Calcitriol increases intestinal calcium absorption several-fold. Which pair of proteins induced by calcitriol mediates transcellular calcium transport across the enterocyte?

• A Apical NPT2a cotransporter and calbindin-D28k
• B Basolateral Na⁺/Ca²⁺ exchanger and the calcium-sensing receptor
• C Apical TRPV6 channel and intracellular calbindin-D9k ✓
• D Apical ENaC channel and cytosolic calmodulin

Explanation

Calcitriol binds the nuclear vitamin D receptor in enterocytes and induces TRPV6, an apical calcium entry channel, together with calbindin-D9k, which buffers and shuttles calcium to the basolateral Ca²⁺-ATPase for extrusion. NPT2a is a proximal tubular phosphate cotransporter downregulated by PTH and FGF23, so option A mixes up organs and ions.

Reference: Ganong's Review of Medical Physiology, 26th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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