Alpha1-Adrenergic Receptor Modulation of Repolarization in Canine Purkinje Fibers

Alpha-Agonists and Repolarization. Introduction: Alpha-adrenergic receptor stimulation increases contractility and prolongs repolarization. These effects are modulated by α₁-adrenergic receptor-mediated inhibition of transsarcolemmal potassium currents.
Methods and Results: We used standard microelectrode techniques to study the actions of 4-aminopyridine (4-AP), which blocks the transient outward current, I_to, and WAY-123,398, which blocks the delayed rectifier, I_k, on canine Purkinje fiber action potential prolongation induced by phenylephrine. At a basic cycle length of 1 second, phenylephrine (0.1 to 10 μ) dose-dependently prolonged action potential duration at 90% repolarization (APD₉₀) from 331 ± 10 msec to 400 ± 12 msec (P < 0.05) at phenylephrine, 10 μ. Phenylephrine did not change phase 1 or plateau height. 4-AP (0.1 mM) decreased phase 1 magnitude, shifted plateau height to more positive potentials (from 0.1 ± 1.8 mV to 14.3 ± 1.1 mV [P < 0.05]), and shortened APD₉₀ from 318 ± 9 msec to 294 ± 8 msec (P < 0.05). 4-AP did not block phenylephrine effects on APD₉₀, which increased, at 10 μ phenylephrine, from 294 ± 8 msec to 342 ± 6 msec (P < 0.05). In contrast, WAY-123,398 (0.1 μ) prolonged APD₉₀ from 360 ± 6 msec to 452 ± 6 msec (P < 0.05), and had no effect on plateau height. In the presence of WAY-123,398, phenylephrine no longer increased APD_9o.
Conclusion: (1) Agents that block I_to shorten APD in Purkinje fibers; and (2) the α-agonist mediated increase of APD in canine Purkinje fibers can be explained by inhibition of I_k.     

Comparison of the Effects of Regional Ischemia and Hyperkalemia on the Membrane Action Potentials of the In Situ Pig Heart

APD During Ischemia. Introduction: This study was designed to determine the role of increased extracellular potassium [K⁺]_e on action potential duration (APD) in the in situ porcine heart during acute regional no-flow ischemia.
Methods and Results: In open chest, anesthetized swine, an arterial shunt from the carotid artery to the mid-left anterior descending coronary artery was created through which a solution of KCl was infused to raise [K⁺]_e, Myocardial [K⁺]_e, was determined by potassium-sensitive electrodes, and transmembrane action potential was recorded by floating glass microelectrode. During the first 2 minutes of ischemia, APD at 90% repolarization (APD₉₀) lengthened by 31.2 ± 1.1 msec (P < 0.05). The comparable increase in [K⁺]_e alone shortened APD₉₀, During the next 6 minutes of ischemia. [K⁺]_e, rose to 11.3 ± 0.3 mM and APD₉₀, showed a decrease. However, the comparable increase in |K⁺]_e, by infusion of KCl caused further shortening of APD₉₀, at similar levels of [K⁺]_e.
Conclusions: Acutely ischemic myocardium showed a brief increase in APD₉₀, during the first 2 minutes of ischemia, followed by a fall in APD₉₀, after 2 minutes of ischemia, but the shortening is less than anticipated by the rise in [K⁺]_e. Thus, we hypothesize that other component(s) of ischemia may inhibit action potential repolarization.     

Comparison of the Rate-Dependent Properties of the Class III Antiarrhythmic Agents Azimilide (NE-10064) and E-4031

