Selective effect of mannitol-induced hyperosmolality on brain interstitial fluid and water content in white matter.

We studied the effect of mannitol-induced hyperosmolality on brain interstitial fluid (ISF) by autoradiography. Adult cats underwent intracerebral infusion of the extracellular marker, 14C-sucrose. Nine animals were given 2g/kg of mannitol intravenously, and another nine animals without mannitol were controls. Plasma and cerebrospinal fluid (CSF) osmolalities were measured. After 2 hr the brains were removed for determination of water and electrolyte content and for preparation of the autoradiograms. Diffusion coefficients were calculated for intracerebral transport with equations for radial diffusion. We found that mannitol increased the plasma osmolality but did not affect that of the CSF. Water and potassium contents were significantly lower in the white matter of mannitol-treated animals than in controls. Diffusion was reduced in the direction of gray matter into the white matter. We conclude that lower doses of mannitol control CSF pressure by selectively removing water from white matter, reducing the CSF volume, and affecting molecular transport at the gray/white interface.