Mechanisms of pacing during diastolic depolarization in a modeled sinoatrial node cell

Spontaneous pacing is an intrinsic property of the pacemaker cells in the sinoatrial node (SAN) of the heart. Models of the SAN cells have been developed over the last two decades to simulate the pacemaker activity. In this study, the most recent and biophysically detailed computational model of the SAN cell (S. Demir et al., Amer. J. Physiol, vol. 266, p. C832-52, 1994) was used to investigate the ionic properties that govern the diastolic depolarization phase of the action potential. The ionic mechanisms in diastole were investigated under both control and stimulated conditions. The control condition represents when the SAN cell is pacing spontaneously without any stimulus. The stimulated condition is when the pacing activity is perturbed by an electrical pulse. Phase-plane analysis techniques, illustrating the changes in current-voltage variables, were utilized in the computer simulations. The simulation data for the control and stimulated conditions demonstrate that the changes in I_K, I_Naca and I _f during the first half of diastole, and changes in I_CaT and I_Na during the second half determine the diastolic depolarization rate and the occurrence of the next action potential