Figure 517

Physiologic response to hypocalcemia. Hypocalcemia stimulates both parathyroid hormone (PTH) release and PTH synthesis. Both hypocalcemia and PTH increase the activity of the 1-a-hydroxylase enzyme in the proximal tubular (PT) cells of the nephron, which increases the synthesis of 1,25-dihy-droxy-vitamin D3 (1,25(OH)2D3). PTH increases bone resorption by osteoclasts. PTH and 1,25(OH)2D3 stimulate Ca reabsorption in the distal convoluted tubule (DCT). 1,25(OH)2D3 increases the fractional absorption of dietary Ca by the gastrointestinal (GI) tract. All these mechanisms aid in returning the serum Ca to normal levels [1].

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