Electrical current-induced atrial and pulmonary vein action potential duration shortening and repetitive activity

The influence of threshold electrical currents (EC) during regular drive on pulmonary vein (PV) and atrial myocardial cell action potential (AP) duration (APD) is unknown. We determined the effects of EC on cellular APD of PV, atria, and ventricles in isolated perfused and superfused male rat hearts (Fisher-344 strain, 3-4 mo old) at 37 degrees C (n = 14). We determined APD changes caused by subthreshold and threshold EC synchronized with a distant pacing electrode and delivered nearby cells from which transmembrane APs were recorded with a glass microelectrode. Progressive APD shortening (P < 0.001) and membrane hyperpolarization (P < 0.05) developed over a 20-s interval in the PV and atrial cells when the EC was delivered at <2 mm but not at >4 mm from the microelectrode. No such effects were seen in ventricular muscle cells. APD fully recovered 25 s after the cessation of EC application. Premature stimuli applied during EC-induced shortening of the APD caused rapid repetitive PV and atrial activity lasting two to five beats. Atropine (2 microM, n = 10) prevented, whereas propranolol (2 microM, n = 5) had no effect, on EC-induced APD shortening or repetitive activity. We conclude that EC shortens the APD and hyperpolarizes the membrane by local release of acetylcholine and causes the steep repolarization gradient in the vicinity of the current source leading to repetitive activity in atrial and PV cells during premature stimulation.     

Frequency-dependent effects of various IKr blockers on cardiac action potential duration in a human atrial model

Rapidly activating K(+) current (I(Kr)) blockers prolong action potential (AP) duration (APD) in a reverse-frequency-dependent manner and may induce arrhythmias, including torsade de pointes in the ventricle. The I(Kr) blocker dofetilide has been approved for treatment of atrial arrhythmias, including fibrillation. There are, however, a limited number of studies on the action of I(Kr) blockers on atrial AP. When we tested a mathematical model of the human atrial AP (M Courtemanche, RJ Ramirez, S Nattel. Am J Physiol Heart Circ Physiol 275: H301-H321, 1998) to examine the effects of dofetilide-type I(Kr) blockade, this model could not reproduce the reverse-frequency-dependent nature of I(Kr) blockade on atrial APD. We modified the model by introducing a slowly activating K(+) current activation parameter. As the slow time constant was increased, dofetilide-type blockade induced more prominent reverse-frequency-dependent APD prolongation. Using the modified model, we also examined the effects of two more types of I(Kr) blockade similar to those of quinidine and vesnarinone. Voltage- and time-dependent block of I(Kr) through the onset of inhibition by quinidine is much faster than by vesnarinone. When we incorporated the kinetics of the effects of these drugs on I(Kr) into the model, we found that quinidine-type blockade caused a reverse-frequency-dependent prolongation of APD that was similar to the effect of dofetilide-type blockade, whereas vesnarinone-type blockade did not. This finding coincides with experimental observations. The lack of the reverse frequency dependence in vesnarinone-type blockade was accounted for by the slow development of I(Kr) blockade at depolarized potentials. These results suggest that the voltage- and time-dependent nature of I(Kr) blockade by drugs may be critical for the phenotype of the drug effect on atrial AP.     

Rate-dependent shortening of action potential duration increases ventricular vulnerability in failing rabbit heart

Congestive heart failure (CHF) predisposes to ventricular fibrillation (VF) in association with electrical remodeling of the ventricle. However, much remains unknown about the rate-dependent electrophysiological properties in a failing heart. Action potential properties in the left ventricular subepicardial muscles during dynamic pacing were examined with optical mapping in pacing-induced CHF (n=18) and control (n=17) rabbit hearts perfused in vitro. Action potential durations (APDs) in CHF were significantly longer than those observed for controls at basic cycle lengths (BCLs)>1,000 ms but significantly shorter at BCLs<400 ms. Spatial APD dispersions were significantly increased in CHF versus control (by 17-81%), and conduction velocity was significantly decreased in CHF (by 6-20%). In both groups, high-frequency stimulation (BCLs<150 ms) always caused spatial APD alternans; spatially concordant alternans and spatially discordant alternans (SDA) were induced at 60% and 40% in control, respectively, whereas 18% and 82% in CHF. SDA in CHF caused wavebreaks followed by reentrant excitations, giving rise to VF. Incidence of ventricular tachycardia/VFs elicited by high-frequency dynamic pacing (BCLs<150 ms) was significantly higher in CHF versus control (93% vs. 20%). In CHF, left ventricular subepicardial muscles show significant APD shortenings at short BCLs favoring reentry formations following wavebreaks in association with SDA. High-frequency excitation itself may increase the vulnerability to VF in CHF.     

