6. Ca2+ regulation

Question 1

under which forms is calcium found in plasma

Answer 1

50% albumin bound

50% free ionised form (only form to be filtered at glomerulus)

Question 2

where and how does Ca2+ reabsorption and secretion take place in the kidney

Answer 2

• Reabsorption: PCT (65%) and TAL (25%) via paracellular pathway - depends on Na+ reabsorption (if Na+ reabsorption is inhibited, eg by diuretics, Ca2+ reabsorption also declines)
• Secretion (Ca2+ fine-tuning): at DCT (8%) via luminal Ca2+ channel (TRPV5) and electrochemical gradient generated by basolateral Ca2+ ATPase or Na+/Ca2+ exchanger

Question 3

how are plasma calcium levels sensed and what are the functions of this sensor

Answer 3

Calcium-sensing receptor (CsR): mainly expressed in parathyroid gland (inhibits PTH release) and kidney tubules (inhibits Ca2+ reabsorption)

Question 4

describe the effects of PTH on plasma [Ca2+]

Answer 4

PTH increases plasma [Ca2+]

1. bone: activates osteoclasts… increased bone resorption and Ca2+ release into circulation
2. kidney: increases Ca2+ reabsorption in DCT by stimulating basolateral Ca2+ ATPase