Role of astrocytes in the clearance of excess extracellular potassium.

The development of concepts describing potassium clearance mechanisms in the mammalian central nervous system in the last 35 years is reviewed. The pattern of excess potassium in the extracellular space is discussed as are the implications of these potassium levels for neuronal excitability. There is a systematic description of the available evidence for astrocytic involvement in situ. The three possible astrocytic potassium clearance mechanisms are introduced: spatial buffer mechanism; carrier-operated potassium chloride uptake as well as channel-operated potassium chloride uptake. The three mechanisms are compared and their compatibility is discussed. Evidence is now available showing that at least two of these if not all three mechanisms co-exist and complement each other. Finally, it is concluded that these potassium movements are not used as a signal system, only as a homeostatic feedback mechanisms. Such a genuine signal system involving glial elements exists--but it is based on calcium waves.