Cerebrospinal fluid anion transport: studies of pertechnetate in unanesthetized sheep

Perfusion of the ventriculocisternal system in unanesthetized sheep was used to quantify the pathways by which the pertechnetate anion (TcO4-) may leave the cerebrospinal fluid space. The largest fraction (about 89%) was removed from the fluid before it reached the cisterna magna; this extraction was significantly inhibited by iodide ion in the perfusion fluid. One-fifth of the extracted pertechnetate remained in the head of the animal for over 3 hours. Only about 3% of the infused pertechnetate was resorbed with the bulk flow of cerebrospinal fluid through the arachnoid villi. These results are consistent with the concept of a specific brain channel for removal of anions. As pertechnetate and iodide ion are thought to be transported by the same mechanisms, the physiological role of this channel may be maintenance of a very low iodide concentration in the extracellular fluid of the brain.