Potassium buffering in the central nervous system.
Rapid changes in extracellular K+ concentration ([K+](o)) in the mammalian CNS are counteracted by simple passive diffusion as well as by cellular mechanisms of K+ clearance. Buffering of [K+](o) can occur via glial or neuronal uptake of K+ ions through transporters or K+-selective channels. The best studied mechanism for [K+](o) buffering in the brain is called K+ spatial buffering, wherein the glial syncytium disperses local extracellular K+ increases by transferring K+ ions from sites of elevated [K+](o) to those with lower [K+](o). In recent years, K+ spatial buffering has been implicated or directly demonstrated by a variety of experimental approaches including electrophysiological and optical methods. A specialized form of spatial buffering named K+ siphoning takes place in the vertebrate retina, where glial Muller cells express inwardly rectifying K+ channels (Kir channels) positioned in the membrane domains near to the vitreous humor and blood vessels. This highly compartmentalized distribution of Kir channels in retinal glia directs K+ ions from the synaptic layers to the vitreous humor and blood vessels. Here, we review the principal mechanisms of [K+](o) buffering in the CNS and recent molecular studies on the structure and functions of glial Kir channels. We also discuss intriguing new data that suggest a close physical and functional relationship between Kir and water channels in glial cells.
Kofuji
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Department of Neuroscience, University of Minnesota, 6-145 Jackson Hall, 321 Church Street SE, Minneapolis, MN 55455, USA. kofuj001@tc.umn.edu
Newman
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EA
D000818:Animals; D020346:Aquaporins / Q000378:metabolism; D002021:Buffers; D002462:Cell Membrane / Q000502:physiology*; D002490:Central Nervous System / Q000502:physiology*; D006801:Humans; D009457:Neuroglia / Q000166:cytology / Q000502:physiology; D011188:Potassium / Q000378:metabolism*; D015221:Potassium Channels / Q000502:physiology*; D012160:Retina / Q000166:cytology / Q000502:physiology; D014882:Water-Electrolyte Balance / Q000502:physiology*
D020346:Aquaporins; D002021:Buffers; D015221:Potassium Channels; D011188:Potassium