Correlation between luminal hydrostatic pressure and proximal tubular fluid reabsorption in the rat kidney

Summary Split-drop experiments were performed to evaluate the effect of changes in luminal hydrostatic pressure on net fluid reabsorption in proximal convoluted tubules of the rat kidney. While hydrostatic pressure in control droplets averaged 28.9±1.03 mm Hg, it increased to a mean of 65.2±3.3 mm Hg during pressure elevation and fell to 10.8±1.04 mm Hg during pressure reduction. In paired measurements in identical tubules net fluid absorption changed from a control value of 2.96±0.14 nl/min·mm to 3.88±0.14 nl/min·mm when luminal pressure was elevated. During pressure reduction net fluid absorption fell from a control of 2.98±0.09 nl/min·mm to 2.26±0.13 nl/min·mm (P<0.001). This dependency of fluid absorption upon hydrostatic pressure was not greatly affected by the finding that microphotography overestimated the true intradroplet volume by 31% during control and by 30.2% and 50% during elevated and reduced pressure respectively. From the relation between the changes of net absorption and luminal hydrostatic pressure an apparent hydraulic conductance of 0.04 nl/min·mm Hg was estimated.