Calcium Metabolism

Calcium - necessary for blood clotting, normal cardiac and skeletal muscle contraction, nerve function, and enzyme cofactor

• 30% increase in free Ca, nerves and muscles become unresponsive
• 35% decrease, nerves overexcitable and convulsions occur; 50% fatal

Sources

• bones and teeth provide huge reservoir of Ca
• 98-99% Ca filtered from blood in kidneys is reabsorbed
• 10-20% Ca consumed as food is absorbed by brush border of intestinal cells

Distribution in the body:

• 99% deposited in bones and teeth
• 0.5% bound to plasma proteins in blood
• 0.5% free ionized calcium in extracellular fluid

Endocrine regulation of calcium:

• Parathyroids are small glands on posterior surface of thyroids, stimulated by low free Ca levels, secrete parathyroid hormone (PTH)
• Thyroid glands wrap around anterior trachea below the larynx, stimulated by high free Ca levels, secrete calcitonin (CT)

Process of calcium regulation:

(figure in lecture)

1. Slight decrease in free ionized Ca level stimulates receptor cells in parathyroid, turns on gene to produce PTH and release it
   1. PTH increases number and activity of osteoclasts, which increases bone resorption (cAMP mech)
   2. PTH increases Ca reabsorption by kidneys and HPO₃^-2 excretion in urine (cAMP mech)
   3. PTH facilitates formation of calcitriol (hormone) from Vitamin D by kidneys and liver
      1. calcitriol increases Ca absorption in intestines
      2. calcitriol increases Ca reabsorption by kidneys
      3. calcitriol increases number of osteoclasts
   4. Result of above activities is an increase in free ionized Ca level
   5. Regulation by negative feedback on free Ca levels, half-life of PTH = 20 min
2. Slight increase in free ionized Ca level stimulates parafollicular cells of thyroid glands to release calcitonin
   1. CT decreases bone resorption by inhibiting osteoclast activity
   2. CT increases kidney excretion of Ca
   3. Result is a decrease in free ionized Ca levels
   4. Regulation by negative feedback on free Ca⁺² levels, half-life of CT = 10 min
   5. Less active in adults, may limit bone loss in pregnant women
3. Other hormones e.g., Growth h, Thyroid h, Estrogens and Testosterone have some effects on Ca metabolism