Endocrine Abstracts

Hepatic encephalopathy (HE), a syndrome of neurological abnormalities caused by impaired liver function, is induced by many chronic liver diseases, such as liver failure or hepatic portosystemic shunts. Whilst multiple substances reach toxic levels in the systemic circulation of HE patients, hyperammonemia (excessive ammonia) is known to be a major contributing factor to the neurological disturbances. Astrocytes utilise ammonia in generating glutamine leading to brain oedema and neurocytotoxicity in HE patients, through unknown mechanisms. Interestingly, astrocytes are known to be targets for C-type natriuretic peptide (CNP) action. In this study, we examine molecular changes occurring in astrocytes during hyperammonaemic conditions. Rat C6 glioma cells were treated with 0 – 10 mM NH₄Cl (an ammonia donor) or 100 nM CNP for up to 72 h before extracting total RNA, and utilising multiplex quantitative RT-PCR to examine gene expression (Gad1, Gfap, S100b, Hmox1, Tspo, Insr, Npr1, Npr2, Npr3, Nppa, Nppb, Nppc, simultaneously). After 72 h, hyperammonaemia caused a significant increase in Insr (**P<0.01), Npr3 (*P<0.05), and Nppb (***P<0.001) expression, but inhibited Gfap (*P<0.05). In contrast, the effects of CNP were more rapid, with significant increases in S100b (*P<0.05), Hmox1 (*P<0.05), Tspo (*P<0.05), and Npr2 (*P<0.05) expression within 24 h. To establish whether hyperammonaemia affects CNP function in astrocytes, C6 cells were pre-treated with 10 mM NH4Cl for 24 or 1 h, prior to stimulation with 100 nM CNP for 0, 5 or 15 min. CNP-stimulated cGMP accumulation underwent homologous downregulation between 5 and 15 mins, regardless of exposure to hyperammonaemia. However, acute (1 h) but not chronic (24 h) induction of hyperammonaemia caused inhibition of cGMP accumulation consistent with heterologous desensitisation. Collectively, these data identify novel molecular targets that may contribute the HE phenotype in astrocytes, and suggest an interaction between ammonia and CNP signalling.