Triggered Ca 2+ Waves Induce Depolarization of Maximum Diastolic Potential and Action Potential Prolongation in Dog Atrial Myocytes

Background: We have identified a novel form of abnormal Ca 2+ wave activity in normal and failing dog atrial myocytes which occurs during the action potential (AP) and is absent during diastole. The goal of this study was to determine if triggered Ca 2+ waves affect cellular electrophysiological properties. Methods: Simultaneous recordings of intracellular Ca 2+ and APs allowed measurements of maximum diastolic potential and AP duration during triggered calcium waves (TCWs) in isolated dog atrial myocytes. Computer simulations then explored electrophysiological behavior arising from TCWs at the tissue scale. Results: At 3.3 to 5 Hz, TCWs occurred during the AP and often outlasted several AP cycles. Maximum diastolic potential was reduced, and AP duration was significantly prolonged during TCWs. All electrophysiological responses to TCWs were abolished by SEA0400 and ORM10103, indicating that Na-Ca exchange current caused depolarization. The time constant of recovery from inactivation of Ca 2+ current was 40 to 70 ms in atrial myocytes (depending on holding potential) so this current could be responsible for AP activation during depolarization induced by TCWs. Modeling studies demonstrated that the characteristic properties of TCWs are potentially arrhythmogenic by promoting both conduction block and reentry arising from the depolarization induced by TCWs. Conclusions: Triggered Ca 2+ waves activate inward NCX and dramatically reduce atrial maximum diastolic potential and prolong AP duration, establishing the substrate for reentry which could contribute to the initiation and maintenance of atrial arrhythmias.

Electrochemical Na+ and Ca2+ gradients drive coupled-clock regulation of automaticity of isolated rabbit sinoatrial nodal pacemaker cells

Coupling of an intracellular Ca2+ clock to surface membrane ion channels, i.e., a “membrane clock, ” via coupling of electrochemical Na+ and Ca2+ gradients ( ENa and ECa, respectively) has been theorized to regulate sinoatrial nodal cell (SANC) normal automaticity. To test this hypothesis, we measured responses of [Na+]i, [Ca2+]i, membrane potential, action potential cycle length (APCL), and rhythm in rabbit SANCs to Na+/K+ pump inhibition by the digitalis glycoside, digoxigenin (DG, 10–20 μmol/l). Initial small but significant increases in [Na+]i and [Ca2+]i and reductions in ENa and ECa in response to DG led to a small reduction in maximum diastolic potential (MDP), significantly enhanced local diastolic Ca2+ releases (LCRs), and reduced the average APCL. As [Na+]i and [Ca2+]i continued to increase at longer times following DG exposure, further significant reductions in MDP, ENa, and ECa occurred; LCRs became significantly reduced, and APCL became progressively and significantly prolonged. This was accompanied by increased APCL variability. We also employed a coupled-clock numerical model to simulate changes in ENa and ECa simultaneously with ion currents not measured experimentally. Numerical modeling predicted that, as the ENa and ECa monotonically reduced over time in response to DG, ion currents ( ICaL, ICaT, If, IKr, and IbNa) monotonically decreased. In parallel with the biphasic APCL, diastolic INCX manifested biphasic changes; initial INCX increase attributable to enhanced LCR ensemble Ca2+ signal was followed by INCX reduction as ENCX ( ENCX = 3 ENa − 2 ECa) decreased. Thus SANC automaticity is tightly regulated by ENa, ECa, and ENCX via a complex interplay of numerous key clock components that regulate SANC clock coupling.

The Major Role of IK1 in Mechanisms of Rotor Drift in the Atria: A Computational Study

Maintenance of paroxysmal atrial fibrillation (AF) by fast rotors in the left atrium (LA) or at the pulmonary veins (PVs) is not fully understood. This review describes the role of the heterogeneous distribution of transmembrane currents in the PVs and LA junction (PV-LAJ) in the localization of rotors in the PVs. Experimentally observed heterogeneities in IK1, IKs, IKr, Ito, and ICaL in the PV-LAJ were incorporated into models of human atrial kinetics to simulate various conditions and investigate rotor drifting mechanisms. Spatial gradients in the currents resulted in shorter action potential duration, less negative minimum diastolic potential, slower upstroke and conduction velocity for rotors in the PV region than in the LA. Rotors under such conditions drifted toward the PV and stabilized at the less excitable region. Our simulations suggest that IK1 heterogeneity is dominant in determining the drift direction through its impact on the excitability gradient. These results provide a novel framework for understanding the complex dynamics of rotors in AF.

Abstract 15961: Syncytial Model of Type 2 Long QT Syndrome Derived From Human iPS Cells Can Be Paced and Responds to Ikr Block and Activation

Type 2 long QT (LQT2) syndrome is a cardiac disorder associated with hERG channel mutation that may lead to tachyarrhythmia and sudden death. We developed a syncytial model of cardiomyocytes (CMs) differentiated from iPS cells derived from an LQT2 patient harboring an A422T mutation. The A422T mutation has been shown to cause a marked decrease in IKr current mainly due to a trafficking defect in the hERG channel. Immunostaining of cTnI revealed that CMs were isotropically distributed with well-formed sarcomeres. Both wild type (WT) and LQT2 monolayers were stable in culture for at least 60 days with spontaneous activity, and could be paced at cycle lengths (CL) from 2000 down to 300 ms. After staining the monolayers with di-4-ANEPPS, optical mapping showed that all monolayers formed a functional syncytium that supported propagating action potentials. LQT2 monolayers had longer APD80s than WT monolayers (332±43 ms, n=6, vs. 267±46 ms, n=18, mean±SD, CL = 700 ms, p<0.005), showing the delayed repolarization expected of the LQT2 phenotype. Reentrant spiral waves were observed in LQT2 (n=2), but not WT monolayers. Application of 0.2μM E-4031 (IKr blocker) slowed repolarization and prolonged APD80 in both WT and LQT2 monolayers, and slowed conduction velocity in WT monolayers (4.7±1.0 cm/s, n=6 vs. 10.4±1.4 cm/s, n=11 control, CL = 1000 ms). The slowing effect may reflect a role of IKr in setting the maximum diastolic potential in these CMs where IK1 is weakly expressed, as has been recently suggested. Application of 10μM ML-T531 (IKr activator) shortened APD80 in LQT2 monolayers (313±36 ms vs. 162±19 ms, n=4, p<0.0001), showing successful reversal of the LQT phenotype. ML-T531 also shortened APD80 in WT monolayers (202±43 ms vs. 154±26 ms, n=4, p<0.04). These results indicate that syncytial models of heritable cardiac disease are feasible and may be useful for studying drug effects that affect electrophysiology and arrhythmogenesis.