Response of Phosphate Transport to Parathyroid Hormone in Segments of Rabbit Nephron

The effects of bovine parathyroid hormone (PTH) on phosphate transport were examined in proximal convoluted tubules, proximal straight tubules, and cortical collecting ducts isolated from the rabbit kidney. The lumen-to-bath flux of phosphate (JlbPO4) and the bath-to-lumen flux (JblPO4) were measured simultaneously with [33P]phosphate and [32P]phosphate. In the proximal convoluted segments perfused with an ultrafiltrate of normal serum, PTH reduced the fluid absorption rate from 1.21 +/- 0.10 to 0.60 nl/mm-min but did not affect JlbPO4, which averaged 5.45 +/- 0.97 pmol/mm-min, or JblPO4, which was 0.50 +/- 0.08 pmol/mm-min. During perfusion with low bicarbonate-high chloride fluids at pH 7.4, the PTH-induced changes in fluid absorption were eliminated but no change occurred in phosphate transport. On the other hand in proximal straight segments JlbP04 was lower at 2.64 +/- 0.41 pmol/mm-min and was directly inhibited by PTH to 1.90 +/- 0.34 pmol/mm-min (P less than 0.001). Net phosphate transport was not observed in cortical collecting ducts in the presence or absence of PTH. These data suggest that phosphate absorption in the proximal tubule involves more than one transport system, that the effects of PTH on fluid absorption are not interdependent with the effects on phosphate transport, and that the proximal straight tubule appears to be an important site of PTH-sensitive phosphate transport.