2+ release (SCR) from the sarcoplasmic reticulum can cause delayed afterdepolarizations and triggered activity, contributing to arrhythmogenesis during β-adrenergic stimulation. Excessive beat-to-beat variability of repolarization duration (BVR) is a proarrhythmic marker. Previous research has shown that BVR is increased during intense β-adrenergic stimulation, leading to SCR.Objective: We aimed to determine ionic mechanisms controlling BVR under these conditions. potential (AP) of the single cardiac myocyte to the QT interval on the body surface. 7-9 Exaggerated BVR has been reported to be a more reliable indicator of arrhythmogenic risk than repolarization prolongation, per se, at least in several experimental ventricular tachycardia models [10][11][12] and in selected human subjects. 8,13 Although BVR has been investigated in multiple studies, the mechanisms underlying this phenomenon at the singlecell level remain to be fully elucidated. Pharmacological interventions influencing ion channels that operate during the AP plateau can markedly alter BVR. 7,14 Despite the fact that inhibition of the slowly activating delayed rectifier K + current (I Ks ) alone has minimal effects on both cellular AP duration (APD) and BVR, 14 we recently have shown that during increased Ca 2+ loading in myocytes subjected to blockade of I Ks in combination with βAR stimulation, BVR is significantly enhanced, even before the occurrence of EADs and TA. 14 In the present study, we investigated the relationship between SCR and BVR using a combined experimental and computational approach in both canine ventricular myocytes and in situ hearts subjected to βAR stimulation. We show that SCRs not only lead to I ti and DAD formation but also lead to a prolonged duration of AP via increased L-type Ca 2+ current (I CaL ), which in turn leads to increased BVR when analyzing multiple consecutive APs. Pharmacological interventions that inhibit SCR (either with reduced or with preserved systolic contraction) prevent this SCR-associated AP prolongation and reduce BVR. Methods and Results: MethodsThis investigation conformed to the Guide for the Care and Use of Laboratory Animals published by the United States National Institutes of Health (National Institutes of Health Publication 85-23, revised 1996). Animal handling was in accordance with the European Directive for the Protection of Vertebrate Animals Used for Experimental and Other Scientific Purposes (86/609/EU). Full details of methods, solutions, and interventions used are given in the onlineonly Data Supplement accompanying this article. A brief summary of the main aspects is provided. Myocyte Isolation and ElectrophysiologyCanine left ventricular (LV) myocytes were isolated as previously described. 15 Transmembrane APs were recorded at ≈37°C using highresistance (30-60 MΩ) glass microelectrodes filled with 3 mol/L KCl. Myocyte contractions were recorded with a video edge motion detector. Calcium MeasurementWe used the perforated patch-clamp technique under current-clamp or vol...

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Heart failure (HF) is commonly associated with reduced cardiac output and an increased risk of atrial arrhythmias particularly during β-adrenergic stimulation. The aim of the present study was to determine how HF alters systolic Ca2 + and the response to β-adrenergic (β-AR) stimulation in atrial myocytes. HF was induced in sheep by ventricular tachypacing and changes in intracellular Ca2 + concentration studied in single left atrial myocytes under voltage and current clamp conditions. The following were all reduced in HF atrial myocytes; Ca2 + transient amplitude (by 46% in current clamped and 28% in voltage clamped cells), SR dependent rate of Ca2 + removal (kSR, by 32%), L-type Ca2 + current density (by 36%) and action potential duration (APD90 by 22%). However, in HF SR Ca2 + content was increased (by 19%) when measured under voltage-clamp stimulation. Inhibiting the L-type Ca2 + current (ICa-L) in control cells reproduced both the decrease in Ca2 + transient amplitude and increase of SR Ca2 + content observed in voltage-clamped HF cells. During β-AR stimulation Ca2 + transient amplitude was the same in control and HF cells. However, ICa-L remained less in HF than control cells whilst SR Ca2 + content was highest in HF cells during β-AR stimulation. The decrease in ICa-L that occurs in HF atrial myocytes appears to underpin the decreased Ca2 + transient amplitude and increased SR Ca2 + content observed in voltage-clamped cells.

Changes of the activity of the sarco-endoplasmic reticulum Ca(2+)-ATPase (SERCA) affect the amplitude of the systolic Ca(2+) transient and thence cardiac contractility. This is thought to be due to alterations of SR Ca(2+) content. Recent work on mice in which the expression of SERCA is decreased found that a large reduction of SERCA expression resulted in a proportionately much smaller decrease of SR Ca(2+) content. The aim of the current work was to investigate the quantitative nature of the dependence of both the amplitude of the systolic Ca(2+) transient and SR Ca(2+) content on SERCA activity during acute partial inhibition of SERCA. Experiments were performed on rat ventricular myocytes. Brief application of thapsigargin (1 μm) resulted in a decrease of SERCA activity as measured from the rate of decay of the systolic Ca(2+) transient. This was accompanied by a decrease in the amplitude of the systolic Ca(2+) transient which was linearly related to that of SERCA activity. However, the fractional decrease in the SR Ca(2+) content was much less than that of SERCA activity. On average SR Ca(2+) content was proportional to SERCA activity raised to the 0.38 ± 0.07 power. This shallow dependence of SR content on SERCA activity arises because Ca(2+) release is a steep function of SR Ca(2+) content. In contrast SR Ca(2+) content was increased 4.59 ± 0.40 (n = 8)-fold by decreasing ryanodine receptor opening with tetracaine (1 mm). Therefore a modest decrease of SR Ca(2+) content results in a proportionately larger fall of Ca(2+) release from the SR which can balance a larger initiating decrease of SERCA. In conclusion, the shallow dependence of SR Ca(2+) content on SERCA activity is expected for a system in which small changes of SR Ca(2+) content produce larger effects on the amplitude of the systolic Ca(2+) transient.

Background: Studies describing the clinical progression of animals with reverse patent ductus arteriosus (PDA) are lacking. Objectives: To describe the signalment, presenting signs, echocardiographic features, and survival in a group of dogs and cats with bidirectional and continuous right-to-left PDA. Animals: Forty-six client-owned animals included, comprising 43 dogs and 3 cats with bidirectional or continuous right-to-left PDA. Methods: Retrospective multicenter study. Medical records and echocardiographic findings reviewed from animals diagnosed with bidirectional or continuous right-toleft PDA. Impact of ductal morphology, spectral Doppler flow profile, PCV, sildenafil treatment at presentation, sildenafil dose, severity of pulmonary hypertension, general anesthesia with or without surgery and the presence of right-sided congestive heart failure (R-CHF) on crude mortality rate were evaluated via Mantel-Cox log rank comparison of Kaplan-Meier survival curves. Univariable and multivariable Cox proportional hazards analysis was performed, and hazard ratio (HR) (95% confidence intervals [CI]) was presented. Results: Hindlimb collapse was the most common presenting sign in dogs (n = 16). Clinical signs in cats were variable. Median survival time was 626 days in dogs (range 1-3628 days). Dogs with R-CHF had a shorter median survival time (58 days vs 1839 days, P = .03). Dogs treated with sildenafil at initial presentation survived longer (1839 days vs 302 days, P = .03), which was the only independent predictor of survival (HR 0.35, CI 0.15-0.86, P = 0.021).