Ammonium Increases Ca2+ Signalling and Up-Regulates Expression of TRPC1 Gene in Astrocytes in Primary Cultures and in the In Vivo Brain

Rapid rise in ammonium concentration in the brain is the major pathogenic factor in hepatic encephalopathy that is manifested by state of confusion, forgetfulness and irritability, psychotic symptoms, delusions, lethargy, somnolence and, in the terminal stages, coma. Primary cultures of mouse astrocytes were used to investigate effects of chronic treatment (3 days) with ammonium chloride (ammonium) at 3 mM, this being a relevant concentration for hepatic encephalopathy condition, on metabotropic receptor agonist-induced increases in free cytosolic Ca2+ concentration [(Ca2+)(i)], measured with fura-2 based microfluorimetry and on store-operated Ca2+ entry (SOCE) activated following treatment with the SERCA inhibitor thapsigargin. The agonists used were the beta-adrenergic agonist isoproterenol, the alpha(2)-adrenergic agonist dexmedetomidine, the InsP(3) receptor (InsP(3)R) agonist adenophostin A and ryanodine receptor agonist 4-Chloro-m-cresol (4-CMC). Agonist-induced [Ca2+] i responses were significantly increased in astrocytes chronically exposed to ammonium. Similarly, the SOCE, meditated by the transient receptor potential channel 1 (TRPC1), was significantly augmented. The ammonium-induced increase in SOCE was a result of an up-regulation of mRNA and protein expression of TRPC1 in astrocytes. Increase in TRPC1 expression and in SOCE were both prevented by ouabain antagonist canrenone. Similar up-regulation of TRPC1 gene expression was found in the brain of adult mice subjected to intraperitoneal injection of urease for 3 days. In transgenic mice tagged with an astrocyte-specific or a neurone-specific markers and treated with intraperitoneal injections of urease for 3 days, the fluorescence-activated cell sorting of neurones and astrocytes demonstrated that TRPC1 mRNA expression was up-regulated in astrocytes, but not in neurones.