The calcium and voltage clocks in sinoatrial node automaticity

Recent evidence indicates that the voltage (cyclic activation and deactivation of membrane ion channels) and Ca(2+) clocks (rhythmic spontaneous sarcoplasmic reticulum Ca(2+) release) jointly regulate sinoatrial node (SAN) automaticity. Since the intact SAN is a heterogeneous structure that includes multiple different cell types interacting with each other, the relative importance of the voltage and Ca(2+) clocks for pacemaking may be variable in different regions of the SAN. Recently, we performed optical mapping in isolated and Langendorff-perfused canine right atria. We mapped the intracellular calcium (Ca(i)) and membrane potentials of the intact SAN simultaneously. Using previously described criteria of the timing of the late diastolic Ca(i) elevation (LDCAE) relative to the action potential upstroke to detect Ca(2+) clock activity, we demonstrated that the sinus rate increased and the leading pacemaker shifted to the superior SAN with the robust LDCAE during beta-adrenergic stimulation. We also showed that the LDCAE was caused by spontaneous diastolic SR Ca(2+) release and was closely related with heart rate changes. We conclude that the Ca(2+) and voltage clocks work synergistically to generate SAN automaticity.