The role of Na and Cl− in fluid reabsorption by the efferent ducts was examined by perfusing individual ducts in vivo with preparations of 160 mM NaCl in which the ions were replaced, together or individually, with organic solutes while maintaining the osmolality at 300 mmol/kg. Progressively replacing NaCl with mannitol reduced net reabsorption of water and the ions in a concentration-dependent manner, and caused net movement into the lumen at concentrations of NaCl less than 80 mM. The net rates of flux were lower for Na than for Cl−. In collectates, [Na ] was greater than [Cl−], indicating that Cl− transport is probably linked with another anion. Replacing either Na or Cl− in perfusates (with choline and isethionate, respectively) while maintaining the other inorganic ion at 160 mM also reduced net rates of reabsorption in a concentration-dependent manner to zero when either ion was completely replaced. There were no significant differences in the osmolality of perfusate and collectate, and collectates contained a mean of 3.4 mM K , indicating a backflux of K into the lumen. It is concluded that fluid reabsorption from the efferent ducts is dependent on the transport of both Na and Cl− from the lumen (from a luminal concentration of at least 70–80 mM), and that Cl− transport is dependent on another anion. The epithelium is permeable to K and has a higher permeability to a range of organic solutes (mannitol, choline, and isethionate) than epithelium in the proximal kidney tubules.
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