Effect of peritubular protein concentration on reabsorption of sodium and water in isolated perfused proxmal tubules.

Micropuncture studies have indicated that variation in peritubular oncotic pressure influences net transport of fluid out of the proximal tubule. The present in vitro studies on isolated perfused rabbit proximal convoluted tubules were designed to examine whether protein concentration gradient must act across the peritubular capillary membrane to influence reabsorption, or whether it can exert a direct effect across the tubular basement membrane 71 proximal tubules were perfused with ultrafiltrate made isosmolal to bathing fluids, the latter having identical electrolyte composition as the perfusing ultrafiltrate, but adjusted to three oncotic pressures: hypooncotic, protein 0.0 g/100 ml; control isooncotic serum, protein 6.4 g/100 ml; and hyperoncotic, protein 12.5 g/100 ml. Net volume flux (nl/mm per min), net Na flux (nEq/mm per min), unidirectional Na flux from bath to lumen (nEq/mm per min), and passive permeability coefficient (x 10(-5) cm/sec) for Na (P(Na)), urea (P(urea)), and sucrose (P(sucrose)) were determined using isotopic techniques. When the bath was hypooncotic, there was (as compared with isooncotic serum) a significant decrease in net volume (38%) and net sodium (40%) flux, but no change in P(Na), P(urea), or transtubular potential; however, P(sucrose) increased significantly (78%). In experiments in which hyperoncotic bath was used, there was (compared with isooncotic serum) an increase in net volume (28%) and net sodium (30%) flux, but transtubular potential difference did not change significantly. These data demonstrated that changes in the ambient protein concentration gradient exert direct effects upon proximal tubular reabsorption. Because penetration of sucrose (an index of intercellular movement) but not urea (an index of transcellular movement) varied with changes in tubular reabsorption, it is suggested that oncotic pressure acts by altering the rate of back-leak of reabsorbate through extracellular pathways between tubular cells. It is concluded that an effect of protein concentration on reabsorption can be exerted directly across the basement membrane, without necessary interposition of the capillary bed.