All living cells are confronted with the necessity to provide osmotic room for their inner machinery due to their fragile cell membrane. This osmotic room is provided by the Na-K-ATPase, which is an electrogenic pump that exchanges three sodium ions for two potassium ions. By doing so, it establishes two actively supported ion gradients (sodium and potassium) and generates a negative membrane potential, which pushes chloride out of the cell leading to a passive chloride gradient and thus provides the required osmotic room. This is an expensive mechanism. For neurons, half of their resting energy consumption is used by the Na-K-ATPase and thus the maintenances of the ion gradients and the membran potential.
The resulting gradients, which can be viewed as large energy stores, are then used to fuel other processes like substrate uptake. Substrates are taken up by the cell against their respective gradients by symport of sodium and/or antiport of potassium. In excitable cells the sodium and potassium gradients are used for another very specific task: the generation and propagation of action potentials. The mechanism of action potential generation uses the sodium and potassium gradient to trigger fast changes of the membrane potential. All changes in the ion gradients must be compensated by the Na-K-ATPase at the expense of higher ATP consumption. Therefore, all these processes strain energy metabolism as well as osmotic balance.
To investigate the mutual interplay of the different processes and their respective feedbacks on the system, we set up an electro-osmotic metabolic model. The electrical part of the model will take ion movements and ion concentrations into account explicitly, which will be used to calculate the osmotic forces needed for the osmotic part. For the metabolic part, enzyme kinetics will be used.