Indirect calcium-level-increasing actions

Indirect calcium level-increasing actions of PTH on the gut are mediated through the vitamin 0 system. Following synthesis in the skin and hydroxylation by the liver, 25(OH)Vit03 is converted... 

Parathyroid hormone is a single-chain polypeptide of 84 amino acids which is produced in the parathyroid glands. It increases serum calcium and decreases serum phosphate. In bone it promotes resorption of calcium. It indirectly increases osteoclastic activity by promoting the action of osteoblasts. It has been shown that in low doses PTH may even increase bone formation without stimulating bone resorption. In the kidney PTH increases resorption of calcium and it increases excretion of phosphate. An other important activity in the kidney is the enhanced synthesis of 1,25-dihydroxyvitamin D. An increased serum calcium level inhibits PTH secretion and increased serum phosphate decreases free serum calcium and thus stimulates PTH secretion. 

Q4 The parathyroids produce a peptide hormone, PTH, which controls the level of calcium in the body. A sensor on the surface of the parathyroid cells monitors blood calcium concentration and PTH is secreted in response to a fall in plasma calcium ion concentration. An increase in the level of PTH leads to hypercalcaemia (raised blood calcium) conversely, a reduction in the level of PTH leads to hypocalcaemia. PTH acts on the kidney to reduce reabsorption of phosphate and at the same time to increase reabsorption of calcium. In addition, it promotes the release of calcium and phosphate into the blood by activating osteoclasts, which break down the inorganic matrix of bone. PTH also increases the absorption of calcium by the mucosal cells of the intestine. The latter is a rather slow, indirect action mediated by PTH stimulation of calcitriol secretion by the kidney. 

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