Selective permeability of [3H]-D-mannitol and [14C]-carboxyl-inulin across the blood-brain barrier and blood-spinal cord barrier in the rabbit.

Selective permeability across the blood-brain and blood-spinal cord barriers was studied in a rabbit experimental model. To maintain steady state levels of the tracers in the circulation, 50muCi each of [3H]-D-mannitol and [14C]-carboxyl-inulin were administered as a bolus and by slow intravenous infusion for 60, 90 and 120 minutes; 90 min proved to be the optimal equilibration time for uptake of radioactive tracers into central nervous system (CNS) regions. Kinetic transfer of [3H]-D-mannitol and [14C]-carboxyl-inulin from blood into CNS was computed as apparent transfer constant (KD). The KD for [3H]-D-mannitol in regions of brain (cerebrum, cerebellum, mid-brain, pons and brain stem), and spinal cord (cervical, thoracic and lumbar) were 0.033 to 0.054 microliter/min/g-1 and 0.053 to 0.065 microliter/min/g-1, respectively. The KD for [14C]-carboxyl-inulin into brain and spinal cord regions were 0.016 to 0.033 and 0.037 to 0.054 microliter/min/g-1, respectively. Of the regions of spinal cord and brain examined, lumbar cord appears to be the most permeable. The KD values pooled for samples of the spinal cord show a significant increase in uptake of [3H]-D-mannitol (p = 0.0043) and [14C]-carboxyl-inulin (p = 0.0001) compared to samples of brain. The blood-spinal cord barrier was more permeable to these substances than the blood-brain barrier.