A MATHEMATICAL-MODEL OF A RABBIT SINOATRIAL NODE CELL

A mathematical model for the electrophysiological responses of a rabbi t sinoatrial node cell that is based on whole cell recordings from enz ymatically isolated single pacemaker cells at 37 degrees C has been de veloped. The ion channels, Na+-K+ and Ca2+ pumps, and Na+-Ca2+ exchang er in the surface membrane (sarcolemma) are described using equations for these known currents in mammalian pacemaker cells. The extracellul ar environment is treated as a diffusion-limited space, and the myopla sm contains Ca2+-binding proteins (calmodulin and troponin). Original features of this model include 1) new equations for the hyperpolarizat ion-activated inward current, 2) assessment of the role of the transie nt-type Ca2+ current during pacemaker depolarization, 3) inclusion of an Na+ current based on recent experimental data, and 4) demonstration of the possible influence of pump and exchanger currents and backgrou nd currents on the pacemaker rate. This model provides acceptable fits to voltage-clamp and action potential data and can be used to seek bi ophysically based explanations of the electrophysiological activity in the rabbit sinoatrial node cell.