[Molecular dynamics of K+ transport and its crucial involvement in signal transduction]

Potassium ion (K+) is the only cation whose concentration in intracellular solution is more abundant than in extracellular fluid. In higher animals, each of the cells, which is bathed in the extracellular fluid containing approximately 5 mM K+, has approximately 140 mM K+ in its cytoplasm. Physiological experiments have reported that the glial cells such as brain astrocytes and retinal Müller cells highly express K+ conductance on their membrane. The K+ conductance is thought to be responsible for transport of K+ from the extracellular regions of high K+, which is elicited by the excitation of neurons, to those of low K+. This so-called glial K(+)-buffering action is essential for maintenance of neuronal activity. Types and distribution of the K+ channels involved in the buffering action, however, have remained unknown for a long time. Recently, we have identified that an inwardly rectifying K+ channel, Kir4.1, is dominated in glial cells and plays a central role in the K(+)-buffering action. We also found that Kir4.1 is enriched on the processes of astrocytes wrapping synapses and blood vessels. Furthermore, some of the Kir4.1 is assembled with Kir5.1, suggesting that a heteromultimer of Kir4.1 and Kir5.1 plays a distinct role in the glia.