Chloride transport in the thick ascending limb of Henle's loop: potassium dependence and stoichiometry of the NaCl cotransport system in plasma membrane vesicles

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Abstract
Cells were isolated from the thick ascending limb of Henle's loop of rabbit kidney outer medulla and a plasma membrane fraction was prepared by differential centrifugation. Sodium and rubidium uptake into the plasma membrane vesicles were determined by a rapid filtration technique. In the presence of a 100 mM KCl gradient and 0.5 mM sodium the vesicles took up 252±82 pmoles sodium/mg protein x 15s; 52% of the uptake was dependent on the presence of chloride or inhibited by 10−3 M bumetanide. If KCl was stepwise replaced by choline chloride, sodium uptake decreased in the absence of bumetanide but was only insignificantly altered in the presence of bumetanide. Potassium exerted a halfmaximum stimulation at 22.3±9.7 mM. In tracer exchange experiments under zero salt gradient conditions, sodium uptake was also strongly reduced in the absence of potassium. Rubidium uptake into the same membrane fraction was highest in the presence of a NaCl gradient, and decreased 41% when sodium was replaced by choline or when 10−3 M bumetanide was present. Replacement of sodium chloride by sodium nitrate also inhibited rubidium uptake. When the sodium, chloride and potassium dependence of the bumetanide sensitive sodium uptake was investigated in more detail, Hill coefficients for sodium of 1.0±0.03, for chloride of 1.8±0.2 and for potassium of 0.98±0.03 were obtained. These results are consistent with the presence of a Na−Cl−K cotransport system in the medullary thick ascending limb of Henle's loop which may operate with a stoichiometry of 1/2/1.