Calcitonin secretion, regulation

Thyroid gland secretes two important hormones, thyroxine (TJ and triiodothyronine (Tj). The third hormone, calcitonin secreted from interstitial cells is physiologically different and is responsible for the regulation of calcium metabolism. 

Regulation of calcitonin secretion is controlled by calcium levels in the blood. The early studies are in accord with this concept. Care (Cl) showed by direct assay of venous eflSuent from perfused pig thyroid that calcitonin release is a function of the calcium level in the perfusate. 

The parathyroids are small, but important, glands involved in regulation of blood calcium levels together with calcitonin secreted by the thyroid gland and vitamin D. Calcitonin has a use in treating Paget s disease and osteoporosis (see Chapter 7). 

REGULATION OF SECRETION Calcitonin secretion increases with hypercalcemia and decreases when plasma Ca " is low the hormone in circulation has a t of 10 minutes. Circulating concentrations are normally low (<15 pg/mL in males and 10 pg/mL in females) but can be markedly elevated with C cell hyperplasia or medullary thyroid cancer. 

Three hormones regulate turnover of calcium in the body (22). 1,25-Dihydroxycholecalciferol is a steroid derivative made by the combined action of the skin, Hver, and kidneys, or furnished by dietary factors with vitamin D activity. The apparent action of this compound is to promote the transcription of genes for proteins that faciUtate transport of calcium and phosphate ions through the plasma membrane. Parathormone (PTH) is a polypeptide hormone secreted by the parathyroid gland, in response to a fall in extracellular Ca(Il). It acts on bones and kidneys in concert with 1,25-dihydroxycholecalciferol to stimulate resorption of bone and reabsorption of calcium from the glomerular filtrate. Calcitonin, the third hormone, is a polypeptide secreted by the thyroid gland in response to a rise in blood Ca(Il) concentration. Its production leads to an increase in bone deposition, increased loss of calcium and phosphate in the urine, and inhibition of the synthesis of 1,25-dihydroxycholecalciferol. 

The CaR regulates numerous biological processes, including the expression of various genes (e.g., PTH) the secretion of hormones (PTH and calcitonin), cytokines (MCP-1), and calcium (e.g., into breast milk) the activities of channels (potassium channels) and transporters (aquaporin-2) cellular shape, motility (of macrophages), and migration cellular adhesion (of hematopoietic stem cells) and cellular proliferation (of colonocytes), differentiation (of keratinocytes), and apoptosis (of H-500 ley dig cancer cells) [3]. 

Calcium is the principal extracellular electrolyte regulated by PTH, calcitonin, and D3. Extracellular calcium is a critical component of signal transduction across the plasma membrane, which regulates a wide spectrum of physiological events including muscle contraction, secretion of neurotransmitters and hormones, and the ac-... 

The normal thyroid gland secretes sufficient amounts of the thyroid hormones—triiodothyronine (T3) and tetraiodothyronine (T4, thyroxine)—to normalize growth and development, body temperature, and energy levels. These hormones contain 59% and 65% (respectively) of iodine as an essential part of the molecule. Calcitonin, the second type of thyroid hormone, is important in the regulation of calcium metabolism and is discussed in Chapter 42. 

Durham, P.L., and Russo, A.F. Regulation of calcitonin gene-related peptide secretion by a serotonergic antimigraine drug, J. Neurosci. 1999, 19, 3423-3429. 

Parathyroid hormone (PTH), vitamin D and calcitonin work in synchrony to regulate calcium homeostasis (not presented in tables). PTH is an 84 amino acid chain secreted by the parathyroid glands in response to low serum calcium. PTH induces bone resorption, which liberates calcium into the bloodstream. These actions are dependent on adequate serum concentrations of 1,25-dihydroxy cholecalciferol (a derivative of vitamin D). Bone resorption is counterregulated by calcitonin, which inhibits osteoclasts (the cells which degrade bone). 

The majority of the thyroid giand contains follicular cells responsible for the production of the thyroid hormones. A second population of endocrine cells within the thyroid known as C (clear) cells, or parafollicular cells, produce the hormone calcitonin (CT), which has an opposing action to that of PTH in that it decreases the Ca concentration in body fluids. It accomplishes this by inhibiting the activity of osteoclasts (i.e., decreasing Ca release from bone by inhibiting bone resorption). The actual biosynthesis and release of CT is regulated by the concentration of Ca in plasma (i.e., when it is high, CT secretion increases). 

The parathyroid glands secrete parathyroid hormone (PTH) that regulate calcium levels in the blood. Calcitonin increases the level of PTH. 

Fig. 5-14. Regulation of calcium levels in blood. A decrease in the calcium levels in blood stimulates parathormone (PTH) secretion and inhibits calcitonin (TCT) secretion and vice versa when the blood calcium increases...

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