Transport of water and electrolytes between brain and ventricular fluid in the rabbit

Abstract

Bidirectional sodium fluxes and net transfer of potassium, chloride, and water across the ventricular ependyma were studied in anesthetized rabbits by closed perfusion of the cerebral aqueduct and anterior fourth ventricle with solutions containing trace amounts of inulin- 3H and 22Na. Inulin dilution and clearance from the perfusate indicated a bulk fluid accretion of 0.37 μl min −1 cm −2 of ependymal surface. The concurrent net sodium movement into ventricular fluid occurred at a rate of .0645 ± .0060 μEq min −1 cm −2 against typical ventriculo-vascular potential gradients of 5–10 mv. Potassium entered the perfusate at concentrations approximating that of normal cerebrospinal fluid (CSF). Chloride accumulated in the ventricular system at a rate of .0356 ± .0057 μEq min −1 cm −2 and its distribution between blood and ventricular fluids approached an electrochemical equilibrium. Water was transported at reduced rates into hypotonic perfusates without associated changes in sodium influx, whereas, both water and sodium movements were inhibited by acetazolamide or cardiac glycosides. These studies imply that movement of CSF across the blood-brain-CSF barrier is an isotonic flow coupled to sodium transport and represents one-third of the total intraventricular formation of CSF.