Transtubular oncotic pressure gradients and net fluid transport in isolated proximal tubules

Abstract

A number of studies have shown that peritubular oncotic forces can directly affect net transport of fluid out of the renal proximal tubule. In a previous report it was shown that peritubular protein concentration does influence net reabsorption out of the isolated perfused proximal convoluted tubule (PCT) without the necessary juxtaposition of the capillary bed. The current studies examine three additional specific issues: Is the protein effect mediated through oncotic forces? Does protein exert a symmetrical effect across the PCT? To what extent is the protein effect dependent on the underlying active transport processes? Using the technique in which rabbit PCT's were perfused in vitro, the following results were obtained: When 3.5 g/100 ml of highly purified polyvinylpyrrolidone (PVP) was added to a bath of serum and the tubules were perfused with an isosmolal ultrafiltrate of the same serum, there was a 67 ± 12% increase in net reabsorption (N=6). When the same tubules were perfused in random order with either isosmolal ultrafiltrate or rabbit serum against a bath of serum, the net reabsorption remained unchanged, 1.07 ± 0.18 and 1.07 ± 0.19 nl/mm min, respectively (N=10). Fractional net water flux (observed change / control) remained the same when 3.5 g/100ml of PVP was added to bath of control serum and serum with 10-5M ouabain (N=15). From these studies at least part of the protein effect is mediated through oncotic forces (though evidence for an additional, more specific effect of proteins on integrity of PCT epithelium is also postulated); the PCT is an anisotropic membrane (rectifier) with respect to protein effects; and active transport pathways are 'coupled' to pathways which are responsive to oncotic forces.