Calcitonin

Thyroid Factor

The Natural Thyroid Diet

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Cells of Origin

Calcitonin is sometimes also called thyrocalcitonin to describe its origin in the parafollicular cells of the thyroid gland. These cells, which are also called C cells, occur singly or in clusters in or between thyroid follicles. They are larger and stain less densely than follicular cells in routine preparations (Fig. 12). Like other peptide hormone-secreting cells, they contain membrane-bound storage granules. Parafollicular cells arise embryologi-cally from neuroectodermal cells that migrate to the last branchial pouch, and, in submammalian vertebrates, give rise to the ultimobranchial glands.

Biosynthesis, Secretion, and Metabolism

Calcitonin consists of 32 amino acids and has amolecu-lar weight of about 3400. Except for a seven-member

Ca2+

Ca2+

FIGURE 11 Regulation of parathyroid hormone secretion by calcium (Ca2+). Heptihelical calcium receptors (CaR) on the surface of chief cells communicate with Ca2+ channels, adenylyl cyclase (AC), and phospholipase C (PLC) by way of guanosine nucleotide binding proteins (Gi and Gq). The resulting increase in Ca2+ and inhibition of adenylyl cyclase lowers cyclic AMP (cAMP) and interferes with protein kinase A (PKA) mediated events that lead to secretion. DAG, diacyl glycerol; IP3; inositol trisphosphate.

FIGURE 11 Regulation of parathyroid hormone secretion by calcium (Ca2+). Heptihelical calcium receptors (CaR) on the surface of chief cells communicate with Ca2+ channels, adenylyl cyclase (AC), and phospholipase C (PLC) by way of guanosine nucleotide binding proteins (Gi and Gq). The resulting increase in Ca2+ and inhibition of adenylyl cyclase lowers cyclic AMP (cAMP) and interferes with protein kinase A (PKA) mediated events that lead to secretion. DAG, diacyl glycerol; IP3; inositol trisphosphate.

FIGURE 12 Low-power photomicrograph of a portion of the thyroid gland of a normal dog. Parafollicular (C) cells are indicated in the walls of the follicles. (From Ham AW, Cormack DH. Histology, 8th ed., Lippincott, Philadelphia, 1979, p 802.)

the mature products have no common amino acid sequences (Fig. 13).

Physiologic Actions of Calcitonin

Calcitonin escaped discovery for many years because no obvious derangement in calcium balance or other homeostatic function results from deficient or excessive production. Thyroidectomy does not produce a tendency toward hypercalcemia, and thyroid tumors that secrete massive amounts of calcitonin do not cause hypocalce-mia. Attention was drawn to the possible existence of a calcium-lowering hormone by the experimental finding that direct injection of a concentrated solution of calcium into the thyroid artery in dogs caused a more rapid fall in blood calcium than parathyroidectomy. Indeed, calcito-nin promptly and dramatically lowers the blood calcium concentration in many experimental animals. Calcitonin is not a major factor in calcium homeostasis in humans, however, and does not participate in minute-to-minute regulation of blood calcium concentrations. Rather, the importance of calcitonin may be limited to protection against excessive bone resorption.

Actions on Bone

Calcitonin lowers blood calcium and phosphate primarily, and perhaps exclusively, by inhibiting osteo-clastic activity. The decrease in blood calcium produced by calcitonin is greatest when osteoclastic bone resorption is most intense and is least evident when osteoclastic activity is minimal. Binding of calcitonin by G-protein-coupled receptors on the osteoclast surface promptly increases cyclic AMP formation, and within minutes the expanse and activity of the ruffled border diminishes. Osteoclasts pull away from the bone surface and begin to dedifferentiate. Synthesis and secretion of lysosomal enzymes is inhibited. In less than an hour, fewer osteoclasts are present, and those that remain have decreased bone-resorbing activity.

Osteoclasts are the principal, and probably only, target cells for calcitonin in bone. Osteoblasts do not have receptors for calcitonin and are not directly affected by it. Curiously, although they are uniquely expressed in either osteoblasts or osteoclasts, receptors for PTH and calcitonin are closely related and have about one-third of their amino acid sequences in common, suggesting they evolved from a common ancestral molecule. Because of the coupling phenomenon in the cycle of bone resorption and bone formation

mRNA

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