Hypokalemia Promotes Arrhythmia by Distinct Mechanisms in Atrial and Ventricular Myocytes

Adverse Clinical Effects Associated With Non-catecholamine Pharmacologic Agents for Treatment of Vasoplegic Syndrome in Adult Cardiac Surgery

Vasoplegic syndrome is a relatively common complication that can happen during and after major adult cardiac surgery. It is associated with a higher rate of complications, including postoperative renal failure, longer duration of mechanical ventilation, and intensive care unit stay, as well as increased mortality. The underlying pathophysiology of vasoplegic syndrome is that of profound vascular hyporesponsiveness, and involves a complex interplay among inflammatory cytokines, cellular surface receptors, and nitric oxide (NO) production. The pharmacotherapy approaches for the treatment of vasoplegia include medications that increase vascular smooth muscle contraction via increasing cytosolic calcium in myocytes, reduce the vascular effects of NO and inflammation, and increase the biosynthesis of and vascular response to norepinephrine. Clinical trials have demonstrated the clinical efficacy of non-catecholamine pharmacologic agents in the treatment of vasoplegic syndrome. With an increase in their use today, it is important for clinicians to understand the adverse clinical outcomes and patient risk profiles associated with these agents, which will allow better-tailored medical therapy.

Potassium levels and the risk of all-cause and cardiovascular mortality among patients with cardiovascular diseases: a meta-analysis of cohort studies

BACKGROUND

Abnormal blood potassium levels are associated with an increased risk of cardiometabolic diseases and mortality in the general population; however, evidence regarding the association between dyskalemia and mortality among patients with cardiovascular disease (CVD) remains inconclusive. This study aimed to evaluate the association of potassium levels with all-cause and cardiovascular mortality among patients with CVD.

METHODS

PubMed, Embase, Web of Science, and Cochrane Library databases were searched up to August 2023 to identify relevant cohort studies among patients with CVD, such as myocardial infarction, stroke, and heart failure. Abnormal potassium levels were considered as hypokalemia or hyperkalemia. The primary outcomes were all-cause mortality based on follow-up length (including in-hospital, short-term and long-term mortality) and cardiovascular mortality. The methodological quality of included studies was assessed by using the Newcastle-Ottawa Scale. The pooled relative risks (RRs) and 95% confidence intervals (CIs) were calculated using random-effects models. Restricted cubic splines were applied to explore the dose-response relationship.

RESULTS

Thirty-one cohort studies involving 227,645 participants with an average age of 68.3 years were included in the meta-analysis, all of which achieved moderate to high quality. Hyperkalemia was significantly associated with an approximately 3.0-fold increased risk of all-cause in-hospital mortality (RR:2.78,95CI%:1.92,4.03), 1.8-fold of all-cause short-term mortality (RR:1.80, 95CI%:1.44,2.27), 1.3-fold of all-cause long-term mortality (RR:1.33, 95CI%:1.19,1.48) and 1.2-fold of cardiovascular mortality (RR:1.19, 95CI%:1.04,1.36). Similar positive associations were also observed between hypokalemia and risk of all-cause mortality and cardiovascular mortality. The RRs of all-cause in-hospital, short-term, long-term mortality and cardiovascular mortality with hyperkalemia were attenuated to 2.21 (95CI%:1.60,3.06), 1.46(95CI%:1.25,1.71), 1.23 (95CI%:1.09,1.39) and 1.13 (95CI%:1.00,1.27) when treating hypokalemia together with normokalemia as the reference group. A U-shaped association was observed between potassium levels and mortality, with the lowest risk at around 4.2 mmol/L.

CONCLUSIONS

Both hypokalemia and hyperkalemia were positively associated with the risk of mortality in patients with CVD. Our results support the importance of potassium homeostasis for improving the CVD management.

REGISTRATION

PROSPERO, CRD42022324337.

Clinical outcomes of serum potassium in patients with percutaneous coronary intervention: insights from a large single-center registry

Background

Serum potassium homeostasis plays an important role in myocardial electrical stability, but the impact of altered serum potassium levels on the major adverse cardiovascular and cerebral events (MACCE) in patients with percutaneous coronary intervention (PCI) has not been evaluated.

Aim

To evaluate the association between serum potassium level and the risk of MACCE in PCI patients.

Materials and methods

This retrospective study involved 8,263 in-patients from a single-center registry who were successfully treated with PCI between January 2003 and December 2020. Clinical data were collected for 24 h after admission. Data were analyzed from June 2003 to December 2021. The primary outcome was MACCE, defined as a composite of all-cause death, myocardial infarction, revascularization, stroke, and heart failure-related hospitalization.

