The contribution of ionic currents to changes in refractoriness of human atrial myocytes associated with chronic atrial fibrillation

<b>Objective:</b> To investigate changes in human atrial single cell functional electrophysiological properties associated with chronic atrial fibrillation (AF), and the contribution to these of accompanying ion current changes. <b>Methods:</b> The whole cell patch clamp technique was used to record action potentials, the effective refractory period (ERP) and ion currents, in the absence and presence of drugs, in enzymatically isolated myocytes from 11 patients with chronic (<6 months) AF and 39 patients in sinus rhythm. <b>Results:</b> Stimulation at high rates (up to 600 beats/min) markedly shortened late repolarisation and the ERP in cells from patients in sinus rhythm, and depolarised the maximum diastolic potential (MDP). Chronic AF was associated with a reduction in the ERP at physiological rate (from 203±16 to 104±15 ms, P<0.05), and marked attenuation in rate effects on the ERP and repolarisation. The abbreviated terminal phase of repolarisation prevented fast rate-induced depolarisation of the MDP in cells from patients with AF. The density of L-type Ca<sup>2+</sup> (<i>I</i><sub>CaL</sub>) and transient outward K<sup>+</sup> (<i>I</i><sub>TO</sub>) currents was significantly reduced in cells from patients with AF (by 60–65%), whilst the inward rectifier K<sup>+</sup> current (IK1) was increased, and the sustained outward current (<i>I</i><sub>KSUS</sub>) was unaltered. Superfusion of cells from patients in sinus rhythm with nifedipine (10 micromol/l) moderately shortened repolarisation, but had no effect on the ERP (228±12 vs. 225±11 ms). 4-Aminopyridine (2 mmol/l) markedly prolonged repolarisation and the ERP (by 35%, P<0.05). However, the combination of these drugs had no effect on late repolarisation or refractoriness. <b>Conclusion:</b> Chronic AF in humans is associated with attenuation in adaptation of the atrial single cell ERP and MDP to fast rates, which may not be explained fully by accompanying changes in <i>I</i><sub>CaL</sub> and <i>I</i><sub>TO</sub>

Ionic basis of a differential effect of adenosine on refractoriness in rabbit AV nodal and atrial isolated myocytes

Methods: The whole cell patch clamp technique was used to record action potentials and ion currents in AV nodal and left atrial myocytes isolated enzymatically from rabbit hearts. Results: Adenosine (10 μM) caused similar hyperpolarisation and shortening of the action potential duration (APD) in both cell types: maximum diastolic potential was hyperpolarised from –59±3 to –66±2 and from –70±2 to –76±2 mV (mean±SEM) and APD90 was shortened by 31±4 and 30±7% in AV nodal (n=14) and atrial cells (n=8), respectively. Adenosine shortened the effective refractory period (ERP) in atrial cells, from 124±15 to 98±14 ms (n=8). In contrast, ERP in AV nodal cells was not significantly affected (112±13 vs. 102±12 ms, n=14), and post-repolarisation refractoriness was prolonged. By contrast, current injection, to induce an equal degree of hyperpolarisation to that produced by adenosine, shortened APD and ERP in both cell types, suggesting an additional action of adenosine in AV nodal cells. Adenosine (10 μM) did not affect peak ICaL in AV nodal cells, but significantly altered the biexponential time course of recovery of ICaL from inactivation. The proportion of recovery in the fast phase (time constant, {tau}=102±10 ms) was reduced from 71±3 to 55±5%, with shift to the slow phase ({tau}=858±168 ms), without altering {tau} in either phase. A similar effect of adenosine was seen in left atrial cells. Conclusion: Adenosine caused hyperpolarisation, APD-shortening and slowing of recovery of ICaL from inactivation, in both AV nodal and atrial cells, but prolonged post-repolarisation refractoriness in AV nodal cells only. This differential effect of adenosine on refractoriness in the two cell types could not be explained by effects on IKAdo, but may be due to slowed reactivation of ICaL, which is the predominant inward current in AV nodal but not left atrial cells

Characterisation of the Na, K pump current in atrial cells from patients with and without chronic atrial fibrillation

