Modulation of proximal tubular hydraulic conductivity by peritubular capillary oncotic pressure

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Abstract
Fluid absorption from the proximal tubular lumen is probably a multifactorial process. A transepithelial hydrostatic and oncotic pressure difference may be the driving force for as much as 30% of the reabsorbed fluid; during saline volume expansion proximal tubular reabsorption declines. Whether this reduction could be caused by changes in the passively driven flux component was investigated [in rats]. The hydraulic conductivity was determined from the reabsorptive rate in split oil droplets with normal and high hydrostatic pressure gradients across the wall; the peritubular capillary net-work was perfused with solutions containing a colloid of high or low concentration. The split oil droplet radius and the relationship between this and pressure were determined; this was 7.3 mm Hg pressure increase/.mu.m increase in radius. The increase in the rate of reabsorption from the droplets due to increased intraluminal hydrostatic pressure was 1.02 .+-. 0.13 nl/min per mm tubular length when a solution with a high colloid concentration was perfused through the capillary net-work; this was compared with 0.41 = 0.11 nl/min per min tubular length when a low colloid containing solution was used for perfusion. The hydraulic conductance in the proximal tubular wall at high colloid perfusion was calculated to be 0.54 nl/min .cntdot. mm .cntdot. mm Hg. This drop in hydraulic conductance might be 1 factor responsible for the decline in fluid absorption in animals exposed to saline volume expansion.