Studies were undertaken to examine the mechanism whereby changes in intraluminal flow rates alter reabsorption in the isolated perfused proximal tubule of the rabbit. All protocols employed the technique of in vitro perfusion of isolated segments of the proximal convoluted tubule. Stepwise elimination of d glucose and l alanine from an artificial perfusate simulating ultrafiltrate decreased the unidirectional flux of sodium, transtubular potential difference, and net water absorption. Using isosmolal ultrafiltrate as the perfusate, net fluid reabsorption and the unidirectional lumen to bath flux of sodium and chloride decreased with a decrease in flow rate below 11 nl/min, but neither net fluid reabsorption nor the unidirectional fluxes of sodium and chloride increased further as the perfusion rate was increased aboven 11 nl/min. The unidirectional flux of 14C urea was not affected by changes of perfusion rate from 1.6 to 44 nl/min. The dependence of net fluid reabsorption and unidirectional fluxes of sodium and chloride on flow rate per se, and not on intraluminal hydrostatic pressure or geometry, was established by demonstrating their decrease despite a rise in intraluminal pressure and inside diameter produced by counterpressure at the collecting end of the tubule, while flow was decreased. Ouabain decreased net fluid reabsorption to near zero at all flow rates, but ouabain had no effect on the flow dependency of unidirectional sodium and chloride fluxes. Furthermore, the flow dependency of the unidirectional flux of sodium was eliminated with a decrease in bicarbonate concentration and removal of d glucose and l alanine from the perfusate. Thus, the present studies demonstrate that net water and unidirectional sodium and chloride fluxes are flow dependent. At flow rates somewhere below 11 nl/min, unidirectional fluxes decreased with decreasing perfusion rates; however, at perfusion rates greater than 11 nl/min, there was no further effect of perfusion rate on either net water absorption or the unidirectional fluxes of sodium or chloride. These effects may be partly mediated through the flow dependent changes in the intraluminal concentration of bicarbonate, d glucose, and l alanine.
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