Which mechanism explains how 1,25-dihydroxyvitamin D increases serum calcium?

Which mechanism explains how 1,25-dihydroxyvitamin D increases serum calcium?

• A. Decreases intestinal absorption
• B. Increases intestinal absorption via calbindin
• C. Inhibits bone resorption
• D. Increases renal calcium excretion

Answer: (B)

Calcitriol raises serum calcium mainly by increasing intestinal calcium absorption. It acts on enterocytes in the gut by binding the vitamin D receptor and upregulating calcium-transporting proteins, notably calbindin, and calcium channels like TRPV6. Calbindin binds Ca2+ inside the cell and shuttles it to the basolateral membrane, where it is pumped into the bloodstream. This enhanced transcellular movement of calcium increases how much calcium enters the circulation after meals or during deficiency. While calcitriol can influence bone cells and renal reabsorption under certain conditions, the primary mechanism for elevating serum calcium is the boosted intestinal uptake via calbindin and related transport pathways.

Calcitriol raises serum calcium mainly by increasing intestinal calcium absorption. It acts on enterocytes in the gut by binding the vitamin D receptor and upregulating calcium-transporting proteins, notably calbindin, and calcium channels like TRPV6. Calbindin binds Ca2+ inside the cell and shuttles it to the basolateral membrane, where it is pumped into the bloodstream. This enhanced transcellular movement of calcium increases how much calcium enters the circulation after meals or during deficiency. While calcitriol can influence bone cells and renal reabsorption under certain conditions, the primary mechanism for elevating serum calcium is the boosted intestinal uptake via calbindin and related transport pathways.