Intracellular calcium dynamics and acceleration of sinus rhythm by β-adrenergic stimulation

Background - Recent evidence indicates that membrane voltage and Ca ²⁺ clocks jointly regulate sinoatrial node (SAN) automaticity. Here we test the hypothesis that sinus rate acceleration by β-adrenergic stimulation involves synergistic interactions between these clock mechanisms. Methods and Results - We simultaneously mapped intracellular calcium (Ca ₁) and membrane potential in 25 isolated canine right atrium, using previously described criteria of the timing of late diastolic Ca₁ 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. Ca₁ 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 β-adrenergic stimulation, interacting synergistically with the voltage clock to increase sinus rate.