Rate Dependence of Azimilide and E-4031. Introduction : Reverse rate-dependence, a lessening in Class III antiarrhythmic agent action potential duration (APD) prolongation as heart rate is increased, has been proposed to be related to an incomplete deactivution of the slow component (I_Ks) of the delayed rectifier K⁺ current (I_K). The rate-dependent properties of block of I_K by azimilide were compared to E-4031, which selectively blocks the rapid component (I_Kr) of I_k, in guinea pig ventricular muscle.
Methods and Results : Azimilide prolonged APD in isolated papillary muscles in a concentration-dependent manner and to a greater degree than E-4031. Both agents prolonged APD less at fast than slow rates, consistent with a similar reverse rate-dependent effect. Isolation of azimilide block of I_Ks by subtraction of APD during E-4031 plus azimilide from E-4031 alone revealed rate-independent prolongation of APD. In voltage clamp experiments on single ventricular myocytes, activation of I_ks was similar following 30 seconds of conditioning pulses of physiological duration (125 to 200 msec) with either a fast (cycle length 250 msec) or slow (cycle length 2000 msec) rate. The block of I_Ks by azimilide 3 μM was greater after a fast conditioning pulse train.
Conclusions : Selective block of I_ks prolongs APD in a rate-independent manner. In voltage clamped myocytes, no evidence of a rate-dependent accumulation of I_Ks was observed. These findings support a mechanism of reverse rate-dependent APD prolongation by Class III antiarrhythmic agents that block I_Kr independent of I_Ks.     

Differential Effects of Cytochalasin D and 2, 3 Butanedione Monoxime on Isometric Twitch Force and Transmembrane Action Potential in Isolated Ventricular Muscle: Implications for Optical Measurements of Cardiac Repolarization

Cytochalasin D Induced Cardiac Contraction Failure. Introduction : 2,3-Butanedione monoxime (BDM) has been widely used to inhibit contraction during optical recordings of cardiac membrane voltage changes, even though it markedly abbreviates cardiac action potentials.
Methods and Results : We compared the effects of BDM and of the F-actin disrupter cytochalasin D (cyto D) on isometric twitch force and transmembrane action potentials in isolated canine right ventricular trabeculae superfused with Tyrode's solution (2 mmol/L CaCl₂, 37°C) and stimulated at 0.5 Hz. BDM at 10 mmol/L and cyto D at 80 μmol/L were equally effective in reducing peak isometric force to 10%± 3% (n = 6; mean ± SEM) and 8%± 1% (n = 8), respectively. Neither agent significantly altered resting tension. While 10 mmol/L BDM markedly shortened the action potential duration at 90% repolarization (APD₉₀) from 198 ± 7 msec to 146 ± 9 msec ( P < 0.001), 80 μmol/L cyto D had no significant effects on APD₉₀ or on any other action potential parameter. The effects of BDM on peak isometric force and APD were completely reversible after 15 minutes of washout, whereas in the cyto D group contractile force continued to be reduced (13%± 3%) and action potential characteristics did not show significant changes from control values after a 60-minute period of superfusion with cyto D-free Tyrode's solution.
Conclusion : We conclude that cyto D should be considered an alternative excitation-contraction uncoupler for optical mapping studies of cardiac repolarization.     

Defibrillation Energy Requirements Differ Between Anesthetic Agents

We examined the effect of anesthesia on the energy requirements for internal defibrillation (DF) in dogs anesthetized with pentobarbital (30 mg/kg IV followed by 2–3 mg/kg/hr constant infusion) (n = 20), fentanyl (25 μg/kg/hour) (n = 25), and enflurane (0.5%–1.5%) (n = 8). Multiple shocks of varying energies were applied through left and right ventricular epicardial patch electrodes to relate delivered energy to percent success in DF. The energies required for 50% success (E₅₀) and 80% success (E₈₀) in DF were estimated using logistic regression. E₅₀ in fentanyl anesthetized animals (3.8 ± 2.3 J) was significantly lower than in those given pentobarbital (6.9 ± 3.0 J) (P < 0.01), and lawer than those given enflurane (5.7 ± 2.8 J) (NS). E₈₀ with fentanyl (6.5 ± 4.0 J) was also lower than that of pentobarbital (10.4 ± 4.9 J) (P < 0.01) and enflurane (7.6 ± 4.3 J) (NS) animals. lawer defibrillation energy requirements (DER) with fentanyl anesthesia, when compared with pentobarbital, were associated with significantly longer ventricular effective refractory periods (VERP) (171 ± 20 versus 142 ± 15 msec: P < 0.01), lower mean arterial pressures (114 ± 20 vs 136 ± 25 mmHg: P < 0.01), and Iower heart rates (90 ± 37 versus 164 ± 19 b/m; P < 0.01).
Anesthetic agents may modify DER; their effects need to be taken into account in the assessment of DER in patients receiving implanted defibrillators and in the evaluation of the results of defibrillation research in anesthetized animals.     

