Kinetics of the sodium pump in the frog choroid plexus.

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Abstract
Active Na+ transport across the choroid plexus is mediated by Na-K pumps in the brush border member of the epithelium. The kinetics of Na+ pumping across the brush border membrane was studied using a 22Na efflux procedure. 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.degree. 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. Ouabain blocked this K+-sensitive Na+ efflux and other monovalent cations substituted for K+ in the sequence Tl+, Rb+, K+, > NH4+ > Cs+ .mchgt. Li+. Active Na transport (.DELTA.k .times. Nac) increased in a sigmoidal and saturable way with intracellular Na+ and with extracellular K+ concentrations. Kinetic parameters were estimated using the model of Garay and Garrahan. The data indicate that there were 2 ligand binding sites for K+ on the outside face and 3 ligand binding sites for Na+ on the cytoplasmic face of the pump. The Km for K and Na were 1.1 and 7 mM. The Na-K pump has a coupling ratio of 1.5. 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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