The purpose of the present studies was to characterize the nature of salt and water transport out of the superficial (SF) and juxtamedullary (JM) straight segments of rabbit proximal tubules as examined by in vitro microperfusion techniques. When the perfusate consisted of a solution simulating ultrafiltrate of plasma, there were no differences between SF and JM straight tubules in either net reabsorption of fluid (SF = 0.47 nl/mm per min; JM = 0.56 nl/mm per min) or in transtubular potential difference (PD) (SF=-2.1 mV; JM=-1.8 mV). Removal of glucose and alanine from the perfusate had no effect on the magnitude of the PD in either straight segment. Ouabain decreased both the net reabsorptive rates and the PD. Isomolal replacement of NaCl by Na cyclamate (a presumed impermeant anion) in the perfusate and the bath caused an increase in luminal negativity in both segments whereas similar substitution of NaCl by choline Cl (nontransported cation) changed the PD to near zero. These studies, therefore, suggest that sodium is transported out of the proximal straight tubules by an active noncoupled process that generates a PD (electrogenic process). When the perfusate consisted of a solution with a high chloride concentration (resulting from greater HCO3 than C1 reabsorption in the proximal convoluted tubule), different PDs in SF and JM tubules were generated: SF=+1.6 ± 0.2 mV; JM=-1.3±0.3 mV. This difference in PD was attributed to relative differences in Na and Cl permeabilities in these two segments. Electrophysiological and isotopic estimates of the chloride to sodium permeability revealed that the SF tubule is about twice as permeant to chloride than to sodium, whereas the JM tubules are approximately twice as permeant to sodium than to chloride. It is concluded that the mechanism of active sodium transport in the straight segment of proximal tubule differs from that of the convoluted segment and that both the SF and JM straight segments differ from each other with respect to sodium and chloride permeability.
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