<b>Objective:</b> To assess the contribution of the Na, K pump current (<i>I</i><sub>p</sub>) to the action potential duration (APD) and effective refractory period (ERP) in human atrial cells, and to investigate whether <i>I</i><sub>p</sub> contributes to the changes in APD and ERP associated with chronic atrial fibrillation (AF). <b>Methods:</b> Action potentials and ion currents were recorded by whole-cell patch clamp in atrial myocytes isolated from consenting patients undergoing cardiac surgery, who were in sinus rhythm (SR) or AF (>3 months). <b>Results:</b> In cells from patients in SR, the <i>I</i><sub>p</sub> blocker, ouabain (10 μM) significantly depolarised the membrane potential, Vm, from -80±2 (mean±S.E.) to -73±2 mV, and lengthened both the APD (174±17 vs. 197±23 ms at 90% repolarisation) and ERP (198±22 vs. 266±14 ms; P<0.05 for each, Student's t-test, <i>n</i>=7 cells, 5 patients). With an elevated pipette [Na<sup>+</sup>] of 30 mM, <i>I</i><sub>p</sub> was measured by increasing extracellular [K<sup>+</sup>] ([K<sup>+</sup>]o) from 0 to 5.4 mM. This produced an outward shift in holding current at -40 mV, abolished by 10 muM ouabain. K± and ouabain-sensitive current densities were similar, at 0.99±0.13 and 1.12±0.11 pA/pF, respectively (P>0.05; <i>n</i>=9 cells), confirming the K±induced current as <i>I</i><sub>p</sub>. <i>I</i><sub>p</sub> increased linearly with increasing Vm between -120 and +60 mV (<i>n</i>=25 cells). Stepwise increments in [K<sup>+</sup>]<sub>o</sub> (between 0 and 10 mM) increased Ip in a concentration-dependent manner (maximum response, <i>E</i><sub>max</sub>=1.19±0.09 pA/pF; EC50=1.71±0.15 mM; n=27 cells, 9 patients). In cells from patients in AF, the sensitivity of Ip to both Vm and [K+]o (<i>E</i><sub>max</sub>=1.02±0.05 pA/pF, EC50=1.54±0.11 mM; <i>n</i>=44 cells, 9 patients) was not significantly different from that in cells from patients in SR. Within the group of patients in AF, long-term digoxin therapy (<i>n</i>=5 patients) was associated with a small, but significant, reduction in <i>E</i><sub>max</sub> (0.92±0.07 pA/pF) and EC<sub>50</sub> (1.35±0.15 mM) compared with non-treatment (<i>E</i><sub>max</sub>=1.13±0.08 pA/pF, EC<sub>50</sub>=1.76±0.14 mM; P<0.05 for each, <i>n</i>=4 patients). In cells from non-digoxin-treated patients in AF, the voltage- and [K<sup>+</sup>]<sub>o</sub>-sensitivity (<i>E</i><sub>max</sub> and EC<sub>50</sub>) were similar to those in cells from patients in SR. <b>Conclusions:</b> The Na, K pump current contributes to the human atrial cell Vm, action potential shape and ERP. However, the similarity in Ip sensitivity to both [K<sup>+</sup>]<sub>o</sub> and <i>V</i><sub>m</sub> between atrial cells from patients with and without chronic AF indicates that <i>I</i><sub>p</sub> is not involved in AF-induced electrophysiological remodelling in patients

Short runs of atrial arrhythmia and stroke risk: a European-wide online survey among stroke physicians and cardiologists

Methods: An online survey of cardiologists and stroke physicians was carried out to assess current management of patients with short runs of atrial arrhythmia within Europe. Results: Respondents included 311 clinicians from 32 countries. To diagnose atrial fibrillation, 80% accepted a single 12-lead ECG and 36% accepted a single run of < 30 seconds on ambulatory monitoring. Stroke physicians were twice as likely to accept < 30 seconds of arrhythmia as being diagnostic of atrial fibrillation (OR 2.43, 95% CI 1.19–4.98). They were also more likely to advocate anticoagulation for hypothetical patients with lower risk; OR 1.9 (95% CI 1.0–3.5) for a patient with CHA2DS2-VASc = 2. Conclusion: Short runs of atrial fibrillation create a dilemma for physicians across Europe. Stroke physicians and cardiologists differ in their diagnosis and management of these patients

