Lecture 20 - Ca2+ and phosphate (Pi) homeostasis

Question 1

Describe the distribution of Ca2+ in the body

Answer 1

• 50% of plasma Ca2+ is free ionized
• 45% is bound to plasma proteins
• 5% is complexed to anions - eg HCO3-, Pi etc

Question 2

What factors influence the distribution of Ca2+ in the plasma?

Answer 2

* PH of plasma - if it is low (ie acidosis) - this displaces Ca2+ from proteins and anions - which increases the plasma conc of ionised Ca2+
* **plasma albumin concentration **- hypoalbuminemia increases ionised Ca2+ and hyperalbuminemia decreases ionised Ca2+

Question 3

What are the 3 main regulators of distribution of Ca2+ between the bone and ECF?

Answer 3

• PTH - mainly effects bone
• Calcitroil (made from vit D) - main effects on intestine
• calcitonin (thyroid gland)

Question 4

Describe the cells relevant to Ca2+ homeostasis in the thyroid gland

Answer 4

• parafollicular cells (C cells) have Ca2+ sensing receptors- they secrete calcitonin when plasma Ca2+ is high
• therefore calcitonin decreases the Ca2+ concentration in the ECF by reducing the release of calcium from the bone - ie it promotes ossification (formation of bone tissue)

Question 5

Describe the cells relevant to Ca2+ homeostasis in the parathyroid gland

Answer 5

* Principal cells also have calcium senssing receptors
* the principal cells secrete PTH when **plasma calcium is low - this increases the Ca2+ concentration** in the ECF by promoting the release of Ca2+ from the bone (ie bone resorption)

Question 6

What hormone is vitamin D converted to and what effect does it have?

Answer 6

• vitamin D is converted to calcitriol by the liver and kineys and its main effect is increasing Ca2+ absorption in the intestines

Question 7

What effect does PTH have on the kidney?

Answer 7

• it increases distal nephron Ca2+ reabsorption

Question 8

How is Ca2+ reabsorped in the PCT?

Answer 8

• paracellular routes - solvent drag and lumen positive voltage
• transcellular - calcium ATPase pump involved

Question 9

How does the TAL reabsorb Ca2+?

Answer 9

• again reabsorbs Ca2+ via transcellular route and also paracellular route (no sovent drag)

Question 10

How does the DCT and CD reabsorb Ca2+?

Answer 10

• reabsorption of Ca2+ in DCT and CD is exclusively transcellular and regulated by PTH
• PTH stimulates Ca2+ opening channels in DCT and causes expression of the NCX1 transporter on the basolateral membrane

Question 11

What is hypocalcemia?

Answer 11

low ionised plasma [Ca2+]

Question 12

What is hypercalcemia?

Answer 12

• elevated ionised plasma [Ca2+]

Question 13

What 3 hormones also regulate phosphate distribution between ECF and ICF?

Answer 13

• PTH
• Calcitonin
• calcitriol

Question 14

What effect does PTH have on Pi reabsorption, in comparison to Ca2+?

Answer 14

• unlike Ca2+ ion reabsorption, Pi reabsorption is decreased by PTH

Question 15

Compare the main effect/ function of PTH vs calcitriol

Answer 15

• PTH is released when Ca2+ conc in plasma is low - it stimulates bone resroption - ie release of Ca2+ from bone
• Calcitriol is also released when plasma Ca2+ conc is low - it stimulates the abroroption of Ca2+ from the intestine

Question 16

How does calcitriol vs PTH effect Pi reabsorption?

Answer 16

• PTH inhibits reabsorption of Pi
• calcitonin stimulates reabsorption of Pi