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1. Active Na+ transport across the choroid plexus is mediated by Na-K pumps in the brush border membrane of the epithelium. We have studied the kinetics of Na+ pumping across the brush border membrane using a 22Na efflux procedure. 2. Frog choroid plexuses were loaded with 22Na in K+ -free, cold Ringer solution containing various Na+ concentrations. The efflux of 22Na from the tissues was monitored at 22 degrees C and the rate constant (k) was estimated for each flux interval. The pump component was obtained from the change in k(delta k) induced by the addition of KCl to the superfusate. 3. Ouabain blocked this K+ -sensitive Na+ efflux and other monovalent cations substituted for K+ in the sequence Tl+, Rb+, K+, greater than NH+4 greater than Cs+ much greater than Li+. Active sodium transport (delta k x Nac) increased in a sigmoidal and saturable way with intracellular Na+ and with extracellular K+ concentrations. 4. Kinetic parameters were estimated using the model of Garay & Garrahan (1973). The data indicate that there were two ligand binding sites for K+ on the outside face and three ligand binding sites for Na+ on the cytoplasmic face of the pump. The Kms for potassium and sodium were 1 . 1 and 7 mM. 5. The results also suggest that the Na-K pump has a coupling ratio of 1 . 5. We conclude that the Na-K pump in this epithelium is very similar to those in single cells such as erythrocytes, nerve and muscle.

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