Kir4.1 channels mediate a depolarization of hippocampal astrocytes under hyperammonemic conditions in situ

Increased ammonium (NH4 +) 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 NH4 +-induced depolarization of astrocytes in mouse hippocampal slices using whole-cell patch-clamp and potassium-selective microelectrodes. At postnatal days (P) 1821, perfusion with 5 mM NH4 + 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 NH4 + perfusion. In voltage-clamped astrocytes, NH4 + induced an inward current and a reduction in membrane resistance. Amplitudes of [K+]o transients and astrocyte depolarization/inward currents increased from P34 to P1821. Perfusion with 100 mu M Ba2+ did not alter [K+]o transients but strongly reduced both astrocyte depolarization and inward currents. NH4 +-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 NH4 + disturbs the extracellular K+ homeostasis. Furthermore, astrocytes depolarize in response to the increase in [K+]o and by influx of NH4 + 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. (c) 2012 Wiley Periodicals, Inc.