Single nephron filtration, luminal flow and tubular fluid reabsorption along the proximal convolution and the pars recta of the rat kidney as influenced by luminal pressure changes.

Intratubular pressures were measured in free flow and after blockade of tubular flow at different distances from the glomerulum in the kidney of Wistar rats. Free flow pressure was ffp = 13.3 +/- 2.5 Torr and stop flow pressure sfp = 41.7 +/- 3.8 Torr. With increasing distance of the blockade from the glomerulum the intratubular pressure decreased being 22.4 +/- 2.1 Torr, when the tubule was blocked at the end of the pars recta. In a second series single nephron filtration rate (gfr) and late proximal flow rates (V) were measured at different intratubular pressures. Free flow gfrf was 26.5 +/- 5.9 nl/min and Vf = 14.7 +/- 4.0 nl/min. The difference of these flow rates divided by tubular length results in a local reabsorption rate of C = 2.9 +/- 0.9 nl/min-mm in the proximal convolution. In the pars recta local reabsorbtion rate was 1.0 +/- 0.3 nl/min-mm. In the proximal convolution C increased with increasing intratubular pressure: deltaC/deltaitp = (2.7 +/- 1.2)-10(-2) nl/min-mm-Torr. Filtration was in disequilibrium in these animals under all conditions examined, hydraulic filtration conductance was K = 1.2 +/- 0.4 nl/min-Torr. Modified methods have been used for intratubular pressure and for flow rate measurements in order to reduce experimental procedure. It is shown, that fractional reabsorption, calculated on the basis of pressure measurements, is a good approximation to results usually obtained by inulin measurements.