Results

The median [interquartile range (IQR)] follow-up for all patients was 4.0 (2.1, 5.8) years, and 1,632 patients (19.7%) were diagnosed with MACCE. High serum potassium levels were associated with a 20% increased risk of MACCE (hazard ratio [HR]: 1.20, 95% confidence interval [CI]: 1.05-1.38, P = 0.008) and 72% increased risk of all-cause death (HR: 1.72, 95% CI: 1.39-2.14, P < 0.001). Multivariate Cox analysis showed that the risk of MACCE was higher in patients at the highest quartile of serum potassium (Q4 vs. Q1: adjusted HR: 1.18, 95% CI: 1.02-1.35, P = 0.026). Moreover, a higher serum potassium level was always associated with a higher risk of all-cause death (Q4 vs. Q1: adjusted HR: 1.50, 95% CI: 1.17-1.91, P = 0.001). A U-shaped relationship between serum potassium levels, MACCE, and all-cause death was derived in patients undergoing PCI. Serum potassium levels, maintained within the range of 3.8-4.0 mmol/L before PCI, exhibited the lowest risk of associated MACCE and all-cause death.

Conclusion

Our results demonstrate that the serum potassium level could be associated with higher risks of MACCE and all-cause death in PCI patients. In particular, serum potassium levels maintained at 3.8-4.0 mmol/L before PCI could lower the risk of MACCE and all-cause death.

Treatment-induced increase in total body potassium in patients at high risk of ventricular arrhythmias; a randomized POTCAST substudy

OBJECTIVE

Hypokalemia is associated with increased risk of arrhythmias and it is recommended to monitor plasma potassium (p-K) regularly in at-risk patients with cardiovascular diseases. It is poorly understood if administration of potassium supplements and mineralocorticoid receptor antagonists (MRA) aimed at increasing p-K also increases intracellular potassium.

METHODS

Adults aged≥18 years with an implantable cardioverter defibrillator (ICD) were randomized (1:1) to a control group or to an intervention that included guidance on potassium rich diets, potassium supplements, and MRA to increase p-K to target levels of 4.5-5.0 mmol/l for six months. Total-body-potassium (TBK) was measured by a Whole-Body-Counter along with p-K at baseline, after six weeks, and after six months.

RESULTS

Fourteen patients (mean age: 59 years (standard deviation 14), 79% men) were included. Mean p-K was 3.8 mmol/l (0.2), and mean TBK was 1.50 g/kg (0.20) at baseline. After six-weeks, p-K had increased by 0.47 mmol/l (95%CI:0.14;0.81), p = 0.008 in the intervention group compared to controls, whereas no significant difference was found in TBK (44 mg/kg (-20;108), p = 0.17). After six-months, no significant difference was found in p-K as compared to baseline (0.16 mmol/l (-0.18;0.51), p = 0.36), but a significant increase in TBK of 82 mg/kg (16;148), p = 0.017 was found in the intervention group compared to controls.

CONCLUSIONS

Increased potassium intake and MRAs increased TBK gradually and a significant increase was seen after six months. The differentially regulated p-K and TBK challenges current knowledge on potassium homeostasis and the time required before the full potential of p-K increasing treatment can be anticipated.

TRIAL REGISTRATION

www.clinicaltrials.gov (NCT03833089).

Recent Progresses in Non-Dialysis Chronic Kidney Disease Patients with Hyperkalemia: Outcomes and Therapeutic Strategies

Chronic kidney disease (CKD) affects about 10% of the world's population. Hyperkalemia is a life-threatening complication in patients with CKD, as it is associated with adverse cardiovascular and kidney outcomes. There are still many challenges and questions to address to improve the currently available therapeutic strategies to treat hyperkalemia, such as how to approach the emergency management of hyperkalemia. In recent years, in addition to novel oral potassium binders, great progress has been made in the application of novel kidney protective strategies, such as mineralocorticoid receptor antagonists and sodium-glucose cotransporter 2 inhibitors (SGLT2i) in hyperkalemia therapy. This review will discuss the recent advances from clinical trials in the effective management of hyperkalemia in non-dialysis CKD patients, enhancing the knowledge of physicians and internists concerning these newer agents and providing a helpful reference for clinical practice.

Spatiotemporal patterns of early afterdepolarizations underlying abnormal T-wave morphologies in a tissue model of the Purkinje-ventricular system

