Brain interstitial composition during acute hyponatremia

Brain cortical water spaces were determined in rat brain following an intraperitoneal infusion of distilled water (15% of body weight). Total water content was determined by gravimetric methods while the size of the interstitial space and the interstitial ion concentrations were measured by ion-selective microelectrodes. Ion content of brain gray matter and plasma were determined by standard methods. During 2 h the plasma osmolarity changed from 301 to 251 mosmol/l while cortical water content increased from 4.04 ml/g dry weight (gdw) to 4.42 ml/gdw. It was observed that the size of the interstitial space remained stable during the two hours of osmotic challenge. Although the brain showed a volume regulatory response by an outward transport of ions, it was not sufficient since there was an increase of brain volume of 10%. The accumulated water was distributed in the interstitial and intracellular compartments according to their individual sizes. It is concluded that cells in brain gray matter are not able to regulate their volume in cytotoxic brain oedema.