SIMULATING CARDIAC SINUS AND ATRIAL NETWORK DYNAMICS ON THE CONNECTION MACHINE

Computational methods for simulating biophysically detailed, large-sca le models of mammalian cardiac sinus and atrial networks on the massiv ely parallel Connection Machine CM-2, and techniques for visualization of simulation data, are presented. Individual cells are modeled using the formulations of Noble et al. Models incorporate properties of vol tage-dependent membrane currents, ion pumps and exchangers, and intern al calcium sequestering and release mechanisms. Network models are use d to investigate factors determining the site of generation and direct ion of propagation of the pacemaker potential. Models of the isolated sinus node are used to show that very few gap junction channels are re quired to support frequency entrainment. When cell membrane properties in the isolated sinus node models are modified to reproduce regional differences in oscillation properties, as described by the data of Kod ama and Boyett, an excitatory wave is generated in the node periphery which propagates towards the node center. This agrees with activation patterns measured in the isolated sinus node by Kirchoff. When the mod el sinus node is surrounded by a region of atrial cells, the site of p acemaker potential generation is shifted away from the periphery towar ds the node center. This is in agreement with activation patterns meas ured by Kirchoff in the intact sinus node of the rabbit heart, and dem onstrates the importance of sinus node boundary conditions on shaping the site of generation and direction of propagation of the pacemaker p otential.