calcium and bone

Question 1

diagram of calcium homeostasis: interaction between PTH and vitamin D

Answer 1

diagram; PTH to increase serum Ca2+: increases Ca2+ reabsorption from bone, causes kidney to reabsorb Ca2+, regulates production of active vitamine D (calcitriol); inactive vitamin D (calcidiol) is from liver and undergoes 1a hydroxylation under control of PTA, increasing serum Ca2+ by increasing Ca2+ absorption from gut

Question 2

phosphate absorption regulation

Answer 2

Na+/PO43- absorbed via cotransporter from urine in proximal convoluted tubule; PTH from blood inhibits this, as does fibroblast growth factor 23 (FGF23 - from osteocytes); this also inhibits calcitriol synthesis, reducing PO43- reabsorption from gut

Question 3

where does phosphate reabsorption occur

Answer 3

via gut and kidneys

Question 4

what does increase phosphate reabsorption cause to Na+

Answer 4

less Na+ excretion in urine (as co-transported in)

Question 5

effect on serum PO43- in hyperparathyroidism

Answer 5

low as increased urine phosphate excretion (PTH inhibits Na+/PO43- transporters)

Question 6

regulation of PTH secretion: high [Ca2+] ECF

Answer 6

Ca2+ binds to Ca2+ sensin receptor on parathyroid cell -> receptor activation leads to inhibition of PTH secretion

Question 7

regulation of PTH secretion: low [Ca2+] ECF

Answer 7

Ca2+ not bound to Ca2+ sensin receptor -> no inhibition -> PTH secreted -> PTH action in body leads to increased [Ca2+] ECF

Question 8

role of vitamin D in calcium homeostasis

Answer 8

diagram

Question 9

where is vitamin D obtained from

Answer 9

vitamin D obtained from diet (ergocalciferol to inactive vitamin D calcidiol in liver) or sunshine (UVB light); in kidney, 1a-hydroxylase (stimulated by PTH) produces calcitriol (active vitamin D)

Question 10

effect of cacitriol on PTH, and Ca2+

Answer 10

negative feedback on PTH; increases Ca2+ gut absorption, maintenance in bone, renal Ca2+ reabsorption

Question 11

5 causes of vitamin D deficiency

Answer 11

malabsorption or dietary insufficiency, lack of sunlight (block UVB light), liver disease (can’t convert to calcidiol), renal disease (no stimulation of renal 1a-hydroxylase by PTH to convert to calcitriol), receptor defects in gut, bone, kidney and parathryoid gland

Question 12

2 examples of diseases causing GI malabsorption of vitamin D

Answer 12

coeliac, inflammatory bowel

Question 13

inheritance and prevalence of vitamin D receptor defects, and response to treatment

Answer 13

autosomal recessive, rare, resistant to vitamin D treatment

Question 14

high EC Ca2+ (hypercalcaemia) effect on nerve and skeletal muscle excitability

Answer 14

Ca2+ blocks Na+ influx as more competition, so less membrane excitability

Question 15

low EC Ca2+ (hypocalcaemia) effect on nerve and skeletal muscle excitability

Answer 15

enables greater Na+ influx as less competition, so more membrane excitability

Question 16

4 signs and symptoms of hypocalcaemia (CATs go numb)

Answer 16

parasthesia (hands, mouth, feet, lips), convulsions, arrythmias, tetany

Question 17

explain symptoms of hypocalcaemia

Answer 17

sensitises excitable tissues, to muscle cramps/tetany, tingling

Question 18

what do Chvostek’s and Trousseau’s signs indicate

Answer 18

neuromuscular irritability due to hypocalcaemia

Question 19

describe Chvostek’s sign

Answer 19

tap facial nerve just below zygomatic arch -> positive response is twitching of facial muscles

Question 20

describe Trousseau’s sign

Answer 20

inflation of BP cuff for several minutes to induce carpopedal spasm

Question 21

4 causes of hypocalcaemia

Answer 21

vitamin D deficiency, low PTH levels (hypoparathyroidism), PTH resistance (e.g. pseudohypoparathryoidism), renail failure

Question 22

3 caues of low PTH levels (hypoparathyroidism)

Answer 22

surgical (neck injury), auto-immune, Mg2+ deficiency

Question 23

how does renal failure cause hypocalcaemia

Answer 23

impaired 1a hydroxylation, decreasing production of 1,25(OH)2D3

Question 24

signs and symptoms of hypercalcaemia

Answer 24

reduced neuronal excitability so atonal muscles; “stones” - renal effects, “abdominal moans” - GI effects and “psychic groans” - CNS effects”

Question 25

renal effects of hypercalcaemia

Answer 25

polyuria and thirst, nephrocalcinosis, renal colic, chronic renal failure

Question 26

GI effects of hypercalcaemia

Answer 26

gut slowed down so: anorexia, nausea, dyspepsia, constipation, pancreatitis

Question 27

CNS effects of hypercalcaemia

Answer 27

fatigue, depression, impaired concentration, altered mentation, coma

Question 28

4 causes of hypercalcaemia

Answer 28

primary hyperparathyroidism, malignancy (these 2 account for 90% of cases), conditions with high bone turnover, vitamin D excess (rare)

Question 29

how might malignancy cause hypercalcaemia

Answer 29

tumours/metastases often secrete a PTH-like peptide; cancer cells in bone cause release of Ca2+

Question 30

conditions with high bone turnover causing hypercalcaemia

Answer 30

hyperthyroidism, Paget’s disease of bone (immobilised patient)

Question 31

diagnostic approach to hypercalcaemia: normal axis

Answer 31

parathyroid cells detect low Ca2+, so increase PTH which raises Ca2+ - negative feedback to switch PTH off when sufficiently raised

Question 32

diagnostic approach to hypercalcaemia: primary hyperparathyroidism

Answer 32

raised Ca2+, low PO43- (PTH inhibits PO43- reabsorption in kidney), raised unsuppressed PTH (autonomous PTH secretion) as no negative feedback

Question 33

diagnostic approach to hypercalcaemia: hypercalcaemia of malignancy

Answer 33

raised Ca2+, suppressed PTH (no issue in parathyroid glands, but cancer deposits in bone causing release of Ca2+ from bone)