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Parafollicular cells secrete calcitonin, a hormone that participates in the regulation of calcium metabolism. 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.
The calcitonin receptor (CT) is a G protein-coupled receptor that binds the peptide hormone calcitonin and is involved in maintenance of calcium homeostasis, [5] particularly with respect to bone formation and metabolism. [6] [7] [8]
The calcitonin receptor is a G protein-coupled receptor localized to osteoclasts [20] as well kidney and brain cells. It is coupled to a G s α subunit , thus stimulating cAMP production by adenylate cyclase in target cells.
The human body is in a constant state of bone remodeling. [4] Bone remodeling is a process which maintains bone strength and ion homeostasis by replacing discrete parts of old bone with newly synthesized packets of proteinaceous matrix. [5] Bone is resorbed by osteoclasts, and is deposited by osteoblasts in a process called ossification. [6]
Osteocalcin is secreted solely by osteoblasts and is thought to play a role in the body's metabolic regulation. [12] In its carboxylated form, calcium is bound directly to the bone and thus concentrates here. In its uncarboxylated form, osteocalcin acts as a hormone in the body, signalling in the pancreas, fat, muscle, testes, and brain. [13]
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]
When osteoclast-inducing cytokines are used to convert macrophages to osteoclasts, very large cells that may reach 100 μm in diameter occur. These may have dozens of nuclei, and typically express major osteoclast proteins but have significant differences from cells in living bone because of the not-natural substrate.
The plasma total calcium concentration is in the range of 2.2–2.6 mmol/L (9–10.5 mg/dL), and the normal ionized calcium is 1.3–1.5 mmol/L (4.5–5.6 mg/dL). [4] The amount of total calcium in the blood varies with the level of plasma albumin, the most abundant protein in plasma, and therefore the main carrier of protein-bound calcium in the blood.