A B S T R A C T Segments of superficial and juxtamedullary proximal convoluted tubules of the rabbit were perfused in vitro to examine the mechanisms responsible for net volume reabsorption. The very early postglomerular segments were not studied. Fluid reabsorptive rates and transepithelial potential differences were compared under various conditions: (a) with perfusate that simulated glomerular filtrate; (b) with perfusate that lacked glucose, amino acids, and acetate and that had HCO3 and Cl concentrations of 5 and 140 mM, respectively; (c) with perfusate that lacked glucose, amino acids, and acetate but with 20 meq ofNaHCO3 replaced with 20 meq ofNa cyclamate; (d) with the same perfusate as in b but in the presence of ouabain in the bath; (e) with ultrafiltrate of rabbit serum titrated with HC1 to final HCO3 and Cl concentrations of 2 and 134 mM, respectively. Tubules were perfused with this titrated ultrafiltrate at 37°C, 21°C, and in the presence of 0.1 mM ouabain in the bath. Bath fluid in all experiments was regular rabbit serum. Under conditions a and b superficial proximal convoluted tubule (SFPCT) and juxtamedullary proximal convoluted tubule (JMPCT) behaved similarly with the exception that SFPCT exhibited a lumen-positive and JMPCT a lumen-negative electrical potential under condition b. However, undeT condition c SFPCT failed to exhibit net volume reabsorption, whereas reabsorption in JMPCT continued unchanged. Ouabain did not affect volume reabsorption in SFPCT under condition

We studied indomethacin as a probe of anion transport across the isolated perfused proximal straight tubule of the rabbit and discovered that a substantial component of transport may occur by anion exchange at the basolateral membrane. Various perturbations involving direct or indirect dissipation of the cellular sodium gradient (ouabain, sodium-or potassium-free solutions, cooling to 18°C) resulted in only a 50% inhibition of indomethacin transport, which raised the question of a co-existent alternative pathway for secretion. Similarly, the anion exchange inhibitor, 4,4'-diisothiocyanostilbene (DIDS), diminished indomethacin secretion by only 50%. Cooling followed by DIDS or the reverse sequence resulted in additive inhibition such that the combination abolished active secretion of indomethacin.We conclude that active secretion of indomethacin by the proximal straight tubule appears to be in part sodium gradient dependent; the remainder may be driven by an anion exchanger on the basolateral membrane.

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