Down-regulation of Kir4.1 in the cerebral cortex of rats with liver failure and in cultured astrocytes treated with glutamine: Implications for astrocytic dysfunction in hepatic encephalopathy

Department of Neurotoxicology, Mossakowski Medical Research Centre, Polish Academy of Sciences, Warsaw, Poland.
Journal of Neuroscience Research (Impact Factor: 2.73). 12/2011; 89(12):2018-27. DOI: 10.1002/jnr.22656
Source: PubMed

ABSTRACT Brain edema in acute hepatic encephalopathy (HE) is due mainly to swelling of astrocytes. Efflux of potassium is implicated in the prevention of glial swelling under hypoosmotic conditions. We investigated whether pathogenic factors of HE, glutamine (Gln) and/or ammonia, induce alterations in the expression of glial potassium channels (Kir4.1, Kir2.1) and Na(+) -K(+) -2Cl(-) cotransporter-1 (NKCC1) in rat cerebral cortex and cultured rat cortical astrocytes and whether these alterations have consequences for potassium efflux and astrocytic swelling. Thioacetamide-induced acute liver failure in rats resulted in significant decreases in the Kir4.1 mRNA and protein contents of cerebral cortex, whereas expression of Kir2.1 and NKCC1 remained unaltered. Incubation of primary cortical astrocytes for 72 hr in the presence of Gln (5 mM), but not of ammonia (5 mM or 10 mM), induced a decrease in the levels of Kir4.1 mRNA and protein. Similarly to incubation with Gln, reduction of Kir4.1 mRNA expression by RNA interference caused swelling of astrocytes as shown by confocal imaging followed by 3D computational analysis. Gln reduced the astrocytic uptake of D-[(3) H]aspartate, but, in contrast to the earlier reported effect of ammonia, this reduction was not accompanied by decreased expression of the astrocytic glutamate transporter GLT-1 mRNA. Both Gln and ammonia decreased hypoosmolarity-induced (86) Rb efflux from the cells, but the effect was more pronounced with Gln. The results indicate that down-regulation of Kir4.1 may mediate distinct aspects of Gln-induced astrocytic dysfunction in HE.

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    • "The above sequence of events is causally related, among other factors, to excessive accumulation of GLU in the extracellular (perisynaptic) space of the brain, seen both in experimental HE animals [7] [8] [9] and human HE patients [10]. Two mutually not exclusive mechanisms have been implicated in the increase of extracellular GLU in HE: (i) impaired reuptake of released GLU to astrocytes associated with downregulation of astrocytic GLU transporters [11] [12] and/or of the inward rectifying astrocytic potassium channel Kir4.1 [13]; (ii) increased synaptic [14] or astrocytic GLU release [15] [16], partly related to excessive activation of NMDA receptors [3] [4]. While the relative contribution of mechanisms (i) and (ii) at different stages of HE has not been accurately assessed, investigations reported up-to-date unambiguously underscore the role of increased NMDA receptor-mediated neuronal GLU release in its acute phase [3] [17]. "
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