On the measurement of extracellular space in slices prepared from different rat brain areas

Extracellular space values were determined with either ¹⁴C-sucrose or ¹⁴C-inulin at different incubation time intervals (30 and 60 min) in rat brain slices prepared from the cerebellum, cortex, striatum and hypothalamus. At both time intervals and in all brain area slices, ¹⁴C-sucrose spaces were larger than ¹⁴C-inulin spaces. Efflux of ¹⁴C-sucrose but not of inulin could be resolved into two linear components with half-times of 1.6 and 15.0 min. Efflux of ²²Na from each of the different brain areas also could be separated into two distinct components, the slower of which was increased by ouabain. Similarities between ²²Na and ¹⁴C-sucrose movements indicate that there is a cellular accumulation of sucrose but not of inulin. An accurate estimation of functional extracellular space in brain slices could be derived from either the ¹⁴C-inulin space or the ¹⁴C-sucrose space corrected for cellular uptake. Values obtained range from approximately 12 ml/100 g (cortex) to 20 ml/100 g (cerebellum).