Question

If blood calcium levels started to rise out of homeostatic range, the body would try to correct this by:

a. Releasing which hormone?
b. From where is this hormone released?
c. This hormone targets the bones. It causes which cell type to become more active (compared to another cell type).   
d. At the bones, altering the activity of the above cell type results in an increase or decrease (choose one)  in storage of calcium into the bone matrix?

e. Name one other direct target site/organ that this hormone affects?

Answer 1

a. Levels of calcium increase in blood due to increase bone resorption. This leads to hypercalcemia. However, the thyroid gland releases calcitonin in order to decrease the levels of calcium in blood. Calcitonin has an opposite action to parathyroid hormone.

b. Calcitonin is released by the parafollicular or C cells of the thyroid gland. In vertebrates that are not mammals, it is secreted by the ultimo-branchial bodies. These cells produce calcitonin as a procalcitonin, a 116 amino acid protein. It is then converted to 32 amino acid calcitonin, which is then secreted.

c. Calcitonin will inhibit the activity of osteoclasts. Osteoclasts are bone cells that degrade bone. Calcitonin will stimulate the activity of osteoclasts. Osteoclasts are bone cells that cause bone formation. Thus, calcium will be increasingly used up from blood, thereby decreasing blood calcium levels. There is also increased excretion of calcium by the kidneys.

d. As osteoclasts are inhibited, there will be decrease bone resorption. Bones will not be degraded by osteoclast. Further, calcium will be used by osteoblasts to synthesize bone. Calcium is stored in bone in the bone matrix where it is complexed with hydroxyapatite and phosphate. Hence, calcitonin will increase bone matrix formation and increased storage of calcium in bone matrix.

e. Calcitonin is also known to increase secretion of calcium by the kidneys. The kidney tubular cells have receptors for calcitonin. Thus, calcitonin will bind to these receptors and the signaling induced will cause increase excretion of calcium in urine. This will decrease blood calcium levels.