Role of individual ionic current systems in the SA node hypothesized by a model study

This paper discusses the development of a cardiac sinoatrial (SA) node pacemaker model. The model successfully reconstructs the experimental action potentials at various concentrations of external Ca2+ and K+. Increasing the amplitude of L-type Ca2+ current (l(CaL)) prolongs the duration of the action potential and thereby slightly decreases the spontaneous rate. On the other hand, a negative voltage shift Of l(CaL) gating by a few mV markedly increases the spontaneous rate. When the amplitude of sustained inward current (l(st)) is increased, the spontaneous rate is increased irrespective of the l(CaL) amplitude. Increasing [Ca2+], shortens the action potential and increases the spontaneous rate. When the spontaneous activity is stopped by decreasing l(CaL) amplitude, the resting potential is nearly constant (-35mV) over 1-15mm [K+](0) as observed in the experiment. This is because the conductance of the inward background non-selective cation current balances with the outward [K+](o)-dependent K+ conductance. The unique role of individual voltage- and time-dependent ion channels is,clearly demonstrated and distinguished from, that of the background current by calculating an instantaneous zero current potential ("lead potential") during the course of the spontaneous activity.