Anti-adrenergic effects of endothelin on human atrial action potentials are potentially anti-arrhythmic

Endothelin-1 (ET-1) is elevated in patients with atrial fibrillation (AF) and heart failure. We investigated effects of ET-1 on human atrial cellular electrophysiological measurements expected to influence the genesis and maintenance of AF. Action potential characteristics and L-type Ca<sup>2+</sup> current (I<sub>CaL</sub>) were recorded by whole cell patch clamp, in atrial isolated myocytes obtained from patients in sinus rhythm. Isoproterenol (ISO) at 0.05 μM prolonged the action potential duration at 50% repolarisation (APD<sub>50</sub>: 54 ± 10 vs. 28 ± 5 ms; <i>P</i> < 0.05, <i>N</i> = 15 cells, 10 patients), but neither late repolarisation nor cellular effective refractory period (ERP) were affected. ET-1 (10 nM) reversed the effect of ISO on APD<sub>50</sub>, and had no basal effect (in the absence of ISO) on repolarisation or ERP. During repetitive stimulation, ISO (0.05 μM) produced arrhythmic depolarisations (<i>P</i> < 0.05). Each was abolished by ET-1 at 10 nM (<i>P</i> < 0.05). ISO (0.05 μM) increased peak I<sub>CaL</sub> from –5.5 ± 0.4 to –14.6 ± 0.9 pA/pF (P < 0.05; N = 79 cells, 34 patients). ET-1 (10 nM) reversed this effect by 98 ± 10% (P < 0.05), with no effect on basal I<sub>CaL</sub>. Chronic treatment of patients with a β-blocker did not significantly alter basal APD50 or I<sub>CaL</sub>, the increase in APD50 or I<sub>CaL</sub> by 0.05 μM ISO, nor the subsequent reversal of this effect on APD50 by 10 nM ET-1. The marked anti-adrenergic effects of ET-1 on human atrial cellular action potential plateau, arrhythmic depolarisations and I<sub>CaL</sub>, without affecting ERP and independently of β-blocker treatment, may be expected to contribute a potentially anti-arrhythmic influence in the atria of patients with AF and heart failure

Chronic beta-adrenoceptor blockade and human atrial cell electrophysiology: evidence of pharmacological remodelling

<b>Objective:</b> Chronic beta-adrenoceptor antagonist (β-blocker) treatment reduces the incidence of reversion to AF in patients, possibly via an adaptive myocardial response. However, the underlying electrophysiological mechanisms are presently unclear. We aimed to investigate electrophysiological changes in human atrial cells associated with chronic treatment with β-blockers and other cardiovascular-acting drugs. <b>Methods:</b> Myocytes were isolated enzymatically from the right atrial appendage of 40 consenting patients who were in sinus rhythm. The cellular action potential duration (APD), effective refractory period (ERP), L-type Ca<sup>2+</sup> current (<i>I</i><sub>CaL</sub>), transient (<i>I</i><sub>TO</sub>) and sustained (<i>I</i><sub>KSUS</sub>) outward K<sup>+</sup> currents, and input resistance (<i>R</i><sub>i</sub>) were recorded using the whole cell patch clamp. Drug treatments and clinical characteristics were compared with electrophysiological measurements using simple and multiple regression analyses. P<0.05 was taken as statistically significant. <b>Results:</b> In atrial cells from patients treated chronically with β-blockers, the APD<sub>90</sub> and ERP (75 beats/min stimulation) were significantly longer, at 213±11 and 233±11 ms, respectively (<i>n</i> = 15 patients), than in cells from non-β-blocked patients, at 176±12 and 184±12 ms (n = 11). These cells also displayed a significantly reduced action potential phase 1 velocity (22±3 vs. 34±3 V/s). Chronic β-blockade was also associated with a significant reduction in the heart rate (58±3 vs. 69±5 beats/min) and in the density of ITO (8.7±1.3 vs. 13.7±2.1 pA/pF), an increase in the Ri (214±24 vs. 132±14 MΩ), but no significant change in <i>I</i><sub>CaL</sub> or <i>I</i><sub>KSUS</sub>. The <i>I</i><sub>TO</sub> blocker 4-aminopyridine largely mimicked the changes in phase 1 and ERP associated with chronic β-blockade, in cells from non-β-blocked patients. Chronic treatment of patients with calcium channel blockers or angiotensin converting enzyme inhibitors (<i>n</i> = 11–13 patients) was not associated with any significant changes in atrial cell electrophysiology. <b>Conclusion:</b> The observed atrial cellular electrophysiological changes associated with chronic β-blockade are consistent with a long-term adaptive response, a type of ‘pharmacological remodelling’, and provide mechanistic evidence supportive of the anti-arrhythmic actions of β-blockade

