Computer simulation of the electrotonic modulation of pacemaker activity in the sinoatrial node by atrial muscle

Electrotonic interaction between the sinoatrial (SA) node and surrounding atrial muscle was investigated in a computer simulation using a modified Oxsoft HEART model (Oxsoft, Oxford, UK). When an SA node cell model was coupled to a passive atrial membrane model (RC circuit) with various coupling conductances (G_c), there was a G_c-dependent prolongation of spontaneous cycle length (SCL). At a sufficiently high value of G_c, the spontaneous activity was finally stopped. A nonlinear relationship between G_c and SCL was obtained, similar to that observed in experiments on rabbit SA node cells. When the muscarinic potassium current (i_K,ACh) was activated in the SA node cell model, the coupling-induced inhibition of pacemaker activity was potentiated. Although coupling current and i_K,ACh were additive, their effects on SCL were more than additive because of the nonlinear dependence of SCL on net current. A decrease in the input resistance of the atrial membrane model to simulate the activation of i_K,ACh in atrial muscle was also shown to potentiate the coupling-induced inhibition of SA node spontaneous activity.