Lecture 27: Calcium & Phosphate Homeostasis

Question 1

Describe the distribution of Ca and inorganic Phosphate in the body

Answer 1

Almost all in the bone. Very little amounts of calcium in the ecf, almost none in the icf. low amounts of turnover.

Question 2

Describe the roles of osteoclasts, osteoblasts, and osteocytes. Describe process of remodelling

Answer 2

osteoclasts break down hydroxyapatite to release Ca and Pi
osteoblasts build bone
osteocytes maintain bone, previously osteoblasts

bone remodelling: osteoblasts release MCSF (monocyte colony stimulating factor) to cause differentiation to osteoblasts. osteoblasts form cavity to break down bone, then osteoblasts come in fill in bone. osteoclasts stopped by osteoprotegerin

Question 3

Describe the three ways there is hormonal regulation of Ca

Answer 3

1) calcitriol (Vitamin D): causes absorption of Ca from SI, causes breakdown of Ca from bone, prevents excretion of Ca, prevents excretion of Pi
2) parathyroid hormone: causes breakdown of bone, prevents excretion of Ca, causes excretion of Pi
3) calcitonin: prevents breakdown of bone, causes excretion of Pi, prevents excretion of Ca

Question 4

Explain how CaSR is involved in Ca balance

Answer 4

CaSR is the primary receptor, acts via Gq and Gi pathways to inhibit production of PTH and exocytosis of PTH. Calcitriol enhances production of CaSR

Question 5

Describe the structure of bone

Answer 5

functional units are osteons, have fibrous periosteum as layer of connective tissue

Question 6

How is Ca and Pi taken up in the kidney and GI?

Answer 6

Kidney: PT - Ca moves paracellularly, Pi goes via sodium/Phosphate cotransporter then via anion-phosphate exchanger.
DT - Ca moves via TRPV5/6, then via NCX or PMCA

GI: Ca moves via TRPV5/6, then PMCA