AJP - Renal Physiology, Vol 248, Issue 5 729-F733, Copyright © 1985 by American Physiological Society
Tubular capacity for phosphate reabsorption in superficial and deep nephrons
A. Haramati
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.
