title: Kir4.1 channels mediate a depolarization of hippocampal astrocytes under hyperammonemic conditions in situ.
abstract: Increased ammonium (NH(4) (+) ) concentration in the brain is the prime candidate responsible for hepatic encephalopathy (HE), a serious neurological disorder caused by liver failure and characterized by disturbed glutamatergic neurotransmission and impaired glial function. We investigated the mechanisms of NH(4) (+) -induced depolarization of astrocytes in mouse hippocampal slices using whole-cell patch-clamp and potassium-selective microelectrodes. At postnatal days (P) 18-21, perfusion with 5 mM NH(4) (+) evoked a transient increase in the extracellular potassium concentration ([K(+) ](o) ) by about 1 mM. Astrocytes depolarized by on average 8 mV and then slowly repolarized to a plateau depolarization of 6 mV, which was maintained during NH(4) (+) perfusion. In voltage-clamped astrocytes, NH(4) (+) induced an inward current and a reduction in membrane resistance. Amplitudes of [K(+) ](o) transients and astrocyte depolarization/inward currents increased from P3-4 to P18-21. Perfusion with 100 μM Ba(2+) did not alter [K(+) ](o) transients but strongly reduced both astrocyte depolarization and inward currents. NH(4) (+) -induced depolarization and inward currents were also virtually absent in slices from Kir4.1 -/- mice, while [K(+) ](o) transients were unaltered. Blocking Na(+) /K(+) -ATPase with ouabain caused an immediate and complex increase in [K(+) ](o) . Taken together, our results are in agreement with the hypothesis that reduced uptake of K(+) by the Na(+) , K(+) -ATPase in the presence of NH(4) (+) disturbs the extracellular K(+) homeostasis. Furthermore, astrocytes depolarize in response to the increase in [K(+) ](o) and by influx of NH(4) (+) through Kir4.1 channels. The depolarization reduces the astrocytes' capacity for channel-mediated flux of K(+) and for uptake of glutamate and might hereby contribute to the pathology of HE.
authors.affiliation: Institute of Neurobiology, Faculty of Mathematics and Natural Sciences, Heinrich Heine University Düsseldorf, Universitätsstrasse 1, Düsseldorf, Germany.
authors.forename: Jonathan
authors.initials: J
authors.lastname: Stephan
authors.forename: Nicole
authors.initials: N
authors.lastname: Haack
authors.forename: Karl W
authors.initials: KW
authors.lastname: Kafitz
authors.forename: Simone
authors.initials: S
authors.lastname: Durry
authors.forename: Daniel
authors.initials: D
authors.lastname: Koch
authors.forename: Peter
authors.initials: P
authors.lastname: Hochstrate
authors.forename: Gerald
authors.initials: G
authors.lastname: Seifert
authors.forename: Christian
authors.initials: C
authors.lastname: Steinhäuser
authors.forename: Christine R
authors.initials: CR
authors.lastname: Rose
mesh_terms: D000367:Age Factors; D000641:Ammonia / Q000378:metabolism; D000818:Animals; D000831:Animals, Newborn; D001253:Astrocytes / Q000187:drug effects* / Q000502:physiology; D001703:Biophysics; D002034:Bumetanide / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004558:Electric Stimulation; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D005665:Furosemide / Q000494:pharmacology; D005904:Glial Fibrillary Acidic Protein / Q000378:metabolism; D006624:Hippocampus / Q000166:cytology*; D066298:In Vitro Techniques; D008564:Membrane Potentials / Q000187:drug effects* / Q000235:genetics; D051379:Mice; D008807:Mice, Inbred BALB C; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000378:metabolism; D026902:Potassium Channel Blockers / Q000494:pharmacology; D024661:Potassium Channels, Inwardly Rectifying / Q000172:deficiency / Q000378:metabolism*; D000644:Quaternary Ammonium Compounds / Q000494:pharmacology*; D026941:Sodium Channel Blockers / Q000494:pharmacology; D049994:Sodium Potassium Chloride Symporter Inhibitors / Q000494:pharmacology; D019789:Tetraethylammonium / Q000494:pharmacology; D013779:Tetrodotoxin / Q000494:pharmacology; D000098854:Kcnj10 Channel
chemical_list: D018691:Excitatory Amino Acid Antagonists; D005904:Glial Fibrillary Acidic Protein; D000098854:Kcnj10 Channel; D026902:Potassium Channel Blockers; D024661:Potassium Channels, Inwardly Rectifying; D000644:Quaternary Ammonium Compounds; D026941:Sodium Channel Blockers; D049994:Sodium Potassium Chloride Symporter Inhibitors; D002034:Bumetanide; D013779:Tetrodotoxin; D019789:Tetraethylammonium; D000641:Ammonia; D005665:Furosemide; D011188:Potassium