Sudden cardiac death (SCD) is a leading cause of death worldwide, and the majority of SCDs are caused by acute ventricular arrhythmias (VAs). Early afterdepolarizations (EADs) are an important trigger of VA under pathological conditions, e.g., inherited or acquired long QT syndrome (LQTS). However, it remains unclear how EAD events at the cellular level are spatially organized at the tissue level to induce and maintain ventricular arrhythmias and whether the spatial-temporal patterns of EADs at the tissue level are associated with abnormal T-wave morphologies that are often observed in LQTS, such as broad-based, notched or bifid; late appearance; and pointed T-waves. Here, a tissue model of the Purkinje-ventricular system (PVS) was developed to quantitatively investigate the complex spatial-temporal dynamics of EADs during T-wave abnormalities. We found that (1) while major inhibition of ICaL can substantially reduce the excitability of the PVS leading to conduction failures, moderate ICaL inhibition can promote occurrences of AP alternans at short cycle lengths (CLs), and EAD events preferentially occur with a major reduction of IKr (>50%) at long CLs; (2) with a minor reduction of ICaL, spatially synchronized steady-state EAD events with inverted and biphasic T-waves can be "weakened" into beat-to-beat concurrences of spatially synchronized EADs and T-wave alternans, and as pacing CLs increase, beat-to-beat concurrences of localized EADs with late-appearing and pointed T-wave morphologies can be observed; (3) under certain conditions, localized EAD events in the midmyocardium may trigger slow uni-directional electric propagation with inverted (antegrade) or upright (retrograde) broad-based T-waves; (4) spatially discordant EADs were typically characterized by desynchronized spontaneous onsets of EAD events between two groups of PVS tissues with biphasic T-wave morphologies, and they can evolve into spatially discordant oscillating EAD patterns with sustained or self-terminated alternating EAD and electrocardiogram (ECG) patterns. Our results provide new insights into the spatiotemporal aspects of the onset and development of EADs and suggest possible mechanistic links between the complex spatial dynamics of EADs and T-wave morphologies.

Association of Intravenous Potassium and Magnesium Administration With Spontaneous Conversion of Atrial Fibrillation and Atrial Flutter in the Emergency Department

IMPORTANCE

Whether the simultaneous intravenous administration of potassium and magnesium is associated with the probability of spontaneous conversion to sinus rhythm (SCV) in the acute treatment of atrial fibrillation (AF) and atrial flutter (AFL) is unknown.

OBJECTIVE

To assess potassium and magnesium administration and SCV probability in AF and AFL in the emergency department.

DESIGN, SETTING, AND PARTICIPANTS

A registry-based cohort study was conducted in the Department of Emergency Medicine of the Medical University of Vienna, Austria. All consecutive patients with AF or AFL were screened between February 6, 2009, and February 16, 2020.

INTERVENTIONS

Intravenous administration of potassium, 24 mEq, and magnesium, 145.8 mg.

MAIN OUTCOMES AND MEASURES

The primary outcome was the probability of SCV during the patient's stay in the emergency department. Multivariable cluster-adjusted logistic regression was used to estimate the association between potassium and magnesium administration and the probability of SCV.

RESULTS

A total of 2546 episodes of nonpermanent AF (median patient age, 68 [IQR, 58-75] years, 1411 [55.4%] men) and 573 episodes of nonpermanent AFL (median patient age, 68 [IQR, 58-75] years; 332 [57.9%] men) were observed. In AF episodes, intravenous potassium and magnesium administration vs no administration was associated with increased odds of SCV (19.2% vs 10.4%; odds ratio [OR], 1.98; 95% CI, 1.53-2.57). In AFL episodes, in contrast, no association was noted for the probability of SCV with potassium and magnesium vs no administration (13.0% vs 12.5%; OR, 1.05; 95% CI, 0.65-1.69).

CONCLUSIONS AND RELEVANCE

The findings of this registry-based cohort study on intravenous administration of potassium and magnesium suggest an increased probability of SCV in nonpermanent AF, but not AFL, during a patients' stay in the emergency department.

Mechanisms of spontaneous Ca2+ release-mediated arrhythmia in a novel 3D human atrial myocyte model: I. Transverse-axial tubule variation

Intracellular calcium (Ca²⁺ ) cycling is tightly regulated in the healthy heart ensuring effective contraction. This is achieved by transverse (t)-tubule membrane invaginations that facilitate close coupling of key Ca²⁺ -handling proteins such as the L-type Ca²⁺ channel and Na⁺ -Ca²⁺ exchanger (NCX) on the cell surface with ryanodine receptors (RyRs) on the intracellular Ca²⁺ store. Although less abundant and regular than in the ventricle, t-tubules also exist in atrial myocytes as a network of transverse invaginations with axial extensions known as the transverse-axial tubule system (TATS). In heart failure and atrial fibrillation, there is TATS remodelling that is associated with aberrant Ca²⁺ -handling and Ca²⁺ -induced arrhythmic activity; however, the mechanism underlying this is not fully understood. To address this, we developed a novel 3D human atrial myocyte model that couples electrophysiology and Ca²⁺ -handling with variable TATS organization and density. We extensively parameterized and validated our model against experimental data to build a robust tool examining TATS regulation of subcellular Ca²⁺ release. We found that varying TATS density and thus the localization of key Ca²⁺ -handling proteins has profound effects on Ca²⁺ handling. Following TATS loss, there is reduced NCX that results in increased cleft Ca²⁺ concentration through decreased Ca²⁺ extrusion. This elevated Ca²⁺ increases RyR open probability causing spontaneous Ca²⁺ releases and the promotion of arrhythmogenic waves (especially in the cell interior) leading to voltage instabilities through delayed afterdepolarizations. In summary, the present study demonstrates a mechanistic link between TATS remodelling and Ca²⁺ -driven proarrhythmic behaviour that probably reflects the arrhythmogenic state observed in disease. KEY POINTS: Transverse-axial tubule systems (TATS) modulate Ca²⁺ handling and excitation-contraction coupling in atrial myocytes, with TATS remodelling in heart failure and atrial fibrillation being associated with altered Ca²⁺ cycling and subsequent arrhythmogenesis. To investigate the poorly understood mechanisms linking TATS variation and spontaneous Ca²⁺ release, we built, parameterized and validated a 3D human atrial myocyte model coupling electrophysiology and spatially-detailed subcellular Ca²⁺ handling governed by the TATS. Simulated TATS loss causes diastolic Ca²⁺ and voltage instabilities through reduced Na⁺ -Ca²⁺ exchanger-mediated Ca²⁺ removal, cleft Ca²⁺ accumulation and increased ryanodine receptor open probability, resulting in spontaneous Ca²⁺ release and promotion of arrhythmogenic waves and delayed afterdepolarizations. At fast electrical rates typical of atrial tachycardia/fibrillation, spontaneous Ca²⁺ releases are larger and more frequent in the cell interior than at the periphery. Our work provides mechanistic insight into how atrial TATS remodelling can lead to Ca²⁺ -driven instabilities that may ultimately contribute to the arrhythmogenic state in disease.

