In sinoatrial node (SAN) cells, electrogenic sodium-calcium exchange (NCX) is the dominant calcium (Ca) efflux mechanism. However, the role of NCX in the generation of SAN automaticity is controversial. To investigate the contribution of NCX to pacemaking in the SAN, we performed optical voltage mapping and high-speed 2D laser scanning confocal microscopy (LSCM) of Ca dynamics in an ex vivo intact SAN/atrial tissue preparation from atrial-specific NCX knockout (KO) mice. These mice lack P waves on electrocardiograms, and isolated NCX KO SAN cells are quiescent. Voltage mapping revealed disorganized and arrhythmic depolarizations within the NCX KO SAN that failed to propagate into the atria. LSCM revealed intermittent bursts of Ca transients. Bursts were accompanied by rising diastolic Ca, culminating in long pauses dominated by Ca waves. The L-type Ca channel agonist BayK8644 reduced the rate of Ca transients and inhibited burst generation in the NCX KO SAN whereas the Ca buffer 1,2-Bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid (acetoxymethyl ester) (BAPTA AM) did the opposite. These results suggest that cellular Ca accumulation hinders spontaneous depolarization in the NCX KO SAN, possibly by inhibiting L-type Ca currents. The funny current (I f ) blocker ivabradine also suppressed NCX KO SAN automaticity. We conclude that pacemaker activity is present in the NCX KO SAN, generated by a mechanism that depends upon I f . However, the absence of NCX-mediated depolarization in combination with impaired Ca efflux results in intermittent bursts of pacemaker activity, reminiscent of human sinus node dysfunction and "tachy-brady" syndrome.sinoatrial node | sodium-calcium exchange | pacemaker activity | arrhythmia | intracellular calcium P hysiological heart rhythm originates in the sinoatrial node (SAN), a cluster of specialized pacemaker cells located on the endocardial surface of the right atrium (RA). SAN dysfunction (SND) leads to serious arrhythmias characterized by pathological pauses, often alternating with rapid heart rates or atrial fibrillation (1). Each year in the United States, close to 200,000 patients affected with SAN disease require surgical implantation of an electronic pacemaker (2). Therefore, advances in our understanding of SAN pacemaker activity are essential for developing new therapies to avoid this costly procedure and its related morbidity.In SAN pacemaker cells, action potentials (APs) are thought to be triggered by spontaneous diastolic depolarization (SDD) produced by a coupled system of cellular "clocks" (3). The first clock, known as the "membrane clock," initiates SDD in response to inward funny current (I f ) carried mostly by HCN4 channels (4) although other ion channels, like voltage-dependent Ca channels, have also been implicated (5). The second (and more controversial) clock is referred to as the "Ca clock." This clock produces a depolarizing current in late diastole when local Ca released by ryanodine receptors (RyRs) on the sarcoplasmic reticulum (SR) is extruded by the e...

Small conductance K (SK) channels have been implicated as modulators of spontaneous depolarization and electrical conduction that may be involved in cardiac arrhythmia. However, neither their presence nor their contribution to sinoatrial node (SAN) pacemaker activity has been investigated. Using quantitative PCR (q-PCR), immunostaining and patch clamp recordings of membrane current and voltage, we identified all three SK isoforms (SK1, SK2 and SK3) in mouse SAN. Inhibition of SK channels with the specific blocker apamin prolonged action potentials (APs) in isolated SAN cells. Apamin also slowed diastolic depolarization and reduced pacemaker rate in isolated SAN cells and intact tissue. We investigated whether the Ca -sensitive nature of SK channels could explain arrhythmic SAN pacemaker activity in the atrial-specific Na /Ca exchange (NCX) knockout (KO) mouse, a model of cellular Ca overload. SAN cells isolated from the NCX KO exhibited higher SK current than wildtype (WT) and apamin prolonged their APs. SK blockade partially suppressed the arrhythmic burst pacing pattern of intact NCX KO SAN tissue. We conclude that SK channels have demonstrable effects on SAN pacemaking in the mouse. Their Ca -dependent activation translates changes in cellular Ca into a repolarizing current capable of modulating regular pacemaking. This Ca dependence also promotes abnormal automaticity when these channels are hyperactivated by elevated Ca . We propose SK channels as a potential target for modulating SAN rate, and for treating patients affected by SAN dysfunction, particularly in the setting of Ca overload.

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