Mechanisms of termination and prevention of atrial fibrillation by drug therapy

Atrial fibrillation (AF) is a disorder of the rhythm of electrical activation of the cardiac atria. It is the most common cardiac arrhythmia, has multiple aetiologies, and increases the risk of death from stroke. Pharmacological therapy is the mainstay of treatment for AF, but currently available anti-arrhythmic drugs have limited efficacy and safety. An improved understanding of how anti-arrhythmic drugs affect the electrophysiological mechanisms of AF initiation and maintenance, in the setting of the different cardiac diseases that predispose to AF, is therefore required. A variety of animal models of AF has been developed, to represent and control the pathophysiological causes and risk factors of AF, and to permit the measurement of detailed and invasive parameters relating to the associated electrophysiological mechanisms of AF. The purpose of this review is to examine, consolidate and compare available relevant data on in-vivo electrophysiological mechanisms of AF suppression by currently approved and investigational anti-arrhythmic drugs in such models. These include the Vaughan Williams class I–IV drugs, namely Na+ channel blockers, β-adrenoceptor antagonists, action potential prolonging drugs, and Ca2+ channel blockers; the “upstream therapies”, e.g., angiotensin converting enzyme inhibitors, statins and fish oils; and a variety of investigational drugs such as “atrial-selective” multiple ion channel blockers, gap junction-enhancers, and intracellular Ca2+-handling modulators. It is hoped that this will help to clarify the main electrophysiological mechanisms of action of different and related drug types in different disease settings, and the likely clinical significance and potential future exploitation of such mechanisms. Keywords: Atrial fibrillation; Cardiac arrhythmia mechanisms: reentry, afterdepolarisations; In-vivo animal models; Pathological electrical remodelling; Pharmacological treatment; Anti-arrhythmic drug mechanisms Abbreviations: ACE, angiotensin-converting enzyme; AF, atrial fibrillation; AFCL, AF cycle length; APD, action potential duration; DAD, delayed afterdepolarisation; EAD, early afterdepolarisation; ERP, effective refractory period; ICaL, L-type Ca2+ current; ICaT, T-type Ca2+ current; If, funny current; IK1, inward rectifier K+ current; IKACh, acetylcholine-activated K+ current; IKr, rapid delayed rectifier K+ current; IKS, slow delayed rectifier K+ current; IKur, ultra-rapid delayed rectifier K+ current; INa, Na+ current; INa/Ca, Na+-Ca2+ exchanger current; INa/H, Na+-H+ exchanger current; INaL, late INa; ISKCa, small conductance Ca2+-activated K+ current; ITO, transient outward K+ curren

Negotiating the coaching landscape: Experiences of Black men and women coaches in the United Kingdom

The current article provides a critical examination of the racialised and gendered processes that reinforce disparities in sport coaching by exploring the experiences of Black men and women coaches in the United Kingdom. The findings are based on in-depth qualitative interviews with coaches from two national governing bodies of sport. Using a Critical Race Theory approach and Black feminist lens, the coaches’ narratives illuminate the complex, multifaceted and dynamic ways in which ‘race’, ethnicity and gender are experienced and negotiated by sport coaches. The coaches’ reflections are discussed under three themes: negotiating identities; privilege and blind spots; and systemic discrimination. The narratives from the coaches’ experiences emphasise the need for key stakeholders in sport to recognise the intersectional, structural and relational experiences that facilitate, as well as constrain, the progression of Black coaches in order to challenge racialised and gendered inequalities

