calcium - Calcium: Homeostasis Bone stores 99% of the total...

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Calcium: Homeostasis ? Bone stores 99% of the total body calcium. ? Blood level of calcium ions is very carefully monitored – a small change in levels can lead to cardiac or respiratory arrest if levels go high or low respectively. ? Calcium also required for nerve function. ? Bone acts as a ‘buffer’ to blood calcium levels; reabsorption occurs when calcium levels are too high whilst low levels of calcium cause release of calcium from the bone pool. ? Calcium metabolism is intimately linked to phosphate metabolism as phosphate concentrations also determine extracellular and intracelluar processes and skeletal mineralisation. ? Calcium in blood is 50% free; whilst 40% is bound to albumin; the remaining 10% is complexed to diffusible anions, of which phosphate is one. ? The binding of positively charged calcium ions to negatively charged anions is pH dependent 4 Hormones Regulating Mineral Metabolism: PTH Parathyroid Hormone: PTH ? Synthesised and secreted by the four parathyroid glands located bilaterally on or near the thyroid gland. ? Synthesised as a pre-pro-PTH precusor which is intracellularly processed to produce intact PTH (84 amino acid polypeptide) which is stored until secreted. ? PTH secretion is primarily regulated by calcium ions by calcium-sensing receptors on the membrane of the parathyroid cells. ? Increase in extracellular free calcium inhibits PTH synthesis and secretion; whilst a decrease has the reverse effect. ? PTH is also under the influence of vitamin D and magnesium. ? PTH influences calcium and phosphate homeostasis directly through its action on bone and kidney and indirectly on the intestine through vitamin D. 5 Hormones Regulating Mineral Metabolism: Vitamin D Vitamin D and its metabolites: ? Vitamin D produced by exposure of human skin to ultraviolet light or acquired by ingestion of foods containing Vitamin D. ? Vitamin D metabolised to 25-OH Vit D in liver by 25-hydroylase and to 1,25-OH Vit D by 1-hyroxylase in kidneys. ? Only 1,25-OH Vitamin D is the biologically active vitamin D. ? 1,25-OH Vit D is tightly regulated by PTH and phosphate. ? 1,25-OH Vit D maintains calcium and phosphate levels via intestinal absorption. ? 1,25-OH Vit D has a direct effect on parathyroids via a Vitamin D receptor, inhibiting synthesis and secretion of PTH. 6 Hormones Regulating Mineral Metabolism: Calcitonin Calcitonin: ? 32 amino acid polypeptide secreted by the parafollicular or C cells of the
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thyroid gland. ? Regulated primarily by free calcium concentration in plasma. ? Secreted by increase in calcium levels. ? Counter-acts PTH activity. ? Decreased calcium and phosphate Hypocalcaemia: Causes ? Low serum calcium (hypocalcaemia) is due to either a reduction in the albumin-bound or free fraction. ? Common causes of hypocalcaemia:
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This note was uploaded on 02/25/2012 for the course CHEM 1341 taught by Professor Compton during the Fall '08 term at Texas State.

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calcium - Calcium: Homeostasis Bone stores 99% of the total...

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