What is the action of PTH in the kidneys?

• A. Decreases calcium reabsorption
• B. Increases tubular resorption of calcium and stimulates kidney to make activated vitamin D
• C. Inhibits vitamin D activation
• D. Reduces phosphate reabsorption

Answer: (B)

PTH acts on the kidney to increase calcium reabsorption in the distal nephron and to stimulate the production of the active form of vitamin D. In the distal tubule, PTH enhances calcium reabsorption by upregulating transport proteins that move calcium from the tubular fluid back into the blood. In the proximal tubule, it increases the activity of 1-alpha-hydroxylase, converting 25(OH) vitamin D to 1,25(OH)2 vitamin D (calcitriol), which raises intestinal calcium absorption and helps raise serum calcium. It’s true that PTH also decreases phosphate reabsorption, but the combination of augmented calcium reabsorption and activated vitamin D production best captures its principal renal actions.