MOVEMENTS OF RADIOACTIVE SODIUM IN CEREBRAL-CORTEX SLICES IN RESPONSE TO ELECTRICAL STIMULATION.

1. Sodium exchange was measured with (24)Na in incubated guinea-pig cerebral-cortex slices maintained under adequate metabolic conditions with a steady content of fluid and ions resembling that of brain in vivo. 2. Evidence was obtained indicating that Na(+) ions behaved in the inulin space as if they were extracellular, and that their entry into the non-inulin space of unstimulated tissue was about 10 times slower and could be separated, on the basis of complete exchangeability, into two components, a ;fast' one, which reacted to electrical stimulation, and a ;slow' one, exchanging at a rate of about 8muequiv./g./hr., which was not affected by stimulation. 3. The average rate of sodium turnover in unstimulated slices was 175-275muequiv./g./hr., whereas that for stimulated slices was approx. 4-6 times this, or 1050-1180muequiv./g./hr. The stimulated rate was equivalent to a turnover of 32% of the sodium in the non-inulin space/min., or 3mmuequiv./g./impulse. 4. Response to the onset of stimulation appeared to be immediate, but after cessation of stimulation increased sodium movements persisted for several minutes before return to unstimulated values. 5. Calculations based on electrochemical gradients suggested that about one-quarter of the energy available from respiration was required for sodium and potassium transport at maximal rates in both unstimulated and stimulated cerebral-cortex slices.