Sodium exchange between blood, brain, and CSF in normal and hydrocephalic cats

The exchange of sodium between blood, brain, and cerebrospinal fluid was studied in normal and kaolin‐induced hydrocephalic cats. The ventricles were perfused to measure fluid formation and Na + exchange rates. 22 Na was added to the perfusion fluid or injected intravenously as a tracer for Na + movement. Na + and 22 Na were also measured in cortical gray and white matter. Na + relative specific activities were calculated for brain, effluent fluid, and serum. With 22 Na in the perfusion fluid, Na + exchange was not different from nascent Na + influx for both normal and hydrocephalic cats. Na + relative specific activities of cortical gray and white matter were 10 times greater in hydrocephalic than in normal cats. This difference in Na + relative specific activity for brain may be due to a higher diffusion constant or to a lower brain capillary permeability. When 22 Na was given intravenously, the Na + diffusional exchange for normal cats was less than that measured when 22 Na + was in the perfusion fluid. In hydrocephalic cats, the Na + diffusional exchange was effectively zero. Na + relative specific activities of cortical gray and white matter were the same for normal and hydrocephalic cats. These findings suggest that the impaired Na + diffusional exchange may be due to pathological changes in the choroid plexus.