ELECTRICAL INTERACTIONS BETWEEN A RABBIT ATRIAL CELL AND A NODAL CELLMODEL

Atrial activation involves interactions between cells with automaticit y and slow-response action potentials with cells that are intrinsicall y quiescent with fast-response action potentials. Understanding normal and abnormal atrial activity requires an understanding of this proces s. We studied interactions of a cell with spontaneous activity, repres ented by a ''real-time'' simulation of a model of the rabbit sinoatria l (SA) node cell, simultaneously being electrically coupled via our '' coupling clamp'' circuit to a real, isolated atrial myocyte with varia tions in coupling conductance (G(c)) or stimulus frequency. The atrial cells were able to be driven at a regular rate by a single SA node mo del (SAN model) cell. Critical G(c) for entrainment of the SAN model c ell to a nonstimulated atrial cell was 0.55 +/- 0.05 nS (n = 7), and t he critical G(c) that allowed entrainment when the atrial cell was dir ectly paced at a basic cycle length of 300 ms was 0.32 +/- 0.01 nS (n = 7). For each atrial cell we found periodic phenomena of synchronizat ion other than 1:1 entrainment when G(c) was between 0.1 and 0.3 nS, b elow the value required for frequency entrainment, when the atrial cel l was directly driven at a basic cycle length of either 300 or 600 ms. In conclusion, the high input resistance of the atrial cells allows s uccessful entrainment of nodal and atrial cells at low values of G(c), but further uncoupling produces arrhythmic interactions.