When increased in the serum, which of the following stimulates the synthesis of 1,25-dihydroxycholecalciferol?

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Quizplus · Accepted Answer

Parathyroid hormone (PTH) stimulates the conversion of 25-hydroxycholecalciferol to 1,25-dihydroxycholecalciferol in the kidneys, increasing its synthesis. Parathyroid hormone (PTH) is synthesized and secreted by chief cells of the parathyroid glands in response to systemic Ca2+ levels. PTH acts by binding to cAMP- and PLCg-activating plasma membrane receptors, initiating a cascade of reactions that culminates in the biological response. There are 2 receptors that recognize PTH identified as the PTH-1 and PTH-2 receptors (PTH1R and PTH2R). The body response to PTH is complex but is aimed in all tissues at increasing Ca²⁺ levels in extracellular fluids. PTH induces the dissolution of bone by stimulating osteoclast activity, which leads to elevated plasma Ca²⁺ and phosphate. In the kidney, PTH reduces renal Ca²⁺ clearance by stimulating its reabsorption; at the same time, PTH reduces the reabsorption of phosphate and thereby increases its clearance. Finally, PTH acts on the liver, kidney, and intestine to stimulate the production of the steroid hormone 1,25-dihydroxycholecalciferol (calcitriol), which is responsible for Ca²⁺ absorption in the intestine.

When Increased in the Serum, Which of the Following Stimulates