INTERSTITIAL SPACE, ELECTRICAL-RESISTANCE AND ION CONCENTRATIONS DURING HYPOTONIA OF RAT HIPPOCAMPAL SLICES

1. The degree to which mammalian brain cells swell in hypotonic enviro nments has not previously been determined. We exposed hippocampal tiss ue slices prepared from anaesthetized rats to artificial cerebrospinal fluid from which varying amounts of NaCl had been deleted. Interstiti al volume (ISV) change was determined fr om the volume of dilution of the marker ions tetramethylammonium (TMA(+)) or tetraethylammonium (TE A(+)). Tissue electrical resistance was measured as the voltage genera ted by constant current pulses. 2. ISV decreased as a function of lowe red extracellular osmolality (osmotic pressure, pi(o)), indicating cel l swelling. After reaching a minimum, ISV recovered partially, suggest ing regulatory volume decrease of cells. After restoring normal pi(o) the ISV expanded, indicating post-hypotonic cell shrinkage. The electr ical resistance of the tissue (R(o)) increased when pi(o) was lowered, due to the reduced ionic strength, as well as restricted ISV. 3. To c ontrol for low NaCl concentration, reduced NaCl was replaced by mannit ol or fructose. In isosmotic, NaCl-deficient solution, ISV showed inco nsistent change, and R(o) corrected for ionic strength tended to decre ase. 4. Extracellular K+ concentration decreased slightly in low pi(o) except when spreading depression caused it to increase. Extracellular Ca2+ concentration decreased substantially, consistently and reversib ly. Administration of isosmotic low-NaCl concentration solutions cause d a similar decrease in extracellular Ca2+ concentrations. We propose that low Na+ concentration in extracellular fluid impaired the extrusi on of Ca2+. 5. In severely hypotonic solution, ISV was reduced to 25% of its control volume, corresponding to a mean cell volume increase of at least 11%, probably more. From plotting relative changes in ISV ag ainst osmolarity we concluded that, within the range tested, hypotonic cell swelling was not opposed by the close approach of plasma membran es of neighbouring cells.