Background—Recent evidence indicates that membrane voltage and Ca²⁺ clocks jointly regulate sinoatrial node (SAN) automaticity. Here we test the hypothesis that sinus rate acceleration by -adrenergic stimulation involves synergistic interactions between these clock mechanisms.

Methods and Results—We simultaneously mapped intracellular calcium (Ca_i) and membrane potential in 25 isolated canine right atrium, using previously described criteria of the timing of late diastolic Ca_i elevation (LDCAE) relative to the action potential upstroke to detect the Ca²⁺ clock. Before isoproterenol, the earliest pacemaking site occurred in the inferior SAN, and LDCAE was observed in only 4 of 25 preparations. Isoproterenol infusion (1 µmol/L) increased sinus rate and shifted pacemaking site to superior SAN, concomitant with the appearance of LDCAE preceding the action potential upstroke by 98±31 ms. Caffeine had similar effects, whereas sarcoplasmic reticulum Ca²⁺ depletion with ryanodine and thapsigargin prevented isoproterenol-induced LDCAE and blunted sinus rate acceleration. Ca_i transient relaxation time during isoproterenol was shorter in superior SAN (124±34 ms) than inferior SAN (138±24 ms; P=0.01) or right atrium (164±33 ms; P=0.001) and was associated with a lower sarcoplasmic reticulum Ca²⁺ ATPase pump to phospholamban protein ratio in SAN than in right atrium. Hyperpolarization-activated pacemaker current (I_f) blockade with ZD 7288 modestly blunted but did not prevent LDCAE or sinus rate acceleration by isoproterenol.

Conclusions—Acceleration of the Ca²⁺ clock in the superior SAN plays an important role in sinus acceleration during -adrenergic stimulation, interacting synergistically with the voltage clock to increase sinus rate.