Electrophysiologic Features of Torsades de Pointes:

Torsades de Pointes in the Isolated Rabbit Heart. Introduction : The exact electrophysiologic mechanism of torsades de pointes (TdP) is under intense investigation. No isolated animal heart model of this particular arrhythmia exists.
Methods and Results : In isolated rabbit hearts, TdP was induced by means of bradyeardia in the presence of a high concentration of d-sotalol (10⁻⁴ M) and shortly after lowering the concentration of potassium and magnesium in the perfusate. Multiple simultaneous epicardial and endocardial monophasic action potentials (MAPs) and volume-conducted 12-lead ECGs were recorded. d-Sotalol prolonged repolarization and increased dispersion of ventricular repolarization compared to baseline recordings. With the onset of low potassium and magnesium concentrations, repolarization was further prolonged and dispersion of repolarization was further increased followed by the occurrence of early afterdepolariZations (EADs) in the majority of MAP recordings, i.e., at both endocardial and epicardial locations of both ventricles, upon increase of EAD amplitude, triggered arrhythmias with TdP of up to 42 heats ensued in 10 of 11 hearts studied. MAP duration at 90% repolarization (APD₉₀), dispersion of APD₉₀, and the incidence of EADs as well as dispersion of the QT interval and T wave area were significantly higher in heats triggering higemini, couplets, or runs of TdP.
Conclusion : TdP observed in this new isolated heart model was associated with markedly increased dispersion of ventricular repolarization and the occurrence of EADs in multiple locations of the heart. TdP is initiated when the amplitude of an EAD reaches threshold for initiation of the first beat of an episode.     

Shock-Induced Dispersion of Ventricular Repolarization:

Shock-induced Dispersion and VF Induction. Introduction: Shock-induced dispersion of ventricular repolarization (SIDR) caused by an electrical field stimulus has been suggested as a mechanism of ventricular fibrillation (VF) induction: however, this hypothesis has not been studied systematically in the intact heart. Likewise, the mechanism underlying the upper (ULV) and lower (LLV) limit of vulnerability remains unclear.
Methods and Results: In eight Langendorff-perfused rabbit hearts, monophasic action potentials were recorded simultaneously from ten different sites of both ventricles. Truncated biphasic T wave shocks were randomly delivered at various coupling intervals and strengths, exceeding the vulnerable window, ULV, and LLV. SIDR, defined as the difference between the longest and shortest postshock repolarization times, was 64 ± 15 msec for sbocks inducing VF. SIDR was 41 ± 17 msec for shocks delivered above the ULV, and 33 ± 14 and 27 ± 8 msec for shocks delivered 10 msec before and after the vulnerable window, respectively (all P < 0.01 vs VF-inducing shocks). Although SIDR was larger for shocks delivered below the LLV(93 ± 24 msec, P < 0.01 vs VF-inducing shocks), the repolarization extension was significantly smaller for shocks below the LLV (10.3%± 3.9% vs 16.3%± 4.9%, P < 0.01).
Conclusion: SIDR is influenced by the shock timing and intensity. Large SIDR within the vulnerable window and an SIDR decrease toward its borders suggest that SIDR is essential for VF induction. The decrease in SIDR toward greater shock strengths may explain the ULV. Small repolarization extension for shocks below the LLV may explain why these shocks, despite producing large SIDR, fail to induce VF.     

