A micropuncture study of renal sodium retention in nephrotic syndrome in rats: evidence for increased resistance to tubular fluid flow.

Micropuncture studies were carried out in surface nephrons of rats with nephrotoxic-serum (NTS)-induced nephrotic syndrome during a period of active sodium and water retention. It was found that hydrostatic pressure and tubular diameter were increased in the proximal tubules (13.4 +/- 0.2 vs. 10.4 +/- 0.2 mm Hg; 31.3 +/- 0.9 vs. 18.4 +/- 0.7 mu), whereas pressure and tubular diameter were normal in the distal tubules. Single nephron glomerular filtration rate (SNGFR) was decreased and fractional reabsorption of fluid was markedly increased in the proximal tubules (74.1 vs. 61.7%). The increased pressure gradient between the proximal and distal tubules suggests a condition of increased resistance to flow between the proximal and distal tubules. Microinfusion of proximal tubules with an isotonic "equilibrium" solution led to little or no rise in intratubular pressure in normal rats but it led to a significant rise in nephrotic rats. When proximal tubules of normal rats were infused with a solution containing 100 mg/100 ml albumin, pressure rose to levels observed in nephrotic rats. The mechanism of the increased resistance to flow appeared to be related, therfore, to the presence of protein in the tubular fluid. Sodium retention in the nephrotic animals might be attributed to the reduction in GFR. In other types of renal disease in animals and man with comparable or greater reductions in GFR, sodium retention does not occur, however, and fractional excretion of sodium in the urine is increased in proportion to the reduction in GFR. Thus, the rise in proximal fractional reabsorption secondary to impaired fluid flow could be an important factor in the sodium retention of this disease.