Cell swelling and volume regulation.

The extracellular space in the brain is typically 20% of the tissue volume and is reduced to at least half its size under conditions of neural insult. Whether there is a minimum size to the extracellular space was discussed. A general model for cell volume regulation was presented, followed by a discussion on how many of the generally involved mechanisms are identified in neural cells and (or) in astrocytes. There seems to be clear evidence suggesting that parallel K+ and Cl- channels mediate regulatory volume decrease in primary cultures of astrocytes, and a stretch-activated cation channel has been reported. The role of the different channels was discussed. A taurine leak pathway is clearly activated after cell swelling both in astrocytes and in neurones. The relations between the effect of glutamate and cell swelling were discussed. Discussion on the clearance of potassium from the extracellular space was continued. Neither the spatial buffering mechanism nor the role of the Na+/K+ pump was discussed here, but additional possibilities for K+ removal were discussed. At a high extracellular K+ concentration there is a possibility of channel-mediated KCl uptake by the glia cells, and a Na-K-2Cl cotransport system is present in astrocytes and seems to play a role at relatively low extracellular K+ concentrations (6-20 mM). The cotransport system is likely to be regulated, but little is known about its regulation in glia cells.