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...

Key pointsr Inositol-1,4,5-trisphosphate receptors (IP 3 Rs) modulate pacemaking in embryonic heart, but their role in adult sinoatrial node (SAN) pacemaking is uncertain. r These findings support development of IP 3 signalling modulators for regulation of heart rate, particularly in heart failure where IP 3 Rs are upregulated.Abstract Cardiac pacemaking initiated by the sinus node is attributable to the interplay of several membrane currents. These include the depolarizing 'funny current' (I f ) and the sodium-calcium exchanger current (I NCX ). The latter is activated by ryanodine receptor (RyR)-mediated calcium (Ca 2+ ) release from the sarcoplasmic reticulum (SR). Another SR Ca 2+ release channel, the inositol-1,4,5-triphosphate receptor (IP 3 R), has been implicated in the generation of spontaneous Ca 2+ release in atrial and ventricular cardiomyocytes. Whether IP 3 R-mediated Ca 2+ release also influences SAN automaticity is controversial, in part due to the confounding influence of periodic Ca 2+ flux through the sarcolemma accompanying each beat. We took advantage of atrial-specific sodium-calcium exchanger (NCX) knockout (KO) SAN cells to study the influence of IP 3 signalling on cardiac pacemaking in a system where periodic intracellular Ca 2+ cycling persists despite the absence of depolarization or Ca 2+ flux across the sarcolemma. We recorded confocal line scans of spontaneous Ca 2+ release in WT and NCX KO SAN cells in the presence or absence of an IP 3 R blocker (2-aminoethoxydiphenyl borate, 2-APB), or during block of IP 3 production by the phospholipase C inhibitor U73122. 2-APB and U73122 decreased the frequency of spontaneous Ca 2+ transients and waves in WT and NCX KO cells, respectively. Alternatively, increased IP 3 production induced by phenylephrine increased Ca 2+ transient and wave frequency. We conclude that IP 3 R-mediated SR Ca 2+ flux is crucial for initiating and modulating the RyR-mediated Ca 2+ cycling that regulates SAN pacemaking. Our results in NCX KO SAN cells also demonstrate that RyRs, but not NCX, are required for IP 3 to modulate Ca 2+ clock frequency.

Patients with Duchenne muscular dystrophy (DMD) are at risk of developing cardiomyopathy and cardiac arrhythmias. Studies in a mouse model of DMD revealed that enhanced sarcoplasmic reticulum (SR) Ca 2þ leak contributes to the pathogenesis of cardiac dysfunction. In view of recent data suggesting the involvement of altered phosphorylation and oxidation of the cardiac ryanodine receptor (RyR2)/ Ca 2þ release channel, we hypothesized that inhibition of RyR2 phosphorylation in a mouse model of DMD can prevent SR Ca 2þ leak by reducing RyR2 oxidation. Confocal Ca 2þ imaging and single RyR2 channel recordings revealed that inhibition of either S2808 or S2814 phosphorylation, or inhibition of oxidation could normalize RyR2 activity in mdx mice. Moreover, genetic inhibition of RyR2 phosphorylation at S2808 or S2814 reduced RyR2 oxidation. Production of reactive oxygen species (ROS) in myocytes from mdx mice was reduced by both inhibition of RyR2 phosphorylation or the ROS scavenger 2-mercaptoproppionylglycin (MPG). Finally, it was shown that ROS production in mdx mice is proportional to the activity of RyR2-mediated SR Ca 2þ leak. We conclude that increased reactive oxygen species (ROS) production in the hearts of mdx mice drives the progression of cardiomyopathy. Inhibition of RyR2 phosphorylation can suppress SR Ca 2þ leak in mdx mouse hearts in part by reducing RyR2 oxidation.

The AngioVac system, designed for suction during extracorporeal bypass, is used to aspirate masses, thrombi, and other undesirable material from the cardiovascular system. To date, it has been used extensively in the venous system and right side of the heart; however, its use in the arterial system has been limited because of smaller vessel sizes and the requirement for a 26F sheath. We report the case of a 45-year-old woman with a history of angiosarcoma who presented with acute embolic events that affected her spleen and lower extremities. We removed a large mobile mass en bloc from her distal thoracic aorta by using the AngioVac system as an alternative to surgical resection. The patient recovered with no recurrence. We discuss the benefits and challenges of using the AngioVac within small vessels of the arterial system.

No abstract

Sinoatrial node (SAN) automaticity is due to the interplay of several membrane currents, including the current produced by Na-Ca exchanger (NCX) in response to Ca cycling. Several lines of evidence suggest that inositol-1,4,5-triphosphate receptors (IP 3 Rs) that mobilize [Ca] i are implicated in generation of automaticity in embryonic and adult cardiomyocytes. Increased IP 3 R expression observed in failing heart may predispose to arrhythmias and could contribute to the elevated heart rates associated with systolic heart failure. However, whether IP 3 R signaling influences SAN pacemaking is still controversial, in part due to the confounding influence of periodic Ca flux through the sarcolemma during every beat. To address this concern, we used NCX knockout (KO) SAN cells to study the role of IP 3 signaling on pacemaker activity. In these cells Ca flux across the sarcolemmal membrane is practically eliminated due to NCX ablation and no Ca flux through L-type Ca channels due to lack of action potentials, yet periodic spontaneous waves are still generated by the localized Ca releases (LCRs) of the “Ca clock”. Thus these waves are “uncoupled” from the membrane because there is no NCX to contribute to the diastolic depolarization in response to the release of [Ca] i . We recorded spontaneous Ca oscillations using line scan confocal microscopy in WT and NCX KO SAN cells in the presence of an IP 3 R blocker or during inhibition of phospholipase C (PLC). We found that superfusion with the IP 3 R blocker, 2-APB (2 μm) decreased the frequency of Ca transients in WT SAN cells by 82.6% (n=9, p<0.05) and Ca waves in NCX KO cells by 66% (n=10, p<0.05). Similar results were also obtained on superfusion with the PLC antagonist, U73122 (1 μm). Alternatively, an increase in IP 3 production using the α-1 adrenergic receptor agonist phenylephrine (10 μm) led to an increase in the frequency of Ca transients in WT SAN cells (n=8. P< 0.05) and Ca waves in NCX KO cells (n=9, p <0.05). This effect was blocked on the subsequent additional application of 2-APB. In summary our results indicate that Ca release from IP 3 Rs can modulate Ca oscillation frequency in NCX KO SAN cells and support our hypothesis that IP 3 signaling modulates the Ca cycling processes that regulate pacemaker frequency in the SAN.

No abstract