Atrial cellular electrophysiological changes in patients with ventricular dysfunction may predispose to AF

<b>Background:</b> Left ventricular systolic dysfunction (LVSD) is a risk factor for atrial fibrillation (AF), but the atrial cellular electrophysiological mechanisms in humans are unclear. Objective This study sought to investigate whether LVSD in patients who are in sinus rhythm (SR) is associated with atrial cellular electrophysiological changes that could predispose to AF. <b>Methods:</b> Right atrial myocytes were obtained from 214 consenting patients in SR who were undergoing cardiac surgery. Action potentials or ion currents were measured using the whole-cell-patch clamp technique. <b>Results:</b> The presence of moderate or severe LVSD was associated with a shortened atrial cellular effective refractory period (ERP) (209 ± 8 ms; 52 cells, 18 patients vs 233 ± 7 ms; 134 cells, 49 patients; P <0.05); confirmed by multiple linear regression analysis. The left ventricular ejection fraction (LVEF) was markedly lower in patients with moderate or severe LVSD (36% ± 4%, n = 15) than in those without LVSD (62% ± 2%, n = 31; P <0.05). In cells from patients with LVEF ≤ 45%, the ERP and action potential duration at 90% repolarization were shorter than in those from patients with LVEF > 45%, by 24% and 18%, respectively. The LVEF and ERP were positively correlated (r = 0.65, P <0.05). The L-type calcium ion current, inward rectifier potassium ion current, and sustained outward ion current were unaffected by LVSD. The transient outward potassium ion current was decreased by 34%, with a positive shift in its activation voltage, and no change in its decay kinetics. <b>Conclusion:</b> LVSD in patients in SR is independently associated with a shortening of the atrial cellular ERP, which may be expected to contribute to a predisposition to AF

Venous thromboembolism in primary nephrotic syndrome - is the risk high enough to justify prophylactic anticoagulation?

Background: The reported incidence of venous thromboembolism (VTE) in patients with nephrotic syndrome (NS) varies widely, as does the approach to prophylactic anticoagulation. We aimed to assess the incidence of VTE in patients with primary NS in order to inform a sample size calculation to determine if a future clinical trial will ever be feasible. Methods: All adults undergoing native renal biopsy for NS between 2008 and 2013 yielding a diagnosis of primary glomerulonephritis were identified. Baseline serum albumin, urine protein:creatinine ratio, estimated glomerular filtration rate, date of biopsy and histological diagnosis were recorded. Episodes of objectively verified VTE were identified using the electronic patient record. Sample size calculations were performed based on 2 independent samples with a dichotomous outcome and to achieve a power of 80% and p < 0.05. Results: Two hundred six patients were included of which 60% were male and mean age at biopsy was 55 years (SD 19). Median follow-up was 2.9 years (interquartile range (IQR) 1.6-4.7). Fourteen (6.8%) patients suffered VTE. Median time to diagnosis of VTE from renal biopsy was 36 days (IQR -22 to 178), with 6 VTEs occurring prior to biopsy and 1 during remission. In a total of 270 patient years of NS, there were 7 VTE that could potentially have been avoided if anticoagulation was given for the duration of NS, that is, 2.6% risk per year of NS; this risk was highest for patients with minimal change nephropathy at 13.3% per year of NS, compared to 0.65% per year of NS for those with idiopathic membranous nephropathy. Assuming a 75% reduction in the incidence of VTE with prophylactic anticoagulation, 972 participants would be required for a future clinical trial to have 80% power. Conclusions: Patients with primary NS are at an increased risk of VTE. The timing of VTE means that only half of episodes would be targeted by prophylactic anticoagulation. Given the low frequency of events, a well-powered clinical trial would be challenging to achieve