Rebound swelling of astroglial cells exposed to hypertonic mannitol

Mannitol is widely used in anesthesia and critical care medicine. Although its clinical effects were originally attributed to osmotic dehydration of brain cells, other mechanisms have also been proposed. Osmotic dehydration of astroglial cells is opposed by powerful volume-regulating mechanisms that involve inward transport of electrolytes. These mechanisms have been studied previously by exposing cells to hypertonic saline gradients. Because of its potential clinical relevance, the volume response of astroglial cells exposed to hypertonic mannitol was investigated.

Rat C6 glioma cells were cultured to confluence, and their volume behavior was observed by laser light scattering. After equilibration at physiologic temperature and pH, cells were abruptly exposed to hypertonic mannitol solutions. In separate experiments, C6 cells were exposed to hypertonic solutions containing radiolabeled mannitol, and its cellular uptake was determined.

Hypertonic mannitol exposure produced initial cell shrinkage followed by rapid volume recovery and rebound swelling. The rebound swelling was similar in magnitude to the initial maximal shrinkage. For +40 mOsm and +70 mOsm mannitol challenges, mean volume recovery was 184+/-31% and 227+/-62%, respectively (where full recovery to baseline volume = 100%). Rebound swelling was substantially inhibited by furosemide. When exposed to mannitol in varying concentrations, uptake was linear, ranging from 82+/-7 nmol/mg to 406+/-26 nmol/mg protein. After 5 min, estimated intracellular concentrations of mannitol were similar to extracellular concentrations.

Unlike hypertonic saline, hypertonic mannitol exposure produces rebound cell swelling. Cellular penetration of mannitol appears to account for much of this phenomenon. The clinical implications of these observations remain to be determined.