Acute volume regulation of brain cells in response to hypertonic challenge

Hypertonic dehydration of the brain through administration of osmotic agents, either alone or in combination with "loop" diuretics, has been a mainstay in the treatment of increased intracranial pressure for decades. Controversy exists, however, as to the mechanism and long-term value of such therapy. Although many cell types possess volume regulatory mechanisms capable of opposing hypertonic dehydration, such behavior in the brain is poorly understood.

As a model for the mammalian central nervous system, the real-time volume behavior of rat C6 glioma cells was observed by laser light scattering during hypertonic challenge. Cells were allowed to equilibrate in isotonic balanced salt solutions at physiologic pH and temperature, and then rapidly exposed to hypertonic solutions. Experiments were conducted in the presence and absence of sodium, chloride, and the loop diuretic bumetanide to assess their roles in volume regulation.

In response to acute, large (70 mOsm) hypertonic exposures, cells immediately shrank and then rapidly regulated their volume completely back to control within minutes. In the presence of the loop diuretic bumetanide, the volume regulatory process was significantly inhibited with only 54% recovery observed at concentrations of 10(-4) M. Volume regulation was also significantly inhibited by removal of extracellular sodium and chloride.

Brain cells possess powerful, electrolyte-dependent and bumetanide-sensitive volume-regulatory mechanisms that directly oppose attempted osmotic shrinkage. These observations suggest a possible new mechanism for the clinically observed synergistic effects of loop and osmotic diuretics in reduction of brain volume.