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 (Gc), there was a Gc-dependent prolongation of spontaneous cycle length (SCL). At a sufficiently high value of Gc, the spontaneous activity was finally stopped. A nonlinear relationship between Gc and SCL was obtained, similar to that observed in experiments on rabbit SA node cells. When the muscarinic potassium current (iK,ACh) was activated in the SA node cell model, the coupling-induced inhibition of pacemaker activity was potentiated. Although coupling current and iK,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 stimulate the activation of iK,ACh in atrial muscle was also shown to potentiate the coupling-induced inhibition of SA node spontaneous activity.