Combined effect of oxygen and ammonia on the kinetics of ammonia elimination and oxygen consumption of adherent rat liver cells

Oxygen is essential for the survival of isolated liver cells and its concentration is known to affect their viability and function. Recent reports have also shown that ammonia is eliminated at a rate depending on its concentration and that high ammonia concentrations may be cytotoxic to rat liver cells. Nonetheless, little quantitative information on the effect of either metabolite on liver cell reaction kinetics is available although important to the design of bioreactors for bioartificial livers (BALs). In this investigation, we characterized the dependence of the rate of oxygen consumption (OCR), ammonia elimination (AER) and urea synthesis (USR) on ammonia concentration at physiological (i.e., 43 and 72 mmHg) and supra-physiological (i.e., 134 mmHg) dissolved oxygen tensions. To this purpose, isolated rat liver cells were cultured in adhesion on collagen in a continuous-flow bioreactor optimised for the kinetic characterisation of liver cell metabolic reactions. Rates of the investigated reactions generally increased with increasing ammonia concentrations. OCR and USR significantly increased with increasing dissolved oxygen tensions, particularly at high ammonia concentrations. The actual dissolved oxygen tension significantly influenced also OCR and USR dependence on ammonia concentration. The best-fit rate equations were used to show that, at the beginning of the treatment with a bioreactor packed with primary liver cells, high ammonia concentration in the blood may cause large hypoxic zones in the bioreactor as a result of its effect on OCR. This suggests that plasma (or blood) detoxification prior to entering the bioreactor might enhance BAL efficacy by preserving a large fraction of the available cell activity for longer times.