An osmotic gradient in ischemic brain edema.

We studied how the osmotic pressure gradient between blood and the brain is related to the development of ischemic brain edema. Focal cerebral ischemia was produced by left MCA occlusion in rats. Brain osmolality was determined with a vapor pressure osmometer, water content of the brain tissue was measured by wet and dry weight, and the tissue sodium and potassium contents were assayed by flame photometry. Permeability of BBB was tested by EB. These measurements were made from the cortical core of MCA territory at various intervals from 1 hr to 14 days after occlusion. Brain osmolality increased from 311 +/- 2 mOsm/kg (M +/- SE) to 329 +/- 2 mOsm/kg (n = 7, p less than 0.01) by 6 hrs after occlusion. Serum osmolality did not significantly change. The osmotic gradient between blood and the brain was about 26 mOsm/kg. Brain osmolality then decreased to 310 +/- 2 mOsm/kg by 12 hr after occlusion and remained about the same level for up to 14 days. Water content progressively increased within 1 day, then gradually decreased by 14 days. Sodium plus potassium content of the brain tissue did not increase and EB extravasation was not seen within 6 hr of occlusion. These findings indicate that an osmotic pressure gradient contributes to the formation of edema only during the early stage of cerebral ischemia. Brain osmolality is not related to tissue electrolyte change and BBB disruption to protein.