The effect of variations in the rate of peritubular capillary removal of tubular epithelial reabsorbate on net reabsorption of fluid from the proximal tubule was studied in rat kidneys by micropuncture techniques. Elevation of the renal venous pressure induced by partial occlusion of the renal vein resulted in decreased proximal tubular reabsorption. This reduction was proportional to the decrease in renal plasma flow. During control conditions an inverse linear relationship was observed between filtration fraction (C_In/Cpah) and the half time of fluid reabsorption, suggesting that an increase in the oncotic pressure in peritubular capillaries facilitates proximal tubular fluid reabsorption. Direct evidence for this hypothesis was obtained in free flow and split drop experiments in which the peritubular capillaries were perfused through micropipettes, with fluid of different oncotic pressures. When the capillary perfusate consisted of colloid-free Ringer’s proximal reabsorption was markedly depressed. AdditioC of 8 g/l00 ml of high molecular Dextran to Ringer’s restored tubular reabsorptive capacity to control levels. The results obtained in these studies demonstrate that a significant portion of tne overall net movement of fluid depends upon the rate of vascular removal of the epithelial reabsorbate.

ExtractAlbumin concentration in the proximal tubules of normal rats and rats with aminonucleoside nephrosis (AMN) was estimated by means of a sensitive radioimmunoassay. Nephrotic animals were studied at the onset of proteinuria and when the nephrotic syndrome was fully established. Albumin concentration in the proximal tubule of normal male rats averaged 2.5 ± 0.3 mg/100 ml, whereas in early nephrotic rats it averaged 4.9 ± 1.3 mg/100 ml and in normal sized tubules of heavily proteinuric rats, 15.7 ± 3.3 mg/100 ml. Control and early AMN animals appeared to have a relatively homogeneous population of surface convolutions, whereas the fully nephrotic rats had markedly increased heterogeneity. A population of dilated proximal tubules with an albumin concentration of 233.8 ± 35.4 mg/100 ml was found in this group. Albumin concentrations were randomly distributed throughout the proximal convolution of all three groups with no clear pattern of changing concentration along the nephron. The data demonstrate that enhanced glomerular permeability to albumin occurs in the experimental nephrotic syndrome in the rat. Speculation

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