Hepatic encephalopathy (HE) is a neuropsychiatric disorder associated with hyperammonemia. Swelling of astrocytes is a primary cause of brain edema in patients and in animal models of HE. Hyperammonemic brain edema is aggravated by hypoosmotic imbalance due to hyponatremia, and is thought to be related not only to ammonia itself, but in the first instance to glutamine (Gln), which is synthesized in astrocytes in excess from ammonia and glutamate (Albrecht and Norenberg, 2006). This study addressed two questions 1) whether and in what degree is the impairment of ion and water homeostasis in astrocytes related to altered potassium transport, 2) if such disturbances occur, in what degree can they be ascribed to Gln accumulation? To this end we investigated the expression of the major potassium transport vehicles: glial inwardly rectifying potassium channels (Kir4.1, Kir2.1) and the Na+-K+-2Cl- cotransporter-1 (NKCC1), in brain of rats with experimentally induced HE and in rat cortical astrocytes treated in culture with ammonia or Gln. Thioacetamide-induced HE decreased the Kir4.1 expression in the rat cerebral cortex at both the mRNA and protein level, while the mRNA level of Kir2.1 and NKCC1 remained unaltered. In primary cortical astrocytes, Gln, but not ammonia, induced a decrease in the levels of Kir4.1 mRNA and protein. Kir2.1 and NKCC1 mRNA levels in cultured astrocytes were unchanged upon Gln administration. Treatment of cultured astrocytes with ammonia or Gln resulted in the reduction of the hypoosmolarity-induced potasium (86Rb) efflux from the cells. Treatment with Gln induced also a decrease in the astrocytic uptake of glutamate (D-[3H]-aspartate). The results suggest that decreased expression of Kir4.1 contributes to disturbances in ion and water homeostasis and impaired Glu clearance accompanying HE, and further bespeak the role of Gln in the pathogenesis of HE. However, direct correlation between Kir4.1 downregulation and impairment of potassium fluxes awaits to be confirmed. Experiments using siRNA duplexes, selectively downregulating Kir4.1 in cultured astrocytes, are conducted to further investigate the involvement of Kir4.1 in astrocytic cell swelling and volume regulation.
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