Impact of protocolized fluid management on electrolyte stability in patients undergoing continuous renal replacement therapy

Objective

Continuous renal replacement therapy (CRRT) is the standard treatment for critically ill patients with acute kidney injury (AKI). Electrolyte disturbance such as hypokalemia or hypophosphatemia occurs paradoxically in patients undergoing CRRT due to high clearance. We developed a fluid management protocol for dialysate and replacement fluid that depends on serum electrolytes and focuses on potassium and phosphate levels to prevent electrolyte disturbance during CRRT. The impact of our new fluid protocol on electrolyte stability was evaluated.

Methods

Adult patients who received CRRT between 2013 and 2017 were included. Patients treated 2 years before (2013–2014; pre-protocol group) and 2 years following development of the fluid protocol (2016–2017; protocol group) were compared. The primary outcomes were individual coefficient of variation (CV) and abnormal event rates of serum phosphate and potassium. Secondary outcomes were frequency of electrolyte replacement and incidence of cardiac arrhythmias. Individual CV and abnormal event rates for each electrolyte were analyzed using the Wilcoxon rank-sum test and Chi-square test with Yates’ continuity correction.

Results

A total of 1,448 patients was included. Both serum phosphate and potassium were higher in the protocol group. The CVs of serum phosphate (pre-protocol vs. protocol, 0.275 [0.207–0.358] vs. 0.229 [0.169–0.304], p &lt; 0.01) and potassium (0.104 [0.081–0.135] vs. 0.085 [0.064–0.110], p &lt; 0.01) were significantly lower in the protocol group. The abnormal event rates of serum phosphate (rate [95% CI], 0.410 [0.400–0.415] vs. 0.280 [0.273–0.286], p &lt; 0.01) and potassium (0.205 [0.199–0.211] vs. 0.083 [0.079–0.087], p &lt; 0.01) were also significantly lower in the protocol group.

Conclusion

The protocolized management of fluid in CRRT effectively prevented hypophosphatemia and hypokalemia by inducing excellent stability of serum phosphate and potassium levels.

Cardiac Transverse Tubules in Physiology and Heart Failure

In mammalian cardiac myocytes, the plasma membrane includes the surface sarcolemma but also a network of membrane invaginations called transverse (t-) tubules. These structures carry the action potential deep into the cell interior, allowing efficient triggering of Ca2+ release and initiation of contraction. Once thought to serve as rather static enablers of excitation-contraction coupling, recent work has provided a newfound appreciation of the plasticity of the t-tubule network's structure and function. Indeed, t-tubules are now understood to support dynamic regulation of the heartbeat across a range of timescales, during all stages of life, in both health and disease. This review article aims to summarize these concepts, with consideration given to emerging t-tubule regulators and their targeting in future therapies.

A Carvedilol Analogue, VK-II-86, Prevents Hypokalaemia-induced Ventricular Arrhythmia through Novel multi-Channel Effects

BACKGROUND AND PURPOSE

QT prolongation and intracellular Ca²⁺ loading with diastolic Ca²⁺ release via ryanodine receptors (RyR2) are the predominant mechanisms underlying hypokalaemia-induced ventricular arrhythmia. We investigated the antiarrhythmic actions of two RyR2 inhibitors: dantrolene and VK-II-86, a carvedilol analogue with no β-blocking activity, in hypokalaemia.

