Phosphate transport in superficial and deep nephrons in phosphate-loaded rats

We tested the hypothesis that greater phosphate delivery from deep nephrons than from superficial nephrons contributes to the addition of phosphate to the collecting system during phosphate loading. In the first group of eight anesthetized Munich-Wistar rats infused with phosphate and parathyroid hormone (PTH), fractional delivery of phosphate (FDP%) from superficial distal tubules was 56 +/- 6%, significantly less than the amount appearing in the urine, 67 +/- 6% (P less than 0.01). In the second group of six rats, we determined whether this addition of phosphate could be accounted for by a higher FDP% from the deep nephrons. Free-flow micropuncture collections were taken from deep nephrons (ascending limb of the loop of Henle in the papilla), superficial nephrons (distal tubules in the cortex), and urine (duct of Bellini). The FDP% to the ascending limb of the loop of Henle in deep nephrons was 78 +/- 10%, significantly greater than to the distal convoluted tubules in superficial nephrons, 51 +/- 6% (P less than 0.005), and the fractional excretion of phosphate in urine, 72 +/- 10% (P less than 0.05). Although a difference between FDP% in superficial and deep nephrons due to reabsorption in the ascending limb of the loop of Henle cannot be ruled out from the present data, other studies indicate that this interpretation is unlikely. We conclude that greater phosphate delivery by deep nephrons contributes to the addition of phosphate to the collecting system of phosphate-loaded rats.