Cerebrospinal fluid transport and the thiocyanate space of the brain

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Abstract
At thio-cyanate plasma levels of 1.0 mmole/liter, the thiocyanate concentration in available brain water is four times that of cerebrospinal fluid and the computed thiocyanate space is less than 10% of total brain water. When the plasma concentration of thiocyanate exceeds 3.0 mmoles/liter, the ratio concentration brain/concentration cerebrospinal fluid approaches unity, and the apparent thiocyanate space increases to approximately 20% of brain water. When sodium iodide (3 mmoles/kg) was given intravenously, or 2,4-dinitrophenol (0.05 mM) was injected into the lateral ventricles, the computed thiocyanate space for rabbit brain was 14-15% at low plasma concentrations (less than 2.0 mmoles/liter). The conclusion, based on this evidence, is that the active transport of thiocyanate out of the cerebrospinal fluid is responsible for maintaining the observed concentration gradient between brain and cerebrospinal fluid, which does not allow CNS in plasma and brain water to reach equilibrium. Altering the effectiveness of this transport process (by saturation, competitive inhibition, or inhibiting cellular respiration) results in no net flux between the blood, brain, and cerebrospinal fluid compartments, and, as a consequence, a more accurate estimation of the thiocyanate space.