EXPERIMENTAL APPROACH

Surface ECG and ventricular action potentials (APs) were recorded from whole-heart murine Langendorff preparations. Ventricular arrhythmia incidence was compared in hearts perfused with low [K⁺ ], and those pre-treated with dantrolene or VK-II-86. Whole-cell patch clamping was used in murine and canine ventricular cardiomyocytes to study the effects of dantrolene and VK-II-86 on AP parameters in low [K⁺ ] and the effects of VK-II-86 on the inward rectifier current (I_K1 ), late sodium current (I_{Na_L} ) and the L-type Ca²⁺ current (I_Ca ). Effects of VK-II-86 on I_Kr were investigated in transfected HEK-293 cells. A fluorogenic probe quantified the effects of VK-II-86 on oxidative stress in hypokalaemia.

KEY RESULTS

Dantrolene reduced the incidence of ventricular arrhythmias induced by low [K⁺ ] in explanted murine hearts by 94%, whereas VK-II-86 prevented all arrhythmias. VK-II-86 prevented hypokalaemia-induced AP prolongation and depolarization, but did not alter AP parameters in normokalaemia. Hypokalaemia was associated with a significant reduction of I_K1 and I_Kr , and increase in I_Na-L , and I_Ca . VK-II-86 prevented all hypokalaemia-induced changes in ion channel activity and oxidative stress.

CONCLUSIONS AND IMPLICATIONS

VK-II-86 prevents hypokalaemia-induced arrhythmogenesis by normalising calcium homeostasis and repolarization reserve. VK-II-86 may provide an exciting treatment in hypokalaemia and other arrhythmias caused by delayed repolarization or Ca²⁺ overload.

Suppression of ventricular arrhythmias by targeting late L-type Ca2+ current

Ventricular arrhythmias, a leading cause of sudden cardiac death, can be triggered by cardiomyocyte early afterdepolarizations (EADs). EADs can result from an abnormal late activation of L-type Ca²⁺ channels (LTCCs). Current LTCC blockers (class IV antiarrhythmics), while effective at suppressing EADs, block both early and late components of I_Ca,L, compromising inotropy. However, computational studies have recently demonstrated that selective reduction of late I_Ca,L (Ca²⁺ influx during late phases of the action potential) is sufficient to potently suppress EADs, suggesting that effective antiarrhythmic action can be achieved without blocking the early peak I_Ca,L, which is essential for proper excitation–contraction coupling. We tested this new strategy using a purine analogue, roscovitine, which reduces late I_Ca,L with minimal effect on peak current. Scaling our investigation from a human Ca_V1.2 channel clone to rabbit ventricular myocytes and rat and rabbit perfused hearts, we demonstrate that (1) roscovitine selectively reduces I_Ca,L noninactivating component in a human Ca_V1.2 channel clone and in ventricular myocytes native current, (2) the pharmacological reduction of late I_Ca,L suppresses EADs and EATs (early after Ca²⁺ transients) induced by oxidative stress and hypokalemia in isolated myocytes, largely preserving cell shortening and normal Ca²⁺ transient, and (3) late I_Ca,L reduction prevents/suppresses ventricular tachycardia/fibrillation in ex vivo rabbit and rat hearts subjected to hypokalemia and/or oxidative stress. These results support the value of an antiarrhythmic strategy based on the selective reduction of late I_Ca,L to suppress EAD-mediated arrhythmias. Antiarrhythmic therapies based on this idea would modify the gating properties of Ca_V1.2 channels rather than blocking their pore, largely preserving contractility.

Modulated Calcium Homeostasis and Release Events Under Atrial Fibrillation and Its Risk Factors: A Meta-Analysis

Background: Atrial fibrillation (AF) is associated with calcium (Ca²⁺) handling remodeling and increased spontaneous calcium release events (SCaEs). Nevertheless, its exact mechanism remains unclear, resulting in suboptimal primary and secondary preventative strategies.

Methods: We searched the PubMed database for studies that investigated the relationship between SCaEs and AF and/or its risk factors. Meta-analysis was used to examine the Ca²⁺ mechanisms involved in the primary and secondary AF preventative groups.

Results: We included a total of 74 studies, out of the identified 446 publications from inception (1982) until March 31, 2020. Forty-five were primary and 29 were secondary prevention studies for AF. The main Ca²⁺ release events, calcium transient (standardized mean difference (SMD) = 0.49; I² = 35%; confidence interval (CI) = 0.33–0.66; p < 0.0001), and spark amplitude (SMD = 0.48; I² = 0%; CI = −0.98–1.93; p = 0.054) were enhanced in the primary diseased group, while calcium transient frequency was increased in the secondary group. Calcium spark frequency was elevated in both the primary diseased and secondary AF groups. One of the key cardiac currents, the L-type calcium current (I_CaL) was significantly downregulated in primary diseased (SMD = −1.07; I² = 88%; CI = −1.94 to −0.20; p < 0.0001) and secondary AF groups (SMD = −1.28; I² = 91%; CI = −2.04 to −0.52; p < 0.0001). Furthermore, the sodium–calcium exchanger (I_NCX) and NCX1 protein expression were significantly enhanced in the primary diseased group, while only NCX1 protein expression was shown to increase in the secondary AF studies. The phosphorylation of the ryanodine receptor at S2808 (pRyR-S2808) was significantly elevated in both the primary and secondary groups. It was increased in the primary diseased and proarrhythmic subgroups (SMD = 0.95; I² = 64%; CI = 0.12–1.79; p = 0.074) and secondary AF group (SMD = 0.66; I² = 63%; CI = 0.01–1.31; p < 0.0001). Sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) expression was elevated in the primary diseased and proarrhythmic drug subgroups but substantially reduced in the secondary paroxysmal AF subgroup.