Ventricular fibrillation induced by stretch pulse: implications for sudden death due to commotio cordis

Introduction: Nonpenetrating chest wall impact (commotio cordis) may lead to sudden cardiac death due to the acute initiation of ventricular fibrillation (VF). VF may result from sudden stretch during a vulnerable window, which is determined by repolarization inhomogeneity.
Methods: We examined action potential morphologies and VF inducibility in response to sudden myocardial stretch in the left ventricle (LV). In six Langendorff perfused rabbit hearts, the LV was instrumented with a fluid-filled balloon. Increasing volume and pressure pulses were applied at different times of the cardiac cycle. Monophasic action potentials (MAPs) were recorded simultaneously from five LV epicardial sites. Inter-site dispersion of repolarization was calculated in the time and voltage domains.
Results: Sudden balloon inflation induced VF when pressure pulses of 208–289 mmHg were applied within a window of 35–88 msec after MAP upstroke, a period of intrinsic increase in repolarization dispersion. During the pressure pulse, MAPs revealed an additional increase in repolarization dispersion (time domain) by 9 ± 6 msec (P < 0.01). The maximal difference in repolarization levels (voltage domain) between sites increased from 19 ± 3% to 26 ± 3% (P < 0.05). Earliest stretch-induced activation was observed near a site with early repolarization, while sites with late repolarization showed delayed activation.
Conclusions: Sudden myocardial stretch can elicit VF when it occurs during a vulnerable window that is based on repolarization inhomogeneity. Stretch pulses applied during this vulnerable window can lead to nonuniform activation. Repolarization dispersion might play a crucial role in the occurrence of fatal tachyarrhythmias during commotio cordis.     

Potent Antiarrhythmic Effects of Chronic Amiodarone in Canine Pulmonary Vein Sleeve Preparations

Objectives: To examine the effects of chronic amiodarone on the electrophysiology of canine pulmonary vein (PV) sleeve preparations and left ventricular wedge preparation.
Background: Amiodarone is commonly used for the treatment of ventricular and supraventricular arrhythmias. Ectopic activity arising from the PV plays a prominent role in the development of atrial fibrillation (AF).
Methods: Standard microelectrode techniques were used to evaluate the electrophysiological characteristics of superfused PV sleeve (left superior or inferior) and arterially perfused left ventricular (LV) wedge preparations isolated from untreated and chronic amiodarone-treated dogs (amiodarone, 40 mg/kg daily for 6 weeks).
Results: In PV sleeves, chronic amiodarone (n = 6) induced a significant increase in action potential duration at 90% repolarization (APD₉₀) and a significant use-dependent reduction in V_max leading to 1:1 activation failure at long cycle lengths (basic cycle length of 124 ± 15 ms in control vs 420 ± 320 ms after chronic amiodarone [P < 0.01]). Diastolic threshold of excitation increased from 0.3 ± 0.2 to 1.8 ± 0.7 mA (P < 0.01). Delayed and late phase 3 early afterdepolarizations and triggered activity could be induced in PV sleeve preparations using acetylcholine (ACh, 1 μM), high calcium ([Ca²⁺]_o= 5.4 mM), isoproterenol (Iso, 1 μM), or their combination in 6 of 6 untreated PV sleeves, but in only 1 of 5 chronic amiodarone-treated PV sleeve preparations. V_max, conduction velocity, and 1:1 activation failure were much more affected in PV sleeves versus LV wedge preparations isolated from amiodarone-treated animals.
Conclusions: The results point to potent effects of chronic amiodarone to preferentially suppress arrhythmogenic substrates and triggers arising from the PV sleeves of the dog.     

Early recurrence of ventricular fibrillation after successful defibrillation during prolonged global ischemia in isolated rabbit hearts

Introduction: The mechanisms that lower the efficacy of electrical defibrillation during prolonged global ischemia remain unclear.
Methods and Results : Epicardial activation patterns during attempted electrical defibrillation were studied in 18 Langendorff-perfused rabbit hearts at baseline, after 5-minute no-flow global ischemia and after 10-minute reperfusion. DFT₅₀ (voltage required to achieve 50% probability of successful defibrillation) was determined at each stage. Defibrillation was considered successful if postshock sinus/idioventricular rhythm was present. Prolonged global ischemia converted type 1 VF (multiple wandering wavelets) into type 2 VF (repetitive epicardial breakthroughs, REBs). The mean DFT₅₀ after 5-minute ischemia (96 ± 39 V) was significantly lower when compared with that at baseline (154 ± 47 V, P < 0.0001) and after 10-minute reperfusion (145 ± 47 V, P < 0.001). However, the incidence of early (within 10 seconds) VF recurrence after successful shock during prolonged global ischemia (23 of 78, 29.5%) was much higher than that at baseline (2 of 60, 3.3%) and after 10-minute reperfusion (5 of 63, 7.9%; P < 0.0001). Mapping data showed that the VF wavefronts during prolonged global ischemia were initially halted by the shock, followed by one to five ventricular escape beats. These beats then triggered REBs and early VF recurrence. In eight out of 11 episodes, the REBs before and after successful shock arose from the same location near the interventricular septum.
Conclusions : There is a significant reduction of DFT₅₀ during prolonged global ischemia. However, defibrillation appears to fail when the preexisting REBs near the interventricular septum induce early VF recurrence. Shock per se cannot eliminate the substrates of these REBs.     

