Gregory S. Hoeker

Electrophysiologic effects of the IK1 inhibitor PA‐6 are modulated by extracellular potassium in isolated guinea pig hearts

The pentamidine analog PA‐6 was developed as a specific inward rectifier potassium current (IK1) antagonist, because established inhibitors either lack specificity or have side effects that prohibit their use in vivo. We previously demonstrated that BaCl2, an established IK1 inhibitor, could prolong action potential duration (APD) and increase cardiac conduction velocity (CV). However, few studies have addressed whether targeted IK1 inhibition similarly affects ventricular electrophysiology. The aim of this study was to determine the effects of PA‐6 on cardiac repolarization and conduction in Langendorff‐perfused guinea pig hearts. PA‐6 (200 nm) or vehicle was perfused into ex‐vivo guinea pig hearts for 60 min. Hearts were optically mapped with di‐4‐ANEPPS to quantify CV and APD at 90% repolarization (APD90). Ventricular APD90 was significantly prolonged in hearts treated with PA‐6 (115 ± 2% of baseline; P < 0.05), but not vehicle (105 ± 2% of baseline). PA‐6 slightly, but significantly, increased transverse CV by 7%. PA‐6 significantly prolonged APD90 during hypokalemia (2 mmol/L [K+]o), although to a lesser degree than observed at 4.56 mmol/L [K+]o. In contrast, the effect of PA‐6 on CV was more pronounced during hypokalemia, where transverse CV with PA‐6 (24 ± 2 cm/sec) was significantly faster than with vehicle (13 ± 3 cm/sec, P < 0.05). These results show that under normokalemic conditions, PA‐6 significantly prolonged APD90, whereas its effect on CV was modest. During hypokalemia, PA‐6 prolonged APD90 to a lesser degree, but profoundly increased CV. Thus, in intact guinea pig hearts, the electrophysiologic effects of the IK1 inhibitor, PA‐6, are [K+]o‐dependent.

The adhesion function of the sodium channel beta subunit (β1) contributes to cardiac action potential propagation

Computational modeling indicates that cardiac conduction may involve ephaptic coupling – intercellular communication involving electrochemical signaling across narrow extracellular clefts between cardiomyocytes. We hypothesized that β1(SCN1B) –mediated adhesion scaffolds trans-activating NaV1.5 (SCN5A) channels within narrow (<30 nm) perinexal clefts adjacent to gap junctions (GJs), facilitating ephaptic coupling. Super-resolution imaging indicated preferential β1 localization at the perinexus, where it co-locates with NaV1.5. Smart patch clamp (SPC) indicated greater sodium current density (INa) at perinexi, relative to non-junctional sites. A novel, rationally designed peptide, βadp1, potently and selectively inhibited β1-mediated adhesion, in electric cell-substrate impedance sensing studies. βadp1 significantly widened perinexi in guinea pig ventricles, and selectively reduced perinexal INa, but not whole cell INa, in myocyte monolayers. In optical mapping studies, βadp1 precipitated arrhythmogenic conduction slowing. In summary, β1-mediated adhesion at the perinexus facilitates action potential propagation between cardiomyocytes, and may represent a novel target for anti-arrhythmic therapies.

Moving beyond the reductionist approach—Time to put the pieces back together in a broken (infarcted) heart

Significant progress has been made in treating patients who have suffered a myocardial infarction (MI); yet current therapies are still lacking, and inhibiting or reversing remodeling and preventing arrhythmias remains a significant clinical problem. Much of what we have learned about post-MI electrophysiologic remodeling has been derived from models in which the MI is induced in otherwise healthy (wild-type [WT] or naive) animals. While the methods for inducing ventricular infarcts (eg, surgical ligation of a coronary artery) may be nonphysiologic processes, these approaches have produced robust and reproducible MIs. As a result, electrical remodeling and arrhythmogenesis associated with MI have been extensively characterized (see Pinto and Boyden for review). This reductionist approach has proven to be invaluable for advancing our understanding of common mechanisms of calcium-mediated and reentrant arrhythmias; however, such experimental simplifications often uncouple the end point (ie, MI) from the disease etiology, bypassing a significant portion of the natural history of the underlying disease processes that lead to MI. Underlying atherosclerosis is the leading cause of MI, with major risk factors including hyperlipidemia, hypertension, diabetes, tobacco use, and male sex. Data from clinical studies suggest that the presence of these comorbidities does not merely serve to precipitate the MI, but actually shapes the prognosis for patients who have survived acute MIs. In a meta-analysis of MI survivors, patients with arterial hypertension were at a greater risk of death, stroke, congestive heart failure, and recurrent MI than were normotensive patients. Elevated monocyte levels, as seen in patients with atherosclerosis, have been shown to be an independent predictor of subsequent pump failure, left ventricular aneurysm, recurrent MI, and sudden death. While it still may not be feasible to study natural lifetime disease remodeling leading to MI in the laboratory setting, to further advance our knowledge in this field, we need to take the next step and

STORM Localizations - Part 1/3

Zip file containing single molecule localization data from STORM. The zip file is divided into 3 volumes. Volume 1/3

STORM and TEM Identify the Cardiac Ephapse: An Intercalated Disk Nanodomain with Previously Unanticipated Functions in Cardiac Conduction

1. Virginia Tech Carilion Research Institute, and Center for Heart and Regenerative Medicine, Virginia Polytechnic University, Roanoke, VA. 2. School of Biomedical Engineering and Sciences, Virginia Polytechnic University, Blacksburg, VA. 3. Dept. of Physiology and Biophysics, Case Western Reserve University, Cleveland, OH. 4. Heart and Vascular Research Center, MetroHealth Medical Center, Case Western Reserve University, Cleveland, OH. 5. Dept. of Medicine, National Heart & Lung Institute, Imperial College London, London, UK.