Conclusions: Our study identified that I_CaL is reduced in both the primary and secondary diseased groups. Furthermore, pRyR-S2808 and NCX1 protein expression are enhanced. The remodeling leads to elevated Ca²⁺ functional activities, such as increased frequencies or amplitude of Ca²⁺ spark and Ca²⁺ transient. The main difference identified between the primary and secondary diseased groups is SERCA expression, which is elevated in the primary diseased group and substantially reduced in the secondary paroxysmal AF subgroup. We believe our study will add new evidence to AF mechanisms and treatment targets.

Mechanisms of phase-3 early afterdepolarizations and triggered activities in ventricular myocyte models

Early afterdepolarizations (EADs) are abnormal depolarizations during the repolarizing phase of the action potential, which are associated with cardiac arrhythmogenesis. EADs are classified into phase-2 and phase-3 EADs. Phase-2 EADs occur during phase 2 of the action potential, with takeoff potentials typically above -40 mV. Phase-3 EADs occur during phase 3 of the action potential, with takeoff potential between -70 and -50 mV. Since the amplitude of phase-3 EADs can be as large as that of a regular action potential, they are also called triggered activities (TAs). This also makes phase-3 EADs and TAs much more arrhythmogenic than phase-2 EADs since they can propagate easily in tissue. Although phase-2 EADs have been widely observed, phase-3 EADs and TAs have been rarely demonstrated in isolated ventricular myocytes. Here we carry out computer simulations of three widely used ventricular action potential models to investigate the mechanisms of phase-3 EADs and TAs. We show that when the T-type Ca2+ current (ICa,T ) is absent (e.g., in normal ventricular myocytes), besides the requirement of increasing inward currents and reducing outward currents as for phase-2 EADs, the occurrence of phase-3 EADs and TAs requires a substantially large increase of the L-type Ca2+ current and the slow component of the delayed rectifier K+ current. The presence of ICa,T (e.g., in neonatal and failing ventricular myocytes) can greatly reduce the thresholds of these two currents for phase-3 EADs and TAs. This implies that ICa,T may play an important role in arrhythmogenesis in cardiac diseases.

Immediate and Delayed Response of Simulated Human Atrial Myocytes to Clinically-Relevant Hypokalemia

Although plasma electrolyte levels are quickly and precisely regulated in the mammalian cardiovascular system, even small transient changes in K⁺, Na⁺, Ca²⁺, and/or Mg²⁺ can significantly alter physiological responses in the heart, blood vessels, and intrinsic (intracardiac) autonomic nervous system. We have used mathematical models of the human atrial action potential (AP) to explore the electrophysiological mechanisms that underlie changes in resting potential (V_r) and the AP following decreases in plasma K⁺, [K⁺]_o, that were selected to mimic clinical hypokalemia. Such changes may be associated with arrhythmias and are commonly encountered in patients (i) in therapy for hypertension and heart failure; (ii) undergoing renal dialysis; (iii) with any disease with acid-base imbalance; or (iv) post-operatively. Our study emphasizes clinically-relevant hypokalemic conditions, corresponding to [K⁺]_o reductions of approximately 1.5 mM from the normal value of 4 to 4.5 mM. We show how the resulting electrophysiological responses in human atrial myocytes progress within two distinct time frames: (i) Immediately after [K⁺]_o is reduced, the K⁺-sensing mechanism of the background inward rectifier current (I_K1) responds. Specifically, its highly non-linear current-voltage relationship changes significantly as judged by the voltage dependence of its region of outward current. This rapidly alters, and sometimes even depolarizes, V_r and can also markedly prolong the final repolarization phase of the AP, thus modulating excitability and refractoriness. (ii) A second much slower electrophysiological response (developing 5-10 minutes after [K⁺]_o is reduced) results from alterations in the intracellular electrolyte balance. A progressive shift in intracellular [Na⁺]_i causes a change in the outward electrogenic current generated by the Na⁺/K⁺ pump, thereby modifying V_r and AP repolarization and changing the human atrial electrophysiological substrate. In this study, these two effects were investigated quantitatively, using seven published models of the human atrial AP. This highlighted the important role of I_K1 rectification when analyzing both the mechanisms by which [K⁺]_o regulates V_r and how the AP waveform may contribute to "trigger" mechanisms within the proarrhythmic substrate. Our simulations complement and extend previous studies aimed at understanding key factors by which decreases in [K⁺]_o can produce effects that are known to promote atrial arrhythmias in human hearts.

