Tubular capacity for phosphate reabsorption in superficial and deep nephrons

The present studies were performed to determine the capacity for phosphate reabsorption in superficial and deep nephron proximal tubules in vivo. Micropuncture experiments were performed in 20 acutely thyroparathyroidectomized (TPTX) Munich-Wistar rats fed a normal phosphate diet (0.7%). Four groups were infused with differing amounts of phosphate (0,2,4, or 6 mumol/min) to increase the filtered phosphate load. The sites selected for micropuncture were the superficial early distal tubule and the deep nephron loop of Henle, which reflect fractional phosphate delivery (FDPi%) from superficial and deep nephron proximal tubules, respectively. In response to phosphate infusions, plasma phosphate increased from 3.03 +/- 0.09 to 7.01 +/- 0.58 mM, and fractional phosphate excretion rose from 2 +/- 1 to 58 +/- 5%. FDPi% increased from both superficial (14 +/- 1 to 58 +/- 2%) and deep nephron proximal tubules (4 +/- 1 to 27 +/- 5%) but always remained lower from deep nephrons, reflecting more avid reabsorption by deep nephron proximal tubules. The maximal rate of phosphate reabsorption (max RPi/SNGFR) in the superficial proximal tubule was significantly less than in the deep nephron proximal tubule (3.2 +/- 0.4 vs. 5.1 +/- 0.1 pmol/nl). In seven of the phosphate-infused rats, parathyroid hormone (PTH, 33 U/kg bolus; 1 U X kg-1 X min-1) was added to the infusion following the initial collections. In the presence of PTH, the RPi/SNGFR was significantly lower in deep than in superficial proximal tubules (0.4 +/- 0.5 vs. 1.6 +/- 0.4 pmol/nl). Thus, the maximum capacity for phosphate reabsorption was greater in deep than in superficial nephrons in TPTX rats. Furthermore, in the presence of phosphate infusions, PTH inhibited phosphate reabsorption to a greater extent in deep than in superficial proximal tubules.