The pacemaker activity of the S-A node cell was explained by reconstructing the pacemaker potential using a Hodgkin-Huxley type mathematical model which was based on the reported voltage clamp data.In this model four dynamic currents, the sodium current, i_Na, the slow inward current, i_s, the potassium current, i_K, and the hyperpolarization-activated current, i_h, in addition to a time-independent leak current, i_l were included. The model simulated the spontaneous action potential the current voltage relationship, and the voltage clamp experiment in normal Tyrode solution of the rabbit S-A node.Furthermore, the changes of activity induced by the potassium current blocker Ba²⁺, by applying constant current, acetylcholine, and epinephrine were also reconstructed.
It was strongly suggested that the pacemaker depolarization in the S-A node cell is mainly due to a gradual increase of i_s during diastole, and that the contribution of i_k is much less compared to the potassium current i_K2 in the Purkinje fiber pacemaker depolarization. The rising phase of the action potential was due to is and the plateau phase is determined by both the inactivation of i_s and activation of i_k.