Phosphate reabsorption in juxtamedullary nephron terminal segments.

Adult Munich Wistar rats undergoing mild salt diuresis (NaCl 20 g x l-1, 0.1 ml x min-1) were injected with tracer doses of 3H-Inulin and 32P-sodium phosphate in thin descending and ascending limbs of Henle's loop, collecting ducts accessible at the surface of the papilla and early distal superficial tubules. Kidneys were prepared for simultaneous papillar microinjection and urinary flow collection. Expressed in percent of the amounts injected, unidirectional phosphate reabsorption fluxes were 5 +/- 1% and 3 +/- 1% for injections into early distal superficial tubules and collecting ducts, respectively. By contrast, the flux was 21.7 +/- 3% for injections into either the descending or ascending thin limbs of juxtamedullary nephrons. We conclude from these results that in the rat, a significant amount of phosphate is reabsorbed by the juxtamedullary distal tubules and/or the subsequent arcades connecting the juxtamedullary distal tubules to the collecting ducts.