Matthew J. Killeen

Arrhythmogenic mechanisms in the isolated perfused hypokalaemic murine heart

Aim:  Hypokalaemia is associated with a lethal form of ventricular tachycardia (VT), torsade de pointes, through pathophysiological mechanisms requiring clarification.

Effects of L‐type Ca2+ channel antagonism on ventricular arrhythmogenesis in murine hearts containing a modification in the Scn5a gene modelling human long QT syndrome 3

Ventricular arrhythmogenesis in long QT 3 syndrome (LQT3) involves both triggered activity and re‐entrant excitation arising from delayed ventricular repolarization. Effects of specific L‐type Ca2+ channel antagonism were explored in a gain‐of‐function murine LQT3 model produced by a ΔKPQ 1505–1507 deletion in the SCN5A gene. Monophasic action potentials (MAPs) were recorded from epicardial and endocardial surfaces of intact, Langendorff‐perfused Scn5a+/Δ hearts. In untreated Scn5a+/Δ hearts, epicardial action potential duration at 90% repolarization (APD90) was 60.0 ± 0.9 ms compared with 46.9 ± 1.6 ms in untreated wild‐type (WT) hearts (P < 0.05; n= 5). The corresponding endocardial APD90 values were 52.0 ± 0.7 ms and 53.7 ± 1.6 ms in Scn5a+/Δ and WT hearts, respectively (P > 0.05; n= 5). Epicardial early afterdepolarizations (EADs), often accompanied by spontaneous ventricular tachycardia (VT), occurred in 100% of MAPs from Scn5a+/Δ but not in any WT hearts (n= 10). However, EAD occurrence was reduced to 62 ± 7.1%, 44 ± 9.7%, 10 ± 10% and 0% of MAPs following perfusion with 10 nm, 100 nm, 300 nm and 1 μm nifedipine, respectively (P < 0.05; n= 5), giving an effective IC50 concentration of 79.3 nm. Programmed electrical stimulation (PES) induced VT in all five Scn5a+/Δ hearts (n= 5) but not in any WT hearts (n= 5). However, repeat PES induced VT in 3, 2, 2 and 0 out of 5 Scn5a+/Δ hearts following perfusion with 10 nm, 100 nm, 300 nm and 1 μm nifedipine, respectively. Patch clamp studies in isolated ventricular myocytes from Scn5a+/Δ and WT hearts confirmed that nifedipine (300 nm) completely suppressed the inward Ca2+ current but had no effect on inward Na+ currents. No significant effects were seen on epicardial APD90, endocardial APD90 or ventricular effective refractory period in Scn5a+/Δ and WT hearts following perfusion with nifedipine at 1 nm, 10 nm, 100 nm, 300 nm and 1 μm nifedipine concentrations. We conclude that L‐type Ca2+ channel antagonism thus exerts specific anti‐arrhythmic effects in Scn5a+/Δ hearts through suppression of EADs.

NF-κβ signaling and chronic inflammatory diseases: exploring the potential of natural products to drive new therapeutic opportunities.

Chronic inflammation has a key role in the pathogenesis of multiple diseases that represent major public health and financial concerns, including heart failure (HF), Alzheimers disease (AD) and arthritis. Nuclear factor kappa β (NF-κβ) is a central component of inflammation; owing to its upstream signaling position, it is considered an attractive target for new anti-inflammatory therapeutics. Hydroxytyrosol is an orally bioavailable polyphenol, obtained from olives, which inhibits NF-κβ activity and has elicited promising efficacy signals in several inflammatory diseases. Here, we further examine the role of NF-κβ in inflammation, provide an introduction to natural products and their anti-inflammatory effects and explore the potential of hydroxytyrosol as a new approach to combating the burden of chronic inflammatory diseases.

Mechanisms of ventricular arrhythmogenesis in mice following targeted disruption of KCNE1 modelling long QT syndrome 5

Mutations within KCNE1 encoding a transmembrane protein which coassembles with K+ channels mediating slow K+, IKs, currents are implicated in cardiac action potential prolongation and ventricular arrhythmogenicity in long QT syndrome 5. We demonstrate the following potentially arrhythmogenic features in simultaneously recorded, left ventricular, endocardial and epicardial monophasic action potentials from Langendorff‐perfused murine KCNE1−/− hearts for the first time. (1) Prolonged epicardial (57.1 ± 0.5 ms cf. 36.1 ± 0.07 ms in wild‐type (WT), P < 0.001; n= 5) and endocardial action potential duration at 90% repolarication (APD90) (54.4 ± 2.4 ms cf. 48.5 ± 0.3 ms, P < 0.05; n= 5). (2) Negative transmural repolarization gradients (ΔAPD90: endocardial minus epicardial APD90) (−2.5 ± 2.4 ms, compared with 12.4 ± 1.1 ms in WT, P < 0.001; n= 5). (3) Frequent epicardial early afterdepolarizations (EADs) and spontaneous ventricular tachycardia (VT) in 4 out of 5 KCNE1−/− hearts but not WT (n= 5). EADs were especially frequent following temporary cessations of ventricular pacing. (4) Monomorphic VT lasting 1.36 ± 0.2 s in 5 out of 5 KCNE1−/− hearts, following premature stimuli but not WT (n= 5). (5) Epicardial APD alternans. Perfusion of KCNE1−/− hearts with 1 μm nifedipine induced potentially anti‐arrhythmic changes including: (1) restored epicardial APD90 (from 57.1 ± 0.5 ms to 42.3 ± 0.4 ms, P < 0.001; n= 5); (2) altered ΔAPD90 to values (11.2 ± 2.6) close to WT (P > 0.05; n= 5); (3) EAD suppression during both spontaneous activity and following cessation of ventricular pacing (n= 5) to give similar features to WT controls (n= 5); (4) suppression of programmed electrical stimulation‐induced VT; and (5) suppression of APD alternans. These findings suggest arrhythmic effects of reduced outward currents expected in KCNE1−/− hearts and their abolition by antagonism of inward L‐type Ca2+ current.