Myocardial Vulnerability to T Wave Shocks: Relation to Shock Strength, Shock Coupling Interval, and Dispersion of Ventricular Repolarization

Myocardial Vulnerability to T Wave Shocks. Introduction: Induction of ventricular fibrillation (VF) by T wave shocks is of clinical interest due to the correlation between the upper limit of vulnerability (ULV) and the defibrillation threshold (DFT). However, the ULV bas not yet been defined precisely in reference to the entire “area of vulnerability” (AOV), which is defined bifunctionally by both shock strengths and shock coupling intervals, nor has it been related to the dispersion of ventricular repolarization, considered to be an important determinant of vulnerability. Methods and Results: In 11 isolated perfused rabbit hearts immersed in a tissue bath containing a 3-lead ECG recording system and two opposite plate electrodes for field shock administration, 7 monophasic action potentials (MAPs) were recorded simultaneously from different epicardial and endocardial regions of the right and left ventricles. An average of 90 ± 25 monophasic waveform shocks of varying shock strengths and coupling intervals were delivered to each heart to determine the horizontal and vertical boundaries of the AOV. The AOV approximated a rhomboid with homogenous VF inducibility. The ULV and lower limit of vulnerability (LLV) represented discrete corners of the AOV with significant changes in VF inducibility if either shock coupling intervals or shock strength were changed by only 10 msec or 10 V. respectively (P < 0.001). The ULV occurred at 7 ± 10 msec shorter coupling intervals than the LLV (P < 0.05), and VF-inducing shock strengths at the left corner of the AOV were 50 ± 67 V higher as compared to the right corner (P < 0.01). The maximal range of VF-inducing coupling intervals coincided (within < 2 msec) with the dispersion of MAPs at 70% repolarization, and the ULV coupling interval coincided (within < 4 msec) with the longest repolarization at 50%. Conclusions: (1) VF vulnerability to monophasic T wave shocks is defined by an AOV that bas the shape of a leftward tilted rhomboid. (2) Both the ULV and LLV are sharply defined upper and lower corners of the AOV rhomboid. (3) The width of the AOV corresponds to the dispersion of ventricular repolarization at the 70% level. (4) Considering the dispersion of ventricular repolarization may yield more precise ULV determinations and a better understanding of the correlation between the ULV and DFT.     

The transport of two iron chelators, desferrioxamine B and L1, across Caco-2 monolayers

Summary. The transport of two iron chelators, desferrioxamine B (DFO) and L1 (1,2-dimethyl-3 hydroxypyridin-4-one) has been studied in vitro using the human adenocarcinoma cell line, Caco-2. The transport of DFO and L1 has also been compared with that of their iron-bound complexes, ferrioxamine (FO) and L1₃-Fe, respectively. We report an apparent permeability coefficient (P_app) value for DFO of 0.170 × 10⁻⁷± 0.080 cm s⁻¹. The P_app value of L1 was 1.297 × 10⁻⁵± 0.133 cm s⁻¹. The P_app values of their iron bound complexes FO and L1₃-Fe are 0.230 × 10⁻⁷± 0.065 cm s⁻¹ and 2.356 × 10⁻⁶ 0.365 cm s⁻¹, respectively. We have shown that the transport of DFO and FO is similar in the Caco-2 cell system. The transport of L1, however, is greatly reduced when complexed to iron. The value for total uptake after 60 min for DFO into the Caco-2 cells was 1.49 ± 0.09 × 10⁻³ nmol per filter. The values for total uptake after 60 min for L1 and L1₃-Fe were 0.37 0.03 nmol per filter and 0.04 ± 0.01 nmol per filter, respectively. Our results indicate that the poor oral bioavailability of DFO can be attributed to the low epithelial permeability of the molecule coupled with its size (mol wt 656). In contrast, the oral bioavailability observed with L1 is due to the high lipophilicity and low molecular weight (mol wt 139) of the molecule. We believe that these differences between the two molecules account for L1 being better orally absorbed than DFO.     

