Calcium homeostasis 2 Flashcards
How much calcium does a young adult body contain?
1100g.
What is extracellular calcium necessary for?
Nerve function, muscle contraction, coagulation, skeletal mineralization and activation of most cell types (signalling pathways).
What is the physiologically active calcium?
It is unbound.
What forms does calcium take in the body?
Complexes with anions, lactate, phosphate, urate, sulfate, bicarbonate, bound to albumin and unbound.
What percentage of calcium in the body is unbound?
45%.
When are calcium requirements higher?
During pregnancy and nursing.
How is calcium excreted?
Through urine and feces.
If 20mmol of calcium is taken in per day, how much is excreted via the feces?
18mmol.
What does calcitonin do and what is it?
It’s a hormone secreted by the thyroid that lowers the blood calcium.
What does parathyroid hormone do?
It increases calcium levels when they are too low.
What is calcitriol?
An active form of vitamin D that is essential for calcium absorption and bone mineralization.
What is the structure of parathyroid hormone?
It is a single chain polypeptide made from 84 amino acids that has a molecular weight of 9500. It is derived from the larger precursor peptides pre-proPTH and proPTH.
Where is parathyroid hormone produced?
Chief cells of the parathyroid gland.
What are the normal plasma levels of PTH?
10-55 pg/ml.
How is PTH secretion regulated on a short-term scale?
Calcium ions act on the G protein coupled calcium sensing receptor. If there is decreased free plasma calcium there is an increase in PTH secretion.
How is PTH secretion regulated on a long term scale?
1,25(OH)2D3 acts directly on the PTH to decrease preproPTH mRNA.
What are the organs in which PTH acts to increase plasma calcium levels?
The kidney, the bone and the GI tract.
What are the ways in which PTH acts on the kidneys to increase plasma calcium levels?
It stimulates calcium reabsorption in the distal tube, it inhibits PO43- reabsorption in the proximal tubule and increases the activity of 1-alpha-hydroxylase and decreases 2,4-hydroxylase and the net effect is a gradual increase in renal production of 1,25(OH)2D3.
What are the ways in which PTH acts on the bone to increase plasma calcium levels?
It stimulates the rapid efflux of calcium from freely exchangeable calcium pool (an effect on osteocytes and bone-lining cells) and increases the number and activity of osteoclasts via action on osteoblasts which causes a gradual increase in bone resorption (calcium and PO43- release).
What are the ways i which PTH acts on the GI tract to increase plasma calcium levels?
It stimulates the absorption of calcium and phosphorus ions. The effect is delayed and indirect.
What is 1,25(OH)2D3?
The active metabolite of vitamin D3 it is a secosteroid (open B ring).
Where is 1,25(OH)2D3 produced?
In the kidney by 1-alpha hydroxylation of 25(OH)D3.
What is transcalciferin?
A vitamin D-binding protein that 1,25(OH)2D3 is bound to.
What happens to vitamin D3 in the body?
It is rapidly converted in the liver to 25(OH)D3.