CNS, CSF, and extradural fluid uptake of various hydrophilic materials in the dogfish

The distribution of 10 radioactively labeled materials from blood to CSF, extradural fluid (EDF), telencephalon (cerebrum), medulla, and spinal cord was studied in the dogfish shark. Plasma volumes, blood flows, and blood-to-tissue or fluid-transfer half-times (t1/2) were calculated from these distribution data. Blood-EDF exchange occurred at slow but similar rates for all tracers. Urea uptake by the CSF was very rapid compared to that of the other compounds and may be facilitated by a special mechanism. The tissue plasma spaces of all three CNS regions were small (1.1-1.5%). The calculated rates of tissue blood flow (in ml/g-min) were 0.11 for telencephalon and medulla and 0.055 for spinal cord. Choroid plexus blood flow was estimated to be 2.3 ml/g-min, a surprisingly high rate. Transport between blood and tissue was most rapid for water ethylene glycol and slowest for inulin. The tissue t1/2's of urea indicated that a significant portion of this compound's net uptake by periventricaul brain tissue occurred via the CSF.