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MSK > Control of Ca and P metabolism > Flashcards

Control of Ca and P metabolism Flashcards

1
Q

Give 5 functions of Calcium

A
  • bone growth and remodelling
  • secretion
  • muscle contraction
  • blood clotting
  • co-enzyme
  • stabilisation of membrane potentials
  • second messenger/stimulus response coupling
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2
Q

Distribution of Calcium in body

A
  • 99% is stored in our skeleton
  • most of the remainder is EC (55% bound, 45% free - controlled by PTH and Vit D)
  • very tightly regulated
  • free ionised Ca can flow in and out of cells and cause cellular signalling
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3
Q

Functions of phosphate

A
  • element in high energy compunds (ATP) and second messengers (cAMP)
  • constituent of DNA/RNA, phospholipid membrane (gives charge allowing them to stick together), and bone
  • IC anion
  • Phosphorylation/activation of enzymes
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4
Q

Distribution of phosphate in the body

A
  • 90% in skeleton
  • of the rest most is IC - 50% ionised and 50% free
  • controlled by PTH and FGF23
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5
Q

Daily turnover of calcium and phosphate

A
  • bone is continually turning over
  • get calcium from diet (Vit D absorption)
  • some secretion in pancreatic and gut fluid too
  • calcium and phosphate are filtered through the kidney - can be switched on and off
  • taken from bone when we need more
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6
Q

Bone remodelling

A
  • osteoclast moves into area of bone and breaks it down
  • osteoblasts move into the area and lays down osteoid (immature bone) which then calcifies to form mature bone
  • then goes back to resting state
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7
Q

Induction of osteoclast differentiation by RANK ligand

A
  • osteoblast stimulates the differentiation of osteoclasts by the production of RANK ligand
  • This activates the RANK receptor on the osteoclast precursor
  • via NF-kappa beta, osteoclasts are differentiated
  • osteogenin is a RANK inhibitor - inhibits differentiation
  • Oestrogen promotes bone growth
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8
Q

Hormonal control of bone remodelling

A
  • major growth signalling peptide is IGF1
  • PTH activates bone resorption
  • Osteoblast sends out ILs, activating the osteoclast starting the resorption of the bone
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9
Q

How is bone laid down?

A
  • bone is laid down along lines of stress
  • weight bearing exercise is good to make bones stronger
  • Astronauts/elderly or disabled people can get osteoperosis as they dont have to do any weight bearing
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10
Q

Bone as an endocrine organ

A
  • osteocytes produce FGF23 - controls the release of phosphate. It decreases Vit D synthesis and increases excretion of inorganic phosphate
  • osteoblasts produce uncarboxylated osteocalcin (uCON) - acts on pancreatic beta-cells and increases insulin secretion. Also acts on adipocytes to increase adiponectin. Acts on muscle to increase insulin sensitivity and glucose uptake
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11
Q

What other hormones are involved in bone turnover and resorption?

A
  • oestrogens, androgens, GH all act to stimulate the osteoblast
  • oestrogen also inhibits the osteoclast, so increases production, but reduces breakdown
  • Thyroxine increases bone turnover - thyrotoxicosis can lead to hypercalcaemia
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12
Q

What are the parathyroid glands?

A
  • 4 glands on upper and lower poles of each lobe of the thyroid gland
  • not uncommon to have more than 4 - can be anywhere down the arch of the aorta
  • Chief cells release the PTH
  • Supplied by blood from inferior thyroid arteries
  • can be damaged by thyroid surgery
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13
Q

PTH synthesis

A
  • prepro part of the hormone allows it to travel to the correct location
  • pre part is a tag to get it sent to the golgi in a vesicle
  • cleaved off in correct location
  • proPTH prevents it from being activated yet, and from being broken down by enzymes (prevents any folding whilst inside the cell)
  • gets cleaved off before being secreted - now an active hormone
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14
Q

The calcium sensing receptor (CaSR)

A
  • Ca is directly sensed by calcium receptor on the surface of chief cell
  • 7TMD - causes increase in PKC (Gq), an increase in Ca influx and a decrease in PKA (Gi). Causes a decrease in PTH
  • As soon as you have enough Ca in the blood, PTH production stops completely
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15
Q

PTH: High vs Low Calcium

A
  • High = Gi inhibits AC activity -> decreased cAMP and PKA. Gq increases IP3 pathway. IC Ca rises, PKA falls and PTH secretion is inhibited
  • Low = decreased IP3, increase PKA and PTH production
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16
Q

