Heart Rate Acceleration of Subsidiary Pacemaker by β-Adrenergic Stimulation

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Backgrounds: The membrane voltage and Ca2+ clocks jointly regulate sinoatrial node (SAN) automaticity. Here we test the hypothesis that heart rate acceleration of subsidiary pacemaker (SP) by β-adrenergic stimulation involves synergistic interactions between both clock mechanisms. Methods: We performed optical mapping and pharmacological interventions in 15 isolated Langendorff-perfused canine right atriums (RA). SP model were produced by ligation of SAN artery at the mid portion of sulcus terminalis. Results: In 6 RAs with intact SAN, isoproterenol infusion of 1 µmol/L increased heart rate from 82±9 to 166±18 bpm (102%) with late diastolic Cai elevation (LDCAE) at superior SAN. However, in 6 SP models, heart rate increased from 55±10 bpm to 106±11 bpm (92%, p=0.005) without LDCAE at the earliest activation site. The isoproterenol induced heart rate increase was reversed to 74±5 bpm (33% from baseline) by funny current blocker, ZD 7288 infusion (3± mol/L, n=3). Whereas, it was suppressed to 69±7 bpm (24% from baseline) by SR Ca2+ emptying with ryanodine (10± mol/L) plus thapsigargin (200 nmol/L, n=3). The isoproterenol induced heart rate increase was completely abolished by combined funny current blocker and SR Ca2+ emptying (n=3). Conclusion: Acceleration of the Ca2+ clock in SP plays an important role in heart rate acceleration during β-adrenergic stimulation, interacting synergistically with the voltage clock to increase heart rate.