Kinetics of Cycle Length Dependence of Ventricular Repolarization:

Cycle Length Dependence of Repolarization. Introduction : Beat-to-beat adaptation of ventricular repolarization duration to cardiac cycle length and autonomic activity has not been previously characterized in the spontaneously beating human heart.
Methods and Results : The ECG of 14 healthy subjects was recorded from the supine and upright positions. Autonomic blockade was accomplished by atropine and propranolol. RR and RT intervals were measured by a computer algorithm, and the impulse response (h) from RR to RT computed. In the supine position the maximal adjustment of the RT interval occurred in the first beat following a change in cycle length (h_peak= 17.8 ± 1.6 msec/sec), but continued to be detectable for 3.8 seconds (2.9–4.7 sec). Propranolol attenuated the peak impulse response to 15.8 ± 4.0 msec/sec (P = NS). In the standing position the peak impulse response was increased to 25.2 ± 5.0 msec/sec (P = 0.004 vs supine), and the impulse response duration (h_dur) shortened to 1.4 seconds (1.3–1.6). This was reversed by beta blockade (h_peak= 10.7 ± 3.6 [P = 0.005 vs standing]; h_dur= 5.5 sec [4.8–6.1]). Parasympathetic and combined autonomic blockade resulted in too little residual heart rate variability to estimate the impulse response accurately. The slope of the regression of δRT and δRR in the supine position was 0.0177 ± 0.0016, which was closely correlated with the peak impulse response (r = 0.91).
Conclusions : System identification techniques can assist in characterizing the cycle dependence of veritricular repolarization and may provide new insights into conditions associated with abnormal repolarization.     

Na+/Ca2+ Exchanger Expression and Function in a Rabbit Model of Myocardial Infarction

Introduction: In general, sarcolemmal Na⁺/Ca²⁺ exchanger (NCX) protein and activity is increased in hearts with ventricular dysfunction. However, in a subset of studies, reduced activity of NCX has been reported. Left ventricular dysfunction (LVD) was induced in the rabbit eight weeks after an apical myocardial infarction.
Methods: Using single microelectrode voltage clamp to assess the NCX activity in isolated ventricular cells, a decrease in NCX activity by ∼30% was observed. Immunoblot analysis indicated increased NCX protein levels by ∼20% in the LVD group. The cause of this paradox is unknown. Overexpression of the protein sorcin increased the activity of NCX without affecting NCX protein levels.
Results: Sorcin protein (dimer) levels were significantly lower in the LVD group (0.67 ± 0.05 n = 15, P < 0.05) compared to sham (1.0 ± 0.16, n = 15). Sorcin monomer levels were not significantly different (sham: 1.0 ± 0.26, LVD: 0.83 ± 0.13). Mathematical modeling of NCX suggests that a reduction of NCX activity during diastole to that in LVD could be achieved by holding the diastolic membrane potential at −60 mV instead of −80 mV. Holding E_m at −60 mV decreased NCX-mediated Ca²⁺ efflux rates to values comparable to those seen in LVD and increased SR Ca²⁺ content and peak systolic [Ca²⁺] in sham and LVD cardiomyocytes.
Conclusions: In conclusion, reduced sorcin expression may be linked to the lower NCX activity in the rabbit model of LVD. Reduced NCX activity during diastole increases SR Ca²⁺ content and Ca²⁺ transient amplitude.     