Actions of PTH

A
  • increased bone resorption (stimulates osteoblasts -> M-CSF and RANKL -> osteoclast differentiation -> bone resorption and increased Ca and PO4)
  • Increased calcium reabsorption in the DCT (when PTH is activated, you get an increase in cAMP. Channels open on luminal side allowing Ca in, then gets transported out into plasma)
  • increased excretion of phosphate (dont want Ca and PO4 in at the same time)
  • Increases 1-alpha hydroxylase in PCT
  • increases Vit D activation - increased Ca absorption from gut
17
Q

PTH and PTH-rp

A
  • PTH-rp is PTH in foetus
  • PTHrp is made in foetal livers as we dont have working thyroid glands - binds to PTH receptors
  • some cancers secrete PTH-rp, causes very high Ca levels through bone resorption
  • premature babies can have calcium imbalance as PTH glands are immature -> low levels
18
Q

Main causes of Hypercalcaemia

A
  • Primary hyperparathyroidism - too much PTH produced - cannot regulate it
  • cancers invade bone and over-activate osteoclast
  • Too much VitD, too much calcium absorbed
  • Renal failure
  • High bone turn over (hyperthyroidism, immobilisation)
19
Q

Symptoms of hypercalcaemia

A
  • Depressed nervous system -> slow reflexes
  • reduced gut motility (abdominal pain, constipation, vomiting)
  • depression
  • weakness
  • polydipsia/uria
  • kidney stones
  • diabetes insipidus
  • kidney failure
  • muscle weakness
  • bone pain
  • osteoperosis
  • QT interval shortening
  • Bradycardia
  • hypertension
20
Q

Active Vit D formation

A
  • small amount of VitD from diet, mainly comes from sunlight falling on skin
  • 7-dehydrocholesterol -> UV light -> cholecalciferol (VitD) -> liver -> Calcidiol (25-hydroxy VitD) -> kidney -> calcitriol (1,25-dihydroxy VitD) - ACTIVE
21
Q

Vit D and calcium homeostasis

A
  • increases Ca absorption from gut
  • Inhibits PTH synthesis
  • Inhibits 1a-hydroxylase (negative feedback)
  • increases osteoclast activity
  • decreases calcium excretion by increasing renal absorption in the PCT
22
Q

Transport of calcium across the epithelial cells of the intestine

A
  • increases expression of calcium channels on the gut cells, then increases active transport of calcium into EC fluid
  • paracellular pathway isnt very efficient
  • transcellular is most important
23
Q

Actions of active VitD

A
  • modulates T and B cells
  • helps regulate cell growth and prevent cancer
  • gives muscle some of its strength
  • decreases the resistance to insulin in pancreatic cells
  • modulates RAAS
24
Q

Causes of VitD deficiency

A
  • renal disease
  • Receptor’s resistance to hormone
  • malabsorption
  • dietary insufficency
  • poor exposure to sunlight
25
Q

Signs of VitD deficiency

A
  • aches and pains in bones
  • Rickets/osteomalacia
  • Proximal myopathy (legs dont work properly)
  • Hypocalcaemia - secondary hyperparathyroidism
26
Q

Endocrine responses to VitD deficiency

A
  • decreased ca absorption in gut
  • decreases blood ca
  • increases PTH levels to get ca from the bone
  • increases FGF23
  • tries to increase 1a-hydroxylase activity
27
Q

FGF23 and calcium/phosphate

A
  • phosphate and activated VitD stimulate FGF23 production

- FGF23 increases phosphate excretion and inhibits 1a-hydroxylase activity

28
Q

PTH and phosphate

A
  • Kidneys - increases Vit D and phosphate excretion

- increases bone resorption and so increases phosphate

29
Q

Causes of hypocalcaemia

A
  • vit D deficiency
  • hypoparathyroidism (thyroid surgery)
  • chelation
  • pseudohypoPT - receptor defect
  • neonatal
  • activating mutation of ca receptor
30
Q

endocrine responses to hypocalcaemia

A
  • increase PTH, increase VITD activation to increase Ca absorption
  • bone resorption is increased, releasing ca into plasma
  • increased Ca reabsorption from kidney
31
Q

Symptoms of hypocalcaemia

A
  • Trousseaus sign (cuff on arm)
  • Chvostek’s sign (smile)
  • muscle cramps
  • tetany
  • cofusion, hallucinasions, seizures

MSK (10 decks)

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