Computer simulation of the Ca/sup ++/ regulation mechanisms in spherical-type cells

We simulate the behaviour of the Na/sup +/-Ca/sup ++/ exchanger as a permanent regulator of calcium homeostasis. As a consequence of a membrane potential or a concentration change, we numerically evaluate the participation of this exchanger in returning to equilibrium cellular conditions. In our model, the Na/sup +/-Ca/sup ++/ exchanger is supposed to work joined to other important regulation mechanisms such as the calcium pump, the calcium channels, the calcium buffers and the passive diffusion of calcium, as described in past work. For evaluating the latter parameter, we are using a previously reported homogenous multi-layer model of a spherical cell. The graphical and numerical results were obtained using a PC-based environment over the Matlab platform, and these were compared to the results obtained with a C-language simulation. The results showed the Na/sup +/-Ca/sup ++/ exchanger is always participating in the transport of calcium ions, and it only modifies its direction and/or its speed as a consequence of a stimuli. Our model also simulates the reversible behaviour of the exchanger for free sodium of calcium variations, including the influence of the membrane potential and the affinity values. Because of the high time constant of the Na/sup +/-Ca/sup ++/ exchanger, the simulation must be robust enough to evaluate all the changes occurring during a long time. With our current simulation programs, it is only possible to do it for the first few seconds. Nevertheless, we are now working on the improvement (precision, speed, resources) of the work described, using an SGI workstation and a parallel processing environment.