β-Adrenergic Stimulation Synchronizes a Broad Spectrum of Action Potential Firing Rates of Cardiac Pacemaker Cells towards a Higher Population Average

The heartbeat is initiated by pacemaker cells residing in the sinoatrial node (SAN). SAN cells generate spontaneous action potentials (APs), i.e. normal automaticity. The sympathetic nervous system increases heart rate commensurate with cardiac output demand via stimulation of SAN β-adrenergic receptors (βAR). While SAN cells reportedly represent a highly heterogeneous cell population, the current dogma is that in response to βAR stimulation all cells increase their spontaneous AP firing rate in a similar fashion. The aim of the present study was to investigate cell-to-cell variability in the responses of a large population of SAN cells. We measured βAR responses among 166 single SAN cells isolated from 33 guinea pig hearts. In contrast to the current dogma, the SAN cell responses to βAR stimulation substantially varied. In each cell, changes in AP cycle length highly correlated (R 2 =0.97) with the AP cycle length before βAR stimulation. While, as expected, on average the cells increased their pacemaker rate, greater responses were observed in cells with slower basal rates, and vice versa, cells with higher basal rates showed smaller responses, no responses, or even decreased their rate. Thus, βAR stimulation synchronizes the operation of the SAN cell population towards a higher average rate, rather than uniformly shifting the rate in each cell, creating a new paradigm of βAR-driven fight-or-flight response among individual pacemaker cells.