ca2+ homeostasis

importance of calcium

• skull, ribcage - vital organ production

• movement - long bones

• neurotransmitter release

• cv system

• hormone secretion

• blood clotting

• intracellular signalling - apoptosis

plasma conc of calcium

• 2.5mM but

• 40% is bound to proteins

• 5% complexes as salts

• free ionised ca2+ conc is 1.2mM

4 calciotropic hormones

• parathyroid hormone

• vitamin D

• calcitonin

• extracellular calcium

what are the paraythroid glands derived from

pharyngeal pouch endoderm driven by Gcm-2

which cells product parathyroid hormone

chief cells of parathyroid gland

relationship between calcium and PTH

• PTH will raise Ca2+ when Ca2+ is low

• steep sigmoidal relationship - tightly regulated

• feedback via calcium sensing receptors on parathyroid cell

where are calcium sensing receptors found

• kidney

• parathyroid glands

main 3 effects of PTH

• bone resorption

  • PTH stimulates mineralised bone → demineralised bone

• inc renal calcium reabsorption

  • acting onthe distal convoluted tubule and thick ascending limb

• inc production of vitamin D

how is vitamin D produced

• pro vitamin D3 → previtamin D3 with UVB

• previtamin D3 → vitamin D3

• vitamin D3 with PTH and OH-ase at 1st alpha → 25(OH)D3 IN LIVER

• 25(OH)D3 → 1,25(OH)2D3 in the kidney

how does calcium affect calcium

• inc net intestinal calcium uptake

• inc serum calcium levels by inc bone resorption and renal calcium reabsorption

intestinal calcium absorption

• most calcium absorption occurs in small intestine

  • passive, paracellular diffusion down electrochemical gradient

• active transcellular transport

  • under control of vitamin D

• CalD9k binds to calcium when it enters the cell then can be pumped out through the basolateral side

calbindins 

• mainly expressed in kidney

• expression is dependent on vitamin D

calcitonin

• secreted from parafollicular thyroid c cells

• dec plasma calcium after meal

  • dec bone resorption so more bone deposition

• contribution in homeostasis is very modest