Absence of Cl- -OH- or Cl- -HCO3- exchange in the rabbit renal proximal tubule

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Abstract
Coupled entry of Na+ and Cl- into the cell may mediate salt and water absorption by the renal proximal tubule. Other leaky epithelia have been shown to have parallel luminal Na+-H+ and Cl--OH- exchangers that can serve in concert as a major pathway for NaCl absorption. Experiments were designed to test whether Cl--OH- or Cl--HCO3- exchange occurs in the isolated perfused rabbit proximal convoluted tubule and thereby whether neutral transcellular NaCl absorption could be a possible mode of active transport in this segment. The studies described here make use of a lumen-to-cell gradient for Cl- to drive OH- from cell to lumen, causing an increase in the rate of H+ disappearance from the luminal fluid. Superficial proximal convoluted tubules were perfused in vitro with acidic solutions (pH 6) that were generally free of organic solutes. The bathing solution contained organics, colloid, < 10% of perfusate H+ concentration. H+ disappearnace was calculated from the difference in pH between perfused and collected fluid, flow rate, and buffer capacity of the perfusate. The removal of Na+ from perfusate and both reduced H+ efflux to 68% of control, a smaller effect than that noted previously in the presence of organic solutes. When Na+-containing solutions were used, bilateral replacement of Cl- by NO3-, isethionate or SO42- failed to reduce H+ efflux, but rather resulted in a 11% increase in H+ flux. When Na+-free solutions were used to prevent any Na+-H+ exchange from shunting the pH change due to Cl--OH- exchange, a similar result was noted, i.e., a 13% increase in H+ efflux above the base line of 2 pmol .cntdot. cm-1 .cntdot. s-1. When the anion-exchange inhibitor SITS [4-acetamido-4-isothiocyanostilbene-2,2''-disulfonic acid] was added to the perfusate at 0.2 mM, no change in H+ efflux was noted in the presence or absence of CO2. The estimated apparent H+ leak permeability of 0.002 cm3 .cntdot. s-1 .cntdot. cm tubule length-1 appears to represent a base-line value. Cl--OH- or Cl--HCO3- exhange is either absent or plays a minor role in influencing apparent H+ permeability, acidification, or NaCl transport in the renal proximal tubule.

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