Effect of peritubular oncotic pressure changes on proximal tubular fluid reabsorption.

SPITZER, ADRIAN, AND ERICH E. WINDHAGER. Efect of peritubular oncotic pressure changes on proximal tubular @id reabsorption. Am. J. Physiol. 218(4): 1188-l 193. 1970.-To evaluate directly the effect of changes in postglomerular capillary blood oncotic pressure on fluid reabsorption by proximal tubules of rat kidneys, an in vivo method of microperfusion of peritubular capillaries was devised. Net fluid reabsorption by tubules receiving their vascular supply from artificially perfused capillaries was studied by tubular micropuncture. In free-flow recollection experiments, the absolute rate of fluid reabsorption decreased by 49% when the capillaries were perfused with colloid-free Ringer solution. When the perfusate was Ringer solution containing 8 % dextran (mol wt 110,000) proximal tubular reabsorptive rates were similar to those obtained during normal blood perfusion of capillaries. A direct relationship was observed, in “split-drop” experiments, between net reabsorption and calculated oncotic pressure of the infusate. The mean reabsorptive capacity of proximal tubules increased from 0.031 nl/sec l mm during capillary perfusion with Ringer, to 0.046 during perfusion with 4oj, dextran in Ringer, and to 0.060 when the concentration of dextran was increased to 8yo.