calcium metabolism Flashcards
endocrine control of calcium: recall the principle hormones which regulate serum calcium concentration, their physiological effects, their mechanism of action and their regulation
how is [Ca2+] increased
parathyroid hormone (PTH) or calcitriol (active form of vitamin D3)
how is [Ca2+] decreased
calcitonin
location of parathyroid glands
4 present; 2 on each lobe of thyroid
what is the hormone secreted by the parathyroid gland
parathyroid hormone (PTH)
what is the hormone secreted by parafollicular cells in the thyroid
calcitonin
what is the Ca2+ sensing receptor and how does it function
G-protein coupled receptor; changes conformation and activates intracellular signalling pathways
Ca2+ and parathyroid hormone (PTH) release
if high [Ca2+], bind to Ca2+ receptor, activating G-protein coupled receptor and activating intracellular signalling pathways to suppress parathyroid hormone release
what is parathyroid hormone (PTH) synthesised as
pre-proPTH polypeptide
what does parathyroid hormone (PTH) bind to and activate
binds to transmembrane G-protein linked receptors and activates adenyl cyclase
parathyroid hormone (PTH) actions in kidney
increases PO4 3- excretion and Ca2+ reabsorption
parathyroid hormone (PTH) action to form vitamin D3
stimulates activity of receptor 1a hydroxylase which forms active vitamin D3
parathyroid hormone (PTH) actions in small intestine
increase PO4 3- and Ca2+ reabsorption
parathyroid hormone (PTH) actions on bone
osteoclasts stimulated and osteoblasts inhibited so increases bone reabsorption
parathyroid hormone (PTH) effect on blood [Ca2+] and how
increase Ca2+ absorption in small intestine and kidneys, and increases bone Ca2+ mobilisation, so increase in blood [Ca2+]
how does parathyroid hormone (PTH) act in bone
PTH receptors on osteoblasts → PTH binds → release osteoclast activating factors such as RANKL → bind to osteoclast receptors → cause bone reabsorption
parathyroid hormone (PTH) regulation following increase in [Ca2+]
negative feedback to parathyroid gland so less release of PTH; vitamin D3 also suppresses PTH;
parathyroid hormone (PTH) regulation following decrease in [Ca2+]
less suppression of PTH release as fewer Ca2+ ions; catecholamines stimulate PTH production
calcitriol (dihydroxy-cholecalciferol; active form of vitamin D3) synthesis
7-dehyrdocholesterol in skin (UV light) and diet → cholecalciferol (vitamin D3) → 25 hydroxy-cholecalciferol synthesised in liver and stored in this form → 1,25 dihydroxy-cholecalciferol (calcitriol) synthesised in kidneys
calcitriol (active vitamin D3) actions on bone
chronic effect of increased osteoblast activity (promote laying down of bone so preserves PO4 3-)
calcitriol (active vitamin D3) actions in kidneys and small intestine
increases Ca2+ and Po4 3- reabsorption in kidneys and small intestine
what does calcitriol reabsorb more as opposed to parathyroid hormone (PTH) and why
reabsorb PO4 3- as concerned with maintaining bone mass
what do high calcitriol (active vitamin D3) levels stimulate
fibroblast growth factor 23
what does fibroblast growth factor 23 regulate
important role in regulating PO4 3- metabolism by lowering [PO4 3-]
how does fibroblast growth factor 23 lower [PO4 3-]
inhibits 1a-hydroxylase so inhibits calcitriol production (negative feedback)