Autonomic tone attenuates drug-induced QT prolongation

Introduction: The QT interval is a predictor of sudden death. Many drugs prolong the QT, primarily through I_Kr block. Autonomic tone directly affects heart rate and ventricular repolarization, but its effects in the setting of I_Kr block are unknown.
Objective: Determine the effects of autonomic tone on heart rate and QT interval after I_Kr block.
Methods and Results: Healthy adults (n = 9) were administered ibutilide with ECGs obtained at regular intervals. On a separate day, subjects were administered intravenous propranolol and atropine to induce autonomic block, resulting in intrinsic heart rate and QT interval; ibutilide was administered again, with serial ECGs obtained at regular intervals. No differences in baseline RR, QT, or QTc intervals were seen between the two study days. Ibutilide in the setting of intact autonomic tone prolonged QTc by 43 ± 8 msec (P = 0.001) and prolonged RR interval by 93 ± 39 msec (P = 0.04). Autonomic block alone on the second day prolonged QTc by 16.7 ± 9.4 msec (P = 0.02). An additional 68 ± 5 msec prolongation of QTc was seen with ibutilide in the setting of double autonomic block (P = 0.036). There was no effect of ibutilide on intrinsic heart rate (P = 0.125).
Conclusion: Autonomic block results in an exaggeration of drug-induced QT prolongation by ibutilide. I_Kr block has no effect on intrinsic heart rate. QT-prolonging drugs may have more deleterious effects in those with underlying autonomic dysfunction, thus increasing the risk for sudden death.     

L-type calcium current reactivation contributes to arrhythmogenesis associated with action potential triangulation

Introduction: The morphology of the mammalian cardiac action potential (AP) is an important factor in the susceptibility to drug-induced early afterdepolarizations (EADs) that may initiate Torsade de Pointes (TdP). AP triangulation has been shown to be an important predictor of drug-induced TdP.
Methods and Results: APs from guinea pig and rabbit left ventricular single myocytes were recorded using a microelectrode-recording technique. I_Ca-L currents were recorded in ventricular myocytes of guinea pig and rabbit using patch-clamping technique. At a stimulus frequency of 0.5 Hz, guinea pig ventricular myocytes displayed a square-like AP, whereas rabbit ventricular myocytes exhibited a triangle-like AP. Dofetilide-induced EADs were observed only in rabbit ventricular myocytes. Under the guinea pig AP clamping condition, the normalized I_Ca-L instant reactivation currents in guinea pig and rabbit myocytes at voltages of –40 mV were 0.13 ± 0.01 and 0.14 ± 0.01, respectively. However, when rabbit AP served as the first clamping voltage, the normalized I_Ca-L reactivation currents at –40 mV in guinea pig and rabbit myocytes were 0.20 ± 0.01, 0.21 ± 0.01, respectively, indicating that the I_Ca-L recovery from inactivation in the rabbit triangular AP condition was significantly faster than in the guinea pig square AP condition. Comparison of the voltage clamp using the triangular waveform with the square waveform further confirmed that triangulation accelerates I_Ca-L recovery from inactivation.
Conclusions: In rabbit ventricular myocardium, AP triangulation accelerates I_Ca-L channel recovery from inactivation, leading to instability of the cell membrane potential during repolarization, which is capable of initiating TdP.     

Sympathomimetic infusion and cardiac repolarization: the normative effects of epinephrine and isoproterenol in healthy subjects