Pharmacological separation of early afterdepolarizations from arrhythmogenic substrate in ΔKPQ Scn5a murine hearts modelling human long QT 3 syndrome

Aim:  To perform an empirical, pharmacological, separation of early afterdepolarizations (EADs) and transmural gradients of repolarization in arrhythmogenesis in a genetically modified mouse heart modelling human long QT syndrome (LQT) 3.

Mouse models of human arrhythmia syndromes

Sudden cardiac death stemming from ventricular arrhythmogenesis is one of the major causes of mortality in the developed world. Congenital and acquired forms of long QT syndrome (LQTS) are in turn associated with life threatening arrhythmias. Over the past decade our understanding of arrhythmogenic mechanisms in the setting of these diseases has increased greatly due to the creation of a number of animal models. Of these, the genetically amenable mouse has proved to be a particularly powerful tool. This review summarizes the congenital and acquired LQTS and describes the various mouse models that have been created to further probe arrhythmogenic mechanisms.

Separation of early afterdepolarizations from arrhythmogenic substrate in the isolated perfused hypokalaemic murine heart through modifiers of calcium homeostasis

Aims:  We resolved roles for early afterdepolarizations (EADs) and transmural gradients of repolarization in arrhythmogenesis in Langendorff‐perfused hypokalaemic murine hearts paced from the right ventricular epicardium.

Drug-induced arrhythmias and sudden cardiac death: implications for the pharmaceutical industry.

Following a series of high profile withdrawals from the market, the ability of medications to induce potentially fatal arrhythmias is a significant problem facing the pharmaceutical industry. Current preclinical cardiac safety assays are based on the assumption that blockade of a single repolarizing K(+) channel alone precipitates drug-induced arrhythmias, however, current findings point to a range of more complex arrhythmogenic mechanisms. This review begins by exploring clinical findings and potential mechanisms underlying drug-induced sudden cardiac death and then goes on to assess current and explore future strategies to detect cardiotoxicity at the preclinical stage.

Effects of potassium channel openers in the isolated perfused hypokalaemic murine heart

Aim:  We explored the anti‐arrhythmic efficacy of K+ channel activation in the hypokalaemic murine heart using NS1643 and nicorandil, compounds which augment IKr and IKATP respectively.

Transient alterations in transmural repolarization gradients and arrhythmogenicity in hypokalaemic Langendorff-perfused murine hearts

Clinical hypokalaemia is associated with acquired electrocardiographic QT prolongation and arrhythmic activity initiated by premature ventricular depolarizations and suppressed by lidocaine (lignocaine). Nevertheless, regular (S1) pacing at a 125 ms interstimulus interval resulted in stable waveforms and rhythm studied using epicardial and endocardial monophasic action potential (MAP) electrodes in Langendorff‐perfused murine hearts whether under normokalaemic (5.2 mm K+) or hypokalaemic (3.0 mm K+) conditions, in both the presence and absence of lidocaine (10 μm). Furthermore, the transmural gradient in repolarization time, known to be altered in the congenital long‐QT syndromes, and reflected in the difference between endocardial and epicardial MAP duration at 90% repolarization (ΔAPD90), did not differ significantly (P > 0.05) between normokalaemic (5.5 ± 4.5 ms, n= 8, five hearts), hypokalaemic (n= 8, five hearts), or lidocaine‐treated normokalaemic (n= 8, five hearts) or hypokalaemic (n= 8, five hearts) hearts. However, premature ventricular depolarizations occurring in response to extrasystolic (S2) stimulation delivered at S1S2 intervals between 0 and 22 ± 6 ms following recovery from refractoriness initiated arrhythmic activity specifically in hypokalaemic (n= 8, five hearts) as opposed to normokalaemic (n= 25, 14 hearts), or lidocaine‐treated hypokalaemic (n= 8, five hearts) or normokalaemic hearts (n= 8, five hearts). This was associated with sharp but transient reversals in ΔAPD90 in MAPs initiated within the 250 ms interval directly succeeding premature ventricular depolarizations, from 3.3 ± 5.6 ms to −31.8 ± 11.8 ms (P < 0.05) when they were initiated immediately after recovery from refractoriness. In contrast the corresponding latency differences consistently remained close to the normokalaemic value (−1.6 ± 1.4 ms, P > 0.05). These findings empirically associate arrhythmogenesis in hypokalaemic hearts with transient alterations in transmural repolarization gradients resulting from premature ventricular depolarizations. This is in contrast to sustained alterations in transmural repolarization gradients present on regular stimulation in long‐QT syndrome models.