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Calcitonin is a polypeptide hormone of 32 amino acids, with a molecular weight of 3454.93 daltons. Its structure comprises a single alpha helix. [40] Alternative splicing of the gene coding for calcitonin produces a distantly related peptide of 37 amino acids, called calcitonin gene-related peptide (CGRP), beta type. [41]
Calcitonin lowers blood levels of calcium by inhibiting the resorption of bone by osteoclasts, and its secretion is increased proportionally with the concentration of calcium. [ 7 ] Parafollicular cells are also known to secrete in smaller quantities several neuroendocrine peptides such as serotonin , somatostatin or CGRP .
Calcitriol is a hormone and the active form of vitamin D, normally made in the kidney. [ 8 ] [ 9 ] [ 10 ] It is also known as 1,25-dihydroxycholecalciferol . It binds to and activates the vitamin D receptor in the nucleus of the cell, which then increases the expression of many genes. [ 11 ]
Calcitonin gene-related peptide (CGRP) is a member of the calcitonin family of peptides consisting of calcitonin, amylin, adrenomedullin, adrenomedullin 2 and calcitonin‑receptor‑stimulating peptide. Calcitonin is mainly produced by thyroid C cells whilst CGRP is secreted and stored in the nervous system. [1]
[11] [12] It follows that an increase in vitamin D 3 intake should lead to a decrease in bone resorption — it has been shown that oral administration of vitamin D does not linearly correlate to increased serum levels of calcifediol, [13] the precursor to calcitriol. Calcitonin is a hormone secreted by the thyroid in humans.
The reduction in the blood calcitriol concentration acts (comparatively slowly) on the epithelial cells (enterocytes) of the duodenum, inhibiting their ability to absorb calcium from the intestinal contents. [2] [5] [28] [29] The low calcitriol levels also act on bone causing the osteoclasts to release fewer calcium ions into the blood plasma. [25]
Cinacalcet, sold under the brand name Sensipar among others, is a medication used to treat primary hyperparathyroidism, tertiary hyperparathyroidism and parathyroid carcinoma.
Calcitriol: secosteroid (1,25-dihydroxyvitamin D 3) skin/proximal tubule of kidneys: VDR: Active form of vitamin D 3. Increase absorption of calcium and phosphate from gastrointestinal tract and kidneys inhibit release of PTH. Calcidiol: secosteroid (25-hydroxyvitamin D 3) skin/proximal tubule of kidneys: VDR: Inactive form of vitamin D 3
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