Introduction: Catecholamines are known to affect cardiac repolarization, and provocation with either isoproterenol or epinephrine has been proposed as a tool for uncovering latent repolarization abnormalities. This study systematically compares the effects of isoproterenol and epinephrine infusions on QT interval (QT), T waves and U waves in normal subjects.
Methods and Results: Twenty-four normal subjects (29 ± 8 years) were evaluated during graded infusions of up to 0.30 μg/kg/minute epinephrine and 5.0 μg/minute isoproterenol. Heart rates at peak doses were 81 ± 13 bpm at 0.28 ± 0.04 μg/kg/minute epinephrine and 104 ± 5 bpm at 2.4 μg/minute isoproterenol. The longest absolute QT increase was 4 ± 5 msec above baseline during isoproterenol (P < 0.001) and 12 ± 23 msec during epinephrine (P = 0.07), while the longest corrected QT interval (QTc) increase was 67 ± 28 msec (P < 0.0001) and 79 ± 40 msec (P < 0.0001) above baseline during isoproterenol and epinephrine, respectively (P = 0.12 for difference). There was a 2-fold increase in U-wave amplitude during each intervention (P < 0.001). The specificity of paradoxical QT prolongation (≥30 msec at 0.05 μg/kg/minute or ≥35 msec at 0.10 μg/kg/minute epinephrine) and an increase in QTc ≥600 msec at any dose epinephrine were 100%. However, the specificity of other proposed criteria that utilized QTc measurement (≥30 msec at 0.10 μg/kg/minute or ≥65 msec at any dose) was poor whether all leads or only lead II were assessed.
Conclusion: Both epinephrine and isoproterenol are associated with QTc prolongation and amplification of the U wave in normal subjects. The specificity of proposed criteria for epinephrine provocation in diagnosis of the long-QT syndrome is variable; however, paradoxical QT prolongation at low-dose epinephrine or a QTc ≥600 msec is highly specific.     

Combined negative and positive selection of mobilized CD34+ blood cells

We tested four negative and two positive selection methods for separation of CD34⁺ cells from mobilized blood cells, and analysed fold-enrichment, purity and recovery of CD34⁺ cells after selection procedures. The elimination of mature CD34⁻ cells was achieved by adhesion to nylon-wool fibre (5.9 ± 1.0 mean fold-enrichment and 65.2 ± 2.3 mean recovery of CD34⁺ cells). Standard or modified Ficoll-Hypaque and Percoll density gradients, as well as phagocytosis with magnetic beads, were less effective in eliminating CD34⁻ cells, both purity and fold-enrichment of CD34⁺ cells being lower than those obtained with separation by nylon-wool. Both positive selection methods tested, Ceprate and MiniMacs System, generated highly purified CD34⁺ cell populations ranging from 80% to 90%. The recovery of CD34⁺ cells was optimal with MiniMacs (77.9±3.6) and low with Ceprate (28.8±2.8). Based on these results, in two large-scale experiments we combined nylon-wool fibre and MiniMacs System in a two-step separation procedure obtaining a 36.9±2.6 mean fold-enrichment and a 50.5±0.3 mean recovery of CD34⁺ cells. In this way we achieved optimal enrichment and recovery of CD34⁺ cells, with a substantial saving of cost compared to either selection method alone.     

Azimilide (NE-10064) Can Prolong or Shorten the Action Potential Duration in Canine Ventricular Myocytes:

Azimilide Effects on Membrane Voltage and Current. Introduction : Azimilide (NE-10064) has antiarrhythmic and antifibrillatory effects in canine models of ventricular arrhythmia. The goal of the present study was to examine the effects of azimilide on action potential and membrane currents of canine ventricular myocytes.
Methods and Results : Membrane voltage and current were recorded using the whole cell, patch clamp method. Azimilide at 1 /μM induced a consistent prolongation of action potential duration (APD): on average APD₉₀ was prolonged by 25% and 17% at stimulation rates of 0.33 and 1 Hz, respectively. Elevating the drug concentration to 5 μM induced APD prolongation in some cells but APD shortening in the others at 0.33 Hz, and a consistent APD shortening at 1 Hz. Azimilide suppressed the following currents (K_d in parenthesis): I_Kr (< 1 μM at -20 mV), I_Ks, (1.8 μM at +30 mV), L-type Ca current (17.8 μM at +10 mV), and Na current (19 μM at -40 mV). Azimilide was a weak blocker of the transient outward and inward rectifier currents (K_d≥ 50 μM at +50 and -140 mV, respectively). Azimilide blocked I_Kr, I_Ks, and I_Na in a use-dependent manner. Furthermore, azimilide reduced a slowly inactivating component of Na current that might be important for maintaining the action potential plateau in canine ventricular myocytes.
Conclusion : Azimilide has variable effects on APD in canine ventricular myocytes due to Us blocking effects on multiple currents with different potencies. Its Class III antiarrhythmic action is most likely seen at low concentrations (< 5 μM).