Calcium

Question 1

list some roles of calcium in the body

Answer 1

Muscle contraction
Nerve excitability
Intracellular messenger 
Regulation of heartbeat 
Clotting of blood 
Enzymes of intermediary metabolism

Question 2

how much calcium do we need to intake each day?

Answer 2

1g (pint of milk)

Question 3

why do total calcium levels vary with albumin levels in the plasma ?

Answer 3

Albumin is the main carrier protein of calcium.
If albumin levels are low then bound calcium levels will be low, and total calcium levels will be low, even though unbound/ionised/free calcium levels may be normal.

Question 4

what can working out the adjusted calcium levels help us do?

Answer 4

measure what the serum total calcium levels would have been like if albumin levels were normal

Question 5

what is the calculation for adjusted calcium?

Answer 5

Ca (adj) = Ca(total) + [0.02(45 - albumin)]

Question 6

what is calcium homeostasis regulated by?

Answer 6

2 hormones:
PTH
1.25 DHCC/calcitriol

Question 7

describe what happens when calcium levels drop?

Answer 7

Calcium Sensing receptors in the parathyroid gland detect a drop in ionised calcium levels.
They inhibit phospholipase C and activate adenylyl cyclase. This increases cAMP levels.

PTH levels are increased, which:
Increase bone resorption
Increase calcium reabsorption in kidney tubules
Stimulate the formation of 1,25 DHCC/calcitriol in the kidney which enhances gut absorption of calcium and bone resorption.

Phosphate reabsorption in the kidneys reduces so that more phosphate is excreted.

These result in an increase in iCa levels.

Question 8

what happens in plasma calcium levels are too high?

Answer 8

Thyroid gland increases secretion of calcitonin which lower calcium levels.

Question 9

describe how vitamin D is absorbed

Answer 9

From supplements, sun or food source.
Vitamin D is converted from cholecalciferol form to 25 hydroxyvitamin D in the liver. In the kidney, this is converted into 1,25 dihydroxyvitamin D, with PTH acting as a catalyst.

Question 10

where and by what mechanisms does calcium absorption in the gut usually occur?

Answer 10

Mainly in the duodenum and jejunum.
Occurs through:
Passive diffusion - which tends on luminal Calcium concentration
Cell mediated active transport - Depends on 1,25 Dihydroxy vitamin D

Question 11

describe the distribution of calcium in the body

Answer 11

Majority (99%) in mineral phase of bone
Extracellular ionised calcium
Intracellular cytosolic Calcium

Question 12

What are the 3 forms that calcium exists in in serum calcium?

Answer 12

Ionised/free/unbound
Bound to albumin
Complexed with other ions

Question 13

what is the population reference range for total calcium?

Answer 13

2.2-2.6

Question 14

What produces PTH and what control its production?

Answer 14

4 small, parathyroid glands produce PTH.

PTH release is regulated by the levels of free/ionised calcium detected by Calcium Sensing Receptors.

Question 15

what type of receptors are calcium sensing receptors and where are they found?

Answer 15

G coupled protein receptors

Found in kidneys, PT glands, thyroid C cells, brain, GI tract

Question 16

what effect does 1,25 hydroxyvitamin D have?

Answer 16

increase calcium absorption in gut

Increases bone resorption

Question 17

when may calcium absorption in the body increase?

Answer 17

If dietary levels fall

During growth, pregnancy and breastfeeding

Question 18

what hormones are involved in phosphate metabolism/homeostasis?

Answer 18

PTH
1,25 DHCC
FGF 23 (fibroblast growth factor)

Question 19

what effect do PTH and FGF 23 have on renal reabsorption of phosphate?

Answer 19

They inhibit reabsorption (so more is excreted in urine)

Question 20

what factors influence phosphate serum values?

Answer 20

Dietary effects

Marked variation rhythm during the day (diurnal) - lowest is before noon, peak after midnight

Question 21

what is the relationship between calcium levels and PTH secretion?

Answer 21

if calcium levels increase, PTH levels drop