Mechanisms of ammonia and ammonium ion toxicity in animal cells: transport across cell membranes.

A model for transport of ammonia and ammonium ions across cell membranes is presented. The model suggests that ammonium ions compete with potassium ions for inward transport, over the cytoplasmic membrane, via potassium transport proteins like the Na+/K(+)-ATPase and the Na+K+2Cl(-)-cotransporter. It also explains the difference between the ammonia/ammonium that is added to the cells and which is formed by the cells during metabolism of amino acids, especially glutamine and glutamate. The ammonium transport and subsequent events lead to predictable intracellular and extracellular pH (pHe) changes. Experiments which verified the model and the predicted consequences were performed by measurements of the pHe in concentrated cell suspensions. Addition of ammonium ions caused a time-dependent pHe increase which was inhibited by potassium ions. The test system is not per se specific for transport measurements but the effect of potassium ions on the pHe strongly favors our suggested model. Simple diffusion of ammonium ions would not be counteracted by potassium ions. The results show that ammonium ion transport in the murine myeloma cell line (Sp2/0-Ag14) used is inhibited by an excess of potassium ions. Results from experiments with specific inhibitors of suggested transport proteins were not conclusive. It is postulated that one important toxic effect of ammonia/ammonium is an increased demand for maintenance energy, caused by the need to maintain ion gradients over the cytoplasmic membrane. The results also suggest that potassium ions can be used to detoxify ammonia/ammonium in animal cell cultivations.