How does PTH affect calcium and phosphate handling in the kidney?

How does PTH affect calcium and phosphate handling in the kidney?

• A. Increases calcium reabsorption in the distal tubule and increases phosphate reabsorption in the proximal tubule
• B. Increases calcium reabsorption in the distal tubule and decreases phosphate reabsorption in the proximal tubule
• C. Decreases calcium reabsorption in the distal tubule and increases phosphate reabsorption in the proximal tubule
• D. Decreases calcium reabsorption in the distal tubule and decreases phosphate reabsorption in the proximal tubule

Answer: (B)

PTH’s action in the kidney is to raise serum calcium while helping dispose of phosphate. It increases calcium reabsorption in the distal tubule by upregulating channels and transporters (notably TRPV5) that move calcium from the tubular lumen into the blood. At the same time, it decreases phosphate reabsorption in the proximal tubule by downregulating the sodium-phosphate cotransporter NaPi-IIa, leading to more phosphate being excreted in urine. This combination—more calcium reabsorbed distally and more phosphate excreted proximally—explains why the correct pattern is higher calcium uptake in the distal nephron and lower phosphate reabsorption in the proximal nephron. (For broader context, PTH also stimulates 1α-hydroxylase to raise calcitriol, which increases intestinal calcium absorption.)

PTH’s action in the kidney is to raise serum calcium while helping dispose of phosphate. It increases calcium reabsorption in the distal tubule by upregulating channels and transporters (notably TRPV5) that move calcium from the tubular lumen into the blood. At the same time, it decreases phosphate reabsorption in the proximal tubule by downregulating the sodium-phosphate cotransporter NaPi-IIa, leading to more phosphate being excreted in urine. This combination—more calcium reabsorbed distally and more phosphate excreted proximally—explains why the correct pattern is higher calcium uptake in the distal nephron and lower phosphate reabsorption in the proximal nephron. (For broader context, PTH also stimulates 1α-hydroxylase to raise calcitriol, which increases intestinal calcium absorption.)