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calcium in the body

• 1000g in total
• 99% in bone mainly as the calcium phosphate compound hydroxyapatite
• calcium means total; Ca2+ means ionized calcium
• cytosolic Ca2+ < 100 nM
• calcium in interstitial fluid 1.5 mM
• calcium in blood 2.5 mM

calcium in blood

• usually total calcium and albumin are measured and an adjusted value can be calculated if albumin deviates from 40g/l
• ionised Ca2+ can be measured with an ion-selective electrode
• acidosis increases plasma Ca2+ because H+ binds to albumin displacing Ca2+
• alkalosis has the opposite effect
• increases in blood phosphate will decrease Ca2+

calcium balance

• intestine : carrier mediated, regulated by 1,25 (OH)2D3
• formation and reabsorption of bone controlled by parathyroid hormone
• Renal : 90% reabsorbed in proximal tubule coupled to Na+; active transport in distal tubule/collecting duct regulated by 1,25(OH)2D3 and parathyroid hormone

parathyroid hormone

• single polypeptide chain of 84 amino acids
• secreted by chief cells of the parathyroid glands
• 1-5 pmol/l in blood
• metabolised by liver and kidney to inactive fragments which can accumulate to higher concentrations than the intact peptide
• presensce of amino terminus critical for activity

secretion

• low extracellular Ca2+ stimulates parathyroid hormone secretion
• high extracellular Ca2+ inhibits parathyroid hormone secretion
• regulation of secretion is mediated by a calcium sensing G-protein- linked receptor on the chief cells
• bimnding of Ca2+ lowers cyclic AMP

action

• parathyroid hormone raises blood calcium
• the receptor for parathyroid hormone is 7-transmemrane G protein receptor linked to both Gas and Gaq

acts on bone

• osteoblasts cause bone formation
• during growth some osteoblasts get trapped and differentiate into osteocytes
• osteoclasts are large multinuclear cells that reabsorb bone

cause bone reabsorption

• parathyroid hormone receptor is on osteoblasts
• osteoblasts release M-CSF and form…