The proarrhythmic effects of hypothermia in atria isolated from 5-HT4-receptor-overexpressing mice

We investigated whether hypothermia would be arrhythmogenic in mice that overexpress the human 5-HT₄ receptor only in their cardiac myocytes (5-HT₄-TG). Contractile studies were performed in isolated, electrically driven (1 Hz) left and spontaneously beating right atrial preparations of 5-HT₄-TG and littermate wild-type control mice (WT). Hypothermia (23 °C) decreased the force of contraction in the mouse right and left atrial preparations. Moreover, the concentration-dependent positive inotropic effects of 5-HT were blunted but still shifted to lower 5-HT concentrations in the left 5-HT₄-TG atria in hypothermia compared to normothermia (37 °C). Furthermore, hypothermia increased the incidence of right atrial arrhythmias in 5-HT₄-TG more than in WT mice. In contrast, at 37 °C, lowering the potassium concentration from 5.2 to 2.0 mM also induced arrhythmias in the right atrium, but with a similar incidence in WT and 5-HT₄-TG mice. In contrast, 10 μM d,l-sotalol and 300 μM erythromycin did not induce arrhythmias. Hypothermia was accompanied by the increased expression of heat shock protein 70 (HSP70) in WT but not in 5-HT₄-TG mice. We concluded that without the stimulation of 5-HT₄-receptors by exogenous agonists, a simple temperature reduction can increase arrhythmias in 5-HT₄-TG mice. It is tempting to speculate that in human patients, 5-HT₄ receptors might contribute to potentially deadly hypothermia-induced arrhythmias.

Targeting of Potassium Channels in Cardiac Arrhythmias

Cardiomyocytes are endowed with a complex repertoire of ion channels, responsible for the generation of action potentials (APs), travelling waves of electrical excitation, propagating throughout the heart and leading to cardiac contractions. Cardiac AP waveforms are shaped by a striking diversity of K⁺ channels. The pivotal role of K⁺ channels in cardiac health and disease is underscored by the dramatic impact that K⁺ channel dysfunction has on cardiac arrhythmias. The development of drugs targeted to specific K⁺ channels is expected to provide an optimized approach to antiarrhythmic therapy. Here, we review the functional roles of cardiac potassium channels under normal and diseased states. We survey current antiarrhythmic drugs (AADs) targeted to voltage-gated and Ca²⁺-activated K⁺ channels and highlight future research opportunities.

In vitro and In silico Models to Study SARS-CoV-2 Infection: Integrating Experimental and Computational Tools to Mimic "COVID-19 Cardiomyocyte"

The rapid dissemination of SARS-CoV-2 has made COVID-19 a tremendous social, economic, and health burden. Despite the efforts to understand the virus and treat the disease, many questions remain unanswered about COVID-19 mechanisms of infection and progression. Severe Acute Respiratory Syndrome (SARS) infection can affect several organs in the body including the heart, which can result in thromboembolism, myocardial injury, acute coronary syndromes, and arrhythmias. Numerous cardiac adverse events, from cardiomyocyte death to secondary effects caused by exaggerated immunological response against the virus, have been clinically reported. In addition to the disease itself, repurposing of treatments by using "off label" drugs can also contribute to cardiotoxicity. Over the past several decades, animal models and more recently, stem cell-derived cardiomyocytes have been proposed for studying diseases and testing treatments in vitro. In addition, mechanistic in silico models have been widely used for disease and drug studies. In these models, several characteristics such as gender, electrolyte imbalance, and comorbidities can be implemented to study pathophysiology of cardiac diseases and to predict cardiotoxicity of drug treatments. In this Mini Review, we (1) present the state of the art of in vitro and in silico cardiomyocyte modeling currently in use to study COVID-19, (2) review in vitro and in silico models that can be adopted to mimic the effects of SARS-CoV-2 infection on cardiac function, and (3) provide a perspective on how to combine some of these models to mimic "COVID-19 cardiomyocytes environment.".

Arrhythmogenic Mechanisms in Hypokalaemia: Insights From Pre-clinical Models

Potassium is the predominant intracellular cation, with its extracellular concentrations maintained between 3. 5 and 5 mM. Among the different potassium disorders, hypokalaemia is a common clinical condition that increases the risk of life-threatening ventricular arrhythmias. This review aims to consolidate pre-clinical findings on the electrophysiological mechanisms underlying hypokalaemia-induced arrhythmogenicity. Both triggers and substrates are required for the induction and maintenance of ventricular arrhythmias. Triggered activity can arise from either early afterdepolarizations (EADs) or delayed afterdepolarizations (DADs). Action potential duration (APD) prolongation can predispose to EADs, whereas intracellular Ca²⁺ overload can cause both EADs and DADs. Substrates on the other hand can either be static or dynamic. Static substrates include action potential triangulation, non-uniform APD prolongation, abnormal transmural repolarization gradients, reduced conduction velocity (CV), shortened effective refractory period (ERP), reduced excitation wavelength (CV × ERP) and increased critical intervals for re-excitation (APD-ERP). In contrast, dynamic substrates comprise increased amplitude of APD alternans, steeper APD restitution gradients, transient reversal of transmural repolarization gradients and impaired depolarization-repolarization coupling. The following review article will summarize the molecular mechanisms that generate these electrophysiological abnormalities and subsequent arrhythmogenesis.

