Active transport of sodium and potassium by the choroid plexus of the rat.

1. Choroid plexus from the lateral ventricle in the adult rat was found to contain approximately 54 m-equiv Na(+) and 89 m-equiv K(+) per kg wet tissue.2. The total water (79%), the extracellular space (21%) and the red blood cell volume (8-9%) in choroid plexus were quantified separately by analysing the distribution of [(14)C]antipyrine, [(14)C]inulin and (51)Cr-tagged erythrocytes, respectively, between this tissue and plasma water.3. The tissue electrolyte data together with the compartmental (space) data were used to calculate an average concentration of Na(+) (39 m-equiv/kg cell H(2)O) and of K(+) (144) in the choroid cell.4. Under various experimental conditions known to stimulate or inhibit the Na(+)-K(+) transport system in other tissues, there were significant changes (10-40 m-equiv/kg cell H(2)O) in the concentrations of both these cations in the plexus epithelial cells.5. Choroid cell K(+) was not independent of the concentration of K(+) in plasma since substantial fluctuations in cell K(+) occurred in rats rendered either hypo- or hyperkalaemic; also, the choroid cell apparently cannot maintain a constant gradient between itself and c.s.f. in the face of kalaemic disturbances.6. Evidence is offered to support the hypothesis that the choroid plexus of the lateral ventricle has a Na(+)-K(+) pump, the operation of which contributes to the maintenance of K(+) homoeostasis in the C.N.S.