids.
PTH:
• Increases the concentration of Ca+2 in the blood.
• Regulates the level of phosphate in the blood.
• PTH increases the concentration of Ca+2 in the blood in three ways:
1-Release of Ca+2 from the huge reservoir in the bones.
2-Reabsorption of Ca+2 from the fluid in the tubules in the kidneys.
3-Absorption of Ca+2 from the contents of the intestine (this action is mediated by calcitriol,
the active form of vitamin D.)
PTH also regulates the level of phosphate in the blood:
Secretion of PTH reduces the efficiency with which phosphate is reclaimed in the proximal
tubules of the kidney causing a drop in the phosphate concentration of the blood.
A. Hyperparathyroidism