High blood levels of
ammonium/
ammonia (NH(4)(+)/NH(3)) are associated with severe neurotoxicity as observed in
hepatic encephalopathy (HE). Astrocytes are the main targets of
ammonium toxicity, while neuronal cells are less vulnerable. In the present study, an
astrocytoma cell line 1321N1 and a
neuroblastoma glioma hybrid cell line NG108-15 were used as model systems for astrocytes and neuronal cells, respectively.
Ammonium salts evoked a transient increase in intracellular
calcium concentrations ([Ca(2+)](i)) in
astrocytoma (EC(50)=6.38 mM), but not in NG108-15 cells. The
ammonium-induced increase in [Ca(2+)](i) was due to an intracellular effect of NH(4)(+)/NH(3) and was independent of extracellular
calcium.
Acetate completely inhibited the
ammonium effect.
Ammonium potently reduced calcium signaling by G(q)
protein-coupled receptors (H(1) and M3) expressed on the cells.
Ammonium (5 mM) also significantly inhibited the proliferation of 1321N1
astrocytoma cells. While
mRNA for the mammalian
ammonium transporters RhBG and RhCG could not be detected in 1321N1
astrocytoma cells, both transporters were expressed in NG108-15 cells. RhBG and RhBC in brain may promote the excretion of NH(3)/NH(4)(+) from neuronal cells. Cellular uptake of NH(4)(+)/NH(3) was mainly by passive diffusion of NH(3). Human 1321N1
astrocytoma cells appear to be an excellent, easily accessible human model for studying HE, which can substitute animal studies, while NG108-15 cells may be useful for investigating the role of the recently discovered Rhesus family type
ammonium transporters in neuronal cells. Our findings may contribute to the understanding of pathologic
ammonium effects in different brain cells, and to the treatment of
hyperammonemia.