The effect of intraluminal flow rate on glomerulotubular balance in the proximal tubule of the rat kidney

To differentiate intraluminal from peritubular factors in mediating the glomerulotubular balance, single nephron filtration rate (SNGFR) and reabsorption were measured by total collection from the last proximal segment before, and during, externally applied mechanical compression of an early proximal convolution of ninety-three rat nephrons. SNGFR fell from 55.6 ± 2.3 nl min_-1 during control conditions to 34.5 ± 2.0 nl min^-1 during compression (P < 0.0001) in sixty tubules ('responders'). Absolute reabsorption fell from 41.8 ± 2.0 to 28.4 ± 1.8 nl min^-1 (P < 0.0001), while percentage reabsorption rose from 75 ± 2 to 82 ± 2% (P < 0.0001). These effects were reversible and independent of whether the compression was applied before or after the control collection. In thirty-three proximal tubules the compression procedure was not successful ('non-responders'), SNGFR remaining unchanged (36.1 ± 2.9 vs. 36.9 ± 2.9 nl min^-1, P > 0.3). Absolute and percentage reabsorptions rose slightly, albeit significantly, from 26.1 ± 2.1 to 30.7 ± 2.4 nl min^-1 (P < 0.0001) and from 75 ± 3 to 85 ± 2% (P < 0.0001). Thus, the response of reabsorption to changes in intraluminal flow is composed of two different adaptive mechanisms. The slowing of flow is present in both sets of tubules and causes a slight increase in resorption; the change in filtration per se is present only in the first set of responders, and causes an SNGFR-dependent consensual change in resorption.