Regulation of calcium and phosphate Flashcards
What is the recommended adult intake for calcium?
1000mg/day
Calcium is the most abundant metal in the body
How is calcium distributed in the body?
Skeleton + teeth (99%)
Intracellular (1%)
Extracellular (0.1%)
How is calcium stored in skeleton and teeth?
calcium hydroxyapatite crystals
How does extracellular calcium exist?
~2.5 mmol/L in plasma
45% is ionised Ca2+
65% is bound Ca2+
Of bound Ca2+: 45% plasma proteins, 65% anions (bicarbonate, phosphate, lactate)
unbound/ionised calcium is the biologically active component
Which hormones increase calcium and phosphate?
Parathyroid hormone (PTH) Vitamin D
Where is parathyroid hormone secreted?
parathyroid glands
Where do we get Vit D from?
synthesised in skin (sunshine)
intake via diet
Where do PTH and vit D act?
kidney, bone and gut
What hormone decrease calcium and phosphate?
Calcitonin
Where is calcitonin secreted?
thyroid parafollicular cells
What does calcitonin do?
Can reduce calcium acutely, but no negative effect if parafollicular cells are removed eg thyroidectomy
What are the key hormones in control of serum calcium and phosphate?
Parathyroid hormone (PTH) Vitamin D
What are the sources of vitamin D?
Vitamin D2 (Ergocalciferol) - diet Vitamin D3 (Cholecalciferol) - skin synthesis/sunshine These two have different slightly different structures
(See diagram 6/32)
What is the mechanism of vit D metabolism?
UVB -> 7-dehydrocholesterol -> pre-vitamin D3 -> Vitamin D3 -> carried from skin to blood to liver into 25(OH)cholecalciferol by 25-hydroxylase -> 1,25(OH)2 cholecaliferol by 1 alpha-hydroxylase 25(OH)cholecalciferol is an inactive precursor
(See diagram 7/32)
What is a good indicator of body vitamin D status?
serum 25-OH vitamin D
How does 1,25(OH)2 vitamin D (calcitriol) regulate its own synthesis?
by decreasing the transcription of 1 alpha hydroxylase
What is the active form of vitamin D?
calcitriol
What are the effects of calcitriol?
↑ Ca2+ reabsorption in bone
↑ Ca2+ and PO43- reabsorption in kidney
↑ Ca2+ absorption in gut
↑ PO43- absorption in gut
(See diagram 8/32)
Which cells secrete PTH?
Chief cells in parathyroid glands
What is PTH secreted as?
a large precursor (pre-pro-PTH) and cleaved to PTH