Neuronal activity leads to rapid fluctuations of the extracellular K+ concentration ([K+]o) because of the restricted volume of the extracellular space. If increases in [K+]o remain uncorrected, the resting potential would become more positive and affect activation of transmembrane ion channels, receptors, and transporters. During neuronal hyperactivity in vivo, [K+]o may increase from 3 mM to a ceiling level of 10–12 mM. Such high [K+]o levels are observed during epileptiform activity. Two different mechanisms are thought to balance [K+]o during neuronal activity: K+ uptake and spatial K+ buffering (for review, see Kofuji and Newman,2004). K+ uptake, mediated by the Na,K-adenosine triphosphatase (Na,K-ATPase) or Na-K-Cl cotransporters, is accompanied by cell swelling and local depolarization of astrocytes. Spatial K+ buffering is driven by the difference between the glial syncytium membrane potential and the local K+ equilibrium potential. K+ enters astrocytes at sites of enhanced [K+]o where the cells' resting potential is negative to the K+ equilibrium potential (due to the electrical coupling with neighbouring cells exposed to normal [K+]o) while efflux of the ions occurs at sites distant to the [K+]o increase. This allows transfer of K+ from regions of elevated [K+]o, through the syncytium, to regions of lower [K+]o (Fig. 1). Spatial buffering depends on proper distribution and function of astrocytic K+ channels and gap junctions.
Figure 1. Space-dependent spatial K+ buffering. Activity in a group of neurons has produced local increase in [K+]o to 12 mM (shaded area, left). This increase produces a depolarization of the membrane potential (Vm) that passively spreads through the coupled astrocytes. The positive shift of the K+ equilibrium potential (EK) is stronger than that of Vm of the cell exposed to high [K+]o because the latter is “clamped” by the neighbouring, more negative astrocytes exposed to lower (normal) [K+]o (4 mM). The difference between EK and Vm drives K+ inward at the region where it is raised and outward at distant regions. The result is a net flux of K+ away from the region where it has accumulated extracellularly. The average [K+]i is not affected. The graph shows the distribution of EK and Vm as a function of distance along the astroglial syncytium. Modified from Orkand, Ann N Y Acad Sci,1986, 481, 269-272, reproduced with permission.
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In astrocytes, the inwardly rectifying K+ channel, Kir4.1, which is regulated by intracellular adenosine triphosphate (ATP), is thought to allow for K+ influx at negative membrane potentials (for review see Butt and Kalsi,2006; Hibino et al.,2010; Olsen and Sontheimer,2008). Genetic inactivation of the Kir4.1 encoding gene, KCNJ10, in animal models substantiated an outstanding role for Kir4.1 in glial K+ buffering (Djukic et al.,2007; Kofuji etal.,2000). Mice with global and conditional knockout of Kir4.1 showed a severe phenotype, including a smaller size, reduced body weight, and deficits in controlling posture and movement. They developed seizures and died about 3 weeks after birth (Djukic et al.,2007; Kofuji et al.,2000). In the CNS, Kir4.1 channels are specifically localized to macroglial cells. Accordingly, conditional knockout under the control of the human GFAP promoter entailed loss of Kir4.1 in NG2 glial cells, oligodendrocytes, and astrocytes. Ablation of Kir4.1 led to spongiform vacuolization, axon swelling, and hypomyelination in the spinal cord, with strongly depolarized oligodendrocytes (Neusch et al.,2001). It has been suggested that Kir4.1 in concert with Cx32/Cx43 activity-dependently contribute to K+ redistribution in the white matter of spinal cord and optic nerve (Menichella et al.,2006). Gap junction coupling between astrocytes and oligodendrocytes has been considered a prerequisite not only for proper K+ buffering, but also for myelin formation and axonal conductivity (Maglione et al.,2010; Magnotti et al.,2011b). In the retina, Kir4.1 knockout leads to depolarization of Müller cell processes contacting blood vessels, and disappearance of the PIII response of the electroretinogram, which is generated by K+ flux (Kofuji et al.,2000). Gray matter astrocytes are also depolarized after Kir4.1 deletion, which not only impaired K+ buffering but glutamate uptake as well (Djukic et al.,2007; Kucheryavykh et al.,2007). These changes led to decreased neuronal activity but enhanced synaptic potentiation in the hippocampus. The key role of Kir4.1 in K+ uptake and spatial buffering has also been confirmed by showing that its deletion causes an epileptic phenotype (Chever et al.,2010; Haj-Yasein et al.,2011).
Ultrastructural analyses in rat demonstrated spatial overlap of Kir4.1 and the water channel aquaporin 4 (AQP4) in astroglial endfeet contacting capillaries (Higashi et al.,2001; Nielsen etal.,1997). This finding gave rise to the hypothesis that K+ clearance through Kir channels might depend on concomitant transmembrane flux of water. Subsequent functional work corroborated this idea by showing that in mice the clearance of extracellular K+ is compromised if the number of perivascular AQP4 channels is decreased (Amiry-Moghaddam et al.,2003). In line with this assumption was also the observation of impaired K+ buffering and prolonged seizure duration in AQP4 knockout mice (Binder et al.,2006) although spatial K+ redistribution was more efficient in these mice, probably due to enhanced astrocyte gap junction coupling and volume regulation (Benfenati et al.,2011; Strohschein et al.,2011). However, more recent data from astrocytes and retinal Müller cells suggested that Kir4.1 channel function is independent of AQP4 expression (Ruiz-Ederra et al.,2007; Zhang and Verkman,2008).