Metabolism of parathyroid hormone (PTH). The PTH gene is located on chromosome 11p15. PTH messenger RNA (mRNA) is transcribed from the DNA fragment and then translated into a 115 amino acid-containing molecule of prepro-PTH. In the rough endoplasmic reticulum, this undergoes hydrolysis to a 90 amino acid-containing molecule, pro-PTH, which undergoes further hydrolysis to the 84 amino acid-containing PTH molecule. PTH is then stored within secretory granules in the cytoplasm for release. PTH is metabolized by hepatic Kupffer cells and renal tubular cells. Transcription of the PTH gene is inhibited by 1,25-dihydroxy-vitamin D3, calcitonin, and hypercalcemia. PTH gene transcription is increased by hypocalcemia, glucocorti-coids, and estrogen. Hypercalcemia also can increase the intracellular degradation of PTH. PTH release is increased by hypocal-cemia, ^-adrenergic agonists, dopamine, and prostaglandin E2. Hypomagnesemia blocks the secretion of PTH [7,12]. VDR— vitamin D receptor; VDRE—vitamin D responsive element. (Adapted from Tanaka and coworkers [12].)

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