A computer simulation of the effect of heart rate on ion concentrations in the heart

At steady-state the passive fluxes of Na+ and K+ across the cell membrane of a heart cell are exactly matched by active fluxes of the two ions in the opposite direction via the Na-K pump, and the concentrations of Na+ and K+ both within the cell and in the clefts between cells are steady. An alteration of the heart rate (or the rate of stimulation) disrupts this balance because the passive fluxes are affected, and there are changes in pump activity as well as the Na+ and K+ concentrations. A computer model incorporating a cell separated from the bathing medium by a restricted extracellular cleft was devised to investigate these changes further. The model was able to simulate the changes observed with a variety of stimulation protocols as well as the effect of block of the Na-K pump. It is concluded that the changes in Na+ and K+ balance with heart rate can be explained in terms of the known properties of cardiac tissue incorporated into the model.