The transport of urea, creatinine and certain monosaccharides between blood and fluid perfusing the cerebral ventricular system of rabbits

Abstract

Artificial cerebrospinal fluid was perfused between the lateral cerebral ventricle and the cisterna magna of the anaesthetized rabbit. This system was used to study the exchanges of urea, creatinine, ID-glucose, D-xylose and D-fructose between blood, brain and cerebrospinal fluid. Urea appears to enter the fluid unidirectionally with the newly formed cerebrospinal fluid at a concentration of about 60% of that in plasma. Diffusional exhanges then occur between blood and cerebrospinal fluid, largely across nervous tissue. Little creatinine enters the fluid with the newly formed cerebrospinal fluid and subsequent diffusional exchanges between blood, brain and fluid occur at a slower rate than those of urea. The major impediment to the diffusion of these substances must lie between blood and brain, rather than between cerebrospinal fluid and brain. Transport of the monosaccharides, D-glucose and D-xylose, and probably D-fructose, in and out of cerebrospinal fluid is carrier-mediated. No evidence was found for transport against a concentration gradient. The affinity of the carrier for the sugars studied varies in the order: glucose > xylose > fructose.