Calcium Homeostasis

Calcium Homeostasis - Hormonal Control of Calcium...

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Hormonal Control of Calcium Homeostasis Calcium homeostasis is important because all physiological processes directly or indirectly involves calcium ions (Ca2+). Thus, the calcium ion levels are maintained strictly between 9mg-11mg/100cc of blood When the blood levels rise above 11mg/100cc of blood, it’s termed HYPERCALCEMIA When the blood levels fall below 9mg/100cc of blood, it’s termed HYPOCALCEMIA 2 hormones involved in maintaining calcium homeostasis are associated with the thyroid gland: Parathyroid hormone released by the parathyroid glands embedded in the posterior aspect of the thyroid hormone Calcitonin released by the parafollicular cells (also known as “C” cells) in the thyroid gland
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Parathyroid hormone (PTH) 84 amino-acid peptide hormone released by the chief cells inside the parathyroid glands. Major stimuli for PTH release: HYPOCALCEMIA; hyperphosphatemia PTH release requires Magnesium PTH binds to PTH receptors on target cells to activate the cAMP-dependent signal transduction mechanism
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Biological Actions of PTH In Bones : PTH INDIRECTLY activates osteoclasts to stimulate bone resorption resulting in the release of calcium and phosphate ions. Osteoclasts lack PTH receptors; Osteoblasts express PTH receptors hence, PTH binds to PTH receptors on osteoblasts and stimulate the osteoblasts to secrete OSTEOCLAST ACTIVATING FACTORS (OAFs). OAFs activate osteoclasts to stimulate bone resorption. In the Kidneys : PTH inhibits calcium excretion and stimulates calcium reabsorption; PTH stimulates the excretion of phosphate ions by the kidneys = phosphaturia. This drops the phosphate levels in blood which prevents calcium phosphate crystals from depositing in tissues/organs. PTH activates 1 alpha hydroxylase , an enzyme produced by the kidneys, which converts 25 hydroxyvitamin D to the more active form, 1,25 hydroxyvitamin D =1,25 (OH) 2 Vit D which is the only hormone that directly stimulates calcium absorption from the small intestine.
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Synthesis of 1,25 (OH) 2 VIT D = cholecalceferol
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Biological Actions of 1,25 (OH) 2 Vit D Vitamin D receptors are nuclear receptors that form heterodimers with RXR at the VRE 1. Directly absorbs calcium and phosphate ions from the GI tract ( from the diet) into blood Synergizes with PTH to stimulate calcium reabsorption from the
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Calcium Homeostasis - Hormonal Control of Calcium...

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