Functional heterogeneity of cell populations increases robustness of pacemaker function in a numerical model of the sinoatrial node tissue

Each heartbeat is initiated by specialized pacemaker cells operating within the sinoatrial node (SAN). While individual cells within SAN tissue exhibit substantial heterogeneity of their electrophysiological parameters and Ca cycling, the role of this heterogeneity for cardiac pacemaker function remains mainly unknown. Here we investigated the problem numerically in a 25x25 square grid of coupled-clock Maltsev-Lakatta cell models and tested the hypothesis that functional heterogeneity of cell populations increases robustness of SAN function. The tissue models were populated by cells with different degree of heterogeneity of the two key model parameters of the coupled-clock system, maximum L-type Ca current conductance (gCaL) and sarcoplasmic reticulum Ca pumping rate (Pup). Our simulations showed that in the areas of Pup-gCaL parametric space at the edge of the system stability where action potential (AP) firing was absent or dysrhythmic in tissues populated by identical cells, rhythmic AP generation was rescued in tissues populated by cells with uniformly random distributions of gCaL or Pup (but keeping the same average values). This effect to increase robust AP generation was synergistic with respect to heterogeneity in both gCaL and Pup and was further strengthened by clustering of cells with higher gCaL or Pup. The effect of functional heterogeneity was not due to a simple summation of activity of intrinsically firing cells naturally present in SAN; rather AP firing cells locally and critically interacted with non-firing/dormant cells. When firing cells prevailed, they recruited many dormant cells to fire, strongly enhancing overall SAN function. And vice versa, prevailing dormant cells suppressed AP firing in cells with intrinsic automaticity and halted SAN automaticity.