Ammonium increases Ca2+ signalling and upregulates expression of Cav1.2 gene in astrocytes in primary cultures and in the invivo brain

AimThe primary aim of this study was to identify the effects of hyperammonaemia on functional expression of Ca(v)1.2L-type Ca2+ channels in astroglia. MethodsPrimary cultures of mouse astrocytes were used to study effects of chronic treatment (1-5days) with ammonium chloride, at 1, 3 and 5mm on depolarization-induced increases in free cytosolic Ca2+ concentration ([Ca2+](i), measured with Fura-2 based microfluorimetry) in control conditions and following treatment with the L-type Ca2+ channel inhibitor, nifedipine, or with ryanodine receptor inhibitor, ryanodine. Expression of Ca(v)1.2 mRNA was identified with RT-PCR, whereas protein content was determined by Western blotting. Sustained hyperammonaemia invivo was induced by daily injections of urease (33unitskg body weight(-1), i.p.) for 3days. ResultsDepolarization-induced [Ca2+](i) transients sensitive to nifedipine (peak of the response) and to ryanodine (plateau phase) were significantly increased in astrocytes chronically exposed to ammonium. The ammonium-induced increase in Ca2+ influx in astrocytes resulted from an upregulation of Ca(v)1.2 channel's expression detected at mRNA and protein levels. Increase in Ca(v)1.2 expression was prevented by ouabain antagonist canrenone. Similar upregulation of Ca(v)1.2 gene expression was found in the brains of adult mice subjected to intraperitoneal injection of urease. In transgenic mice tagged with an astrocyte-specific or neurone-specific markers and treated with intraperitoneal injections of urease, the fluorescence-activated cell sorting of neurones and astrocytes demonstrated that Ca(v)1.2 mRNA expression was upregulated in astrocytes, but not in neurones. ConclusionsAmmonium-induced deregulation of astroglial Ca2+ signalling, is, in part, associated with upregulation of Ca(v)1.2L-type calcium channels.