Adenosine Reduces Sinoatrial Node Cell AP Firing Rate by uncoupling its Membrane and Calcium Clocks

Abstract

AO_SCPLOWBSTRACTC_SCPLOWThe spontaneous action potential (AP) firing rate of sinoatrial nodal cells (SANC) is regulated by a system of intracellular Ca2+ and membrane ion current clocks driven by Ca2+-calmodulin-activated adenylyl cyclase-protein kinase A (PKA) signaling. The mean AP cycle length (APCL) and APCL variability inform on the effectiveness of clock coupling. Endogenous ATP metabolite adenosine (ado) binds to adenosine receptors that couple to Gi protein-coupled receptors, reducing spontaneous AP firing rate via G{beta}{gamma} signaling that activates an membrane-clock outward current, IKACh. Ado also inhibits adenylyl cyclase activity via Gi signaling, impacting cAMP-mediated PKA-dependent protein phosphorylation and intracellular Ca2+ cycling. We hypothesize that in addition to IKAdo activation, ado signaling impacts Ca2+ via Gi signaling and that both effects reduce AP firing rate by reducing the effectiveness of the Ca2+ and membrane clock coupling. To this end, we measured Ca2+ and membrane potential characteristics in enzymatically isolated single rabbit SANC. 10 {micro}M ado substantially increased both the mean APCL (on average by 43%, n=10) and AP beat-to-beat variability from 5.1{+/-}1.7% to 7.2{+/-}2.0% (n=10) measured via membrane potential and 5.0{+/-}2.2 to 10.6{+/-}5.9 (n=40) measured via Ca2+ (assessed as the coefficient of variability, CV=SD/mean). These effects were mediated by hyperpolarization of the maximum diastolic membrane potential (membrane clock effect) and suppression of diastolic spontaneous, local Ca2+ releases (LCRs) (Ca2+ clock effect): as LCR size distributions shifted from larger to smaller values, the time of LCR occurrence during diastolic depolarization (LCR period) became prolonged, and the ensemble LCR Ca2+ signal became reduced. The tight linear relationship of coupling between LCR period to the APCL in the presence of ado "drifted" upward and leftward, i.e. for a given LCR period, APCL was prolonged, becoming non-linear indicating clock uncoupling. An extreme case of uncoupling occurred at higher ado concentrations (>100 {micro}M): small stochastic LCRs of the Ca2+ clock failed to self-organize and synchronize to the membrane clock, thus creating a failed attempt to generate an AP resulting in arrhythmia and cessation of AP firing. Thus, the effects of ado to activate G{beta}{gamma} and IKACh, Ado and to activate Gi, suppressing adenylyl cyclase activity, both contribute to the ado-induced increase in the mean APCL and APCL variability by reducing the fidelity of clock coupling and AP firing rate.