Cardiac transmembrane ion channels and action potentials: cellular physiology and arrhythmogenic behavior

Cardiac arrhythmias are among the leading causes of mortality. They often arise from alterations in the electrophysiological properties of cardiac cells and their underlying ionic mechanisms. It is therefore critical to further unravel the pathophysiology of the ionic basis of human cardiac electrophysiology in health and disease. In the first part of this review, current knowledge on the differences in ion channel expression and properties of the ionic processes that determine the morphology and properties of cardiac action potentials and calcium dynamics from cardiomyocytes in different regions of the heart are described. Then the cellular mechanisms promoting arrhythmias in congenital or acquired conditions of ion channel function (electrical remodeling) are discussed. The focus is on human-relevant findings obtained with clinical, experimental, and computational studies, given that interspecies differences make the extrapolation from animal experiments to human clinical settings difficult. Deepening the understanding of the diverse pathophysiology of human cellular electrophysiology will help in developing novel and effective antiarrhythmic strategies for specific subpopulations and disease conditions.

Dynamics of Atrial Fibrillation Mechanisms and Comorbidities

Atrial fibrillation (AF) contributes to morbidity and mortality of millions of individuals. Its molecular, cellular, neurohumoral, and hemodynamic pathophysiological mechanisms are complex, and there is increasing awareness that a wide range of comorbidities can contribute to AF-promoting atrial remodeling. Moreover, recent research has highlighted that AF risk is not constant and that the temporal variation in concomitant conditions contributes to the complexity of AF dynamics. In this review, we provide an overview of fundamental AF mechanisms related to established and emerging comorbidities or risk factors and their role in the AF-promoting effects. We focus on the accumulating evidence for the relevance of temporally dynamic changes in these risk factors and the consequence for AF initiation and maintenance. Finally, we highlight the important implications for future research and clinical practice resulting from the dynamic interaction between AF risk factors and mechanisms.

Inhibition of KCa2 Channels Decreased the Risk of Ventricular Arrhythmia in the Guinea Pig Heart During Induced Hypokalemia

BACKGROUND

Hypokalemia reduces the cardiac repolarization reserve. This prolongs the QT-interval and increases the risk of ventricular arrhythmia; a risk that is exacerbated by administration of classical class 3 anti-arrhythmic agents.Small conductance Ca2+-activated K+-channels (KCa2) are a promising new atrial selective target for treatment of atrial fibrillation. Under physiological conditions KCa2 plays a minor role in ventricular repolarization. However, this might change under hypokalemia because of concomitant increases in ventriculay -60r intracellur Ca2+.

PURPOSE

To study the effects of pharmacological KCa2 channel inhibition by the compounds AP14145, ICA, or AP30663 under hypokalemic conditions as compared to dofetilide and hypokalemia alone time-matched controls (TMC).

METHODS

The current at +10 mV was compared in HEK293 cells stably expressing KCa2.3 perfused first with normo- and then hypokalemic solutions (4 mM K+ and 2.5 mM K+, respectively). Guinea pig hearts were isolated and perfused with normokalemic (4 mM K+) Krebs-Henseleit solution, followed by perfusion with drug or vehicle control. The perfusion was then changed to hypokalemic solution (2.5 mM K+) in presence of drug. 30 animals were randomly assigned to 5 groups: ICA, AP14145, AP30663, dofetilide, or TMC. QT-interval, the interval from the peak to the end of the T wave (Tp-Te), ventricular effective refractory period (VERP), arrhythmia score, and ventricular fibrillation (VF) incidence were recorded.

RESULTS

Hypokalemia slightly increased KCa2.3 current compared to normokalemia. Application of KCa2 channel inhibitors and dofetilide prolonged the QT interval corrected for heart rate. Dofetilide, but none of the KCa2 channel inhibitors increased Tp-Te during hypokalemia. During hypokalemia 4/6 hearts in the TMC group developed VF (two spontaneously, two by S1S2 stimulation) whereas 5/6 hearts developed VF in the dofetilide group (two spontaneously, three by S1S2 stimulation). In comparison, 0/6, 1/6, and 1/6 hearts developed VF when treated with the KCa2 channel inhibitors AP30663, ICA, or AP14145, respectively.

CONCLUSION

Hypokalemia was associated with an increased incidence of VF, an effect that also seen in the presence of dofetilide. In comparison, the structurally and functionally different KCa2 channel inhibitors, ICA, AP14145, and AP30663 protected the heart from hypokalemia induced VF. These results support that KCa2 inhibition may be associated with a better safety and tolerability profile than dofetilide.