Electrotonic interactions between fibroblasts and pacemaker cells within the SA node are investigated using a two-dimensional lattice model of a circular SA node embedded within a surrounding square grid of atrial cells. In the simulations reported here, the SA node was 33 cells in diameter, and was positioned in the centre of a 64×64 grid of surrounding atrial cells. Cell-to-cell gap junction coupling within the node was set at a low uniform value of 10 nS. Coupling within the surrounding atrium was set at a higher, uniform value of 100 nS. Gap junction coupling between border SA node and atrial cells was also set to 100 nS.This step discontinuity in coupling strength between the SA node and atrium, as well as approximately uniform coupling within the node, are supported by recent studies using im-munofluorescent antibodies to label the connexin-43 gap junction protein . A coupling value of 10 nS within the node is sufficient to assure complete frequency entrain-ment of node cells , yields central and peripheral conduction velocities similar to those measured experimentally and is close to the coupling value estimated in pairs of dissociated SA node cells . The coupling value of 100 nS used within the atrium and at the SA node/atrial junction is large enough to provide a substantial electrotonic load to the model SA node, but is almost certainly smaller than that which exists in the real rabbit atrium. Coupling values in the range of 1000 nS reproduce experimentally determined atrial conduction velocities , but would – in order to counterbalance the increased ‘realistic’ atrial load by an increased ‘realistic’ SA node size – require a substantial increase of grid size without yielding additional fundamental insights into fibroblast-SA node interaction. You will always be offered cialis super active online pharmacy at the pharmacy you can trust.