Reduction in number of sarcolemmal KATP channels slows cardiac action potential duration shortening under hypoxia

The cardiovascular system operates under demands ranging from conditions of rest to extreme stress. One mechanism of cardiac stress tolerance is action potential duration shortening driven by ATP-sensitive potassium (K_ATP) channels. K_ATP channel expression has a significant physiologic impact on action potential duration shortening and myocardial energy consumption in response to physiologic heart rate acceleration. However, the effect of reduced channel expression on action potential duration shortening in response to severe metabolic stress is yet to be established. Here, transgenic mice with myocardium-specific expression of a dominant negative K_ATP channel subunit were compared with littermate controls. Evaluation of K_ATP channel whole cell current and channel number/patch was assessed by patch clamp in isolated ventricular cardiomyocytes. Monophasic action potentials were monitored in retrogradely perfused, isolated hearts during the transition to hypoxic perfusate. An 80–85% reduction in cardiac K_ATP channel current density results in a similar magnitude, but significantly slower rate, of shortening of the ventricular action potential duration in response to severe hypoxia, despite no significant difference in coronary flow. Therefore, the number of functional cardiac sarcolemmal K_ATP channels is a critical determinant of the rate of adaptation of myocardial membrane excitability, with implications for optimization of cardiac energy consumption and consequent cardioprotection under conditions of severe metabolic stress.     

Ischemic shortening of action potential duration as a result of KATP channel opening attenuates myocardial stunning by reducing calcium influx

Action potential duration (APD) shortening due to opening of sarcolemmal ATP-dependent potassium (KATP) channels has been postulated to protect the myocardium against postischemic damage by reducing Ca²⁺ influx. This hypothesis was assessed, assuming that increased postischemic stunning due to KATP channel inhibition with glibenclamide could be reverted by the addition of the Ca²⁺ channel blocker diltiazem. Percent wall thickening fraction (% WTh, conscious sheep) and APD (open-chest sheep) were obtained from the following groups: control: 12 min ischemia by anterior descending coronary artery occlusion followed by 2 h reperfusion; glibenclamide: same as control, with glibenclamide (0.4 mg/kg) infused 30 min before ischemia; diltiazem: same as control, with diltiazem (100 g/kg) administered prior to ischemia; glibenclamide+diltiazem: both drugs infused as in glibenclamide and diltiazem groups. APD was reduced in control ischemia. Conversely, KATP-channel blockade by glibenclamide lengthened APD and increased postischemic stunning (p < 0.01 vs. control); glibenclamide+diltiazem did not shorten APD but enhanced functional recovery (p < 0.01 vs. glibenclamide). Ca²⁺ channel blockade improvement of increased stunning provoked by KATP channel inhibition supports the hypothesis that APD shortening due to opening of KATP channels protects against postischemic stunning by limiting Ca²⁺ influx.     

Control of action potential duration by calcium ions in cardiac Purkinje fibers

It is well known that cardiac action potentials are shortened by increasing the external calcium concentration (Cao). The shortening is puzzling since Ca ions are thought to carry inward current during the plateau. We therefore studied the effects of Cao on action potentials and membrane currents in short Purkinje fiber preparations. Two factors favor the earlier repolarization. First, calcium-rich solutions generally raise the plateau voltage; in turn, the higher plateau level accelerates time- and voltage-dependent current changes which trigger repolarization. Increases in plateau height imposed by depolarizing current consistently produced shortening of the action potential. The second factor in the action of Ca ions involves iK1, the background K current (inward rectifier). Raising Cao enhances iK1 and thus favors faster repolarization. The Ca-sensitive current change was identified as an increase in iK1 by virtue of its dependence on membrane potential and Ko. A possible third factor was considered and ruled out: unlike epinephrine, calcium-rich solutions do not enhance slow outward plateau current, ikappa. These results are surprising in showing that calcium ions and epinephrine act quite differently on repolarizing currents, even though they share similar effects on the height and duration of the action potential.     

Taurine prevents cardiomyocyte death by inhibiting NADPH oxidase-mediated calpain activation

Taurine has been shown to prevent cardiomyocyte apoptosis. This study investigated the effects of taurine on NADPH oxidase and calpain activation in mediating apoptosis in cardiomyocytes. Apoptosis was induced by norepinephrine (NE) in cultured adult rat ventricular cardiomyocytes. NE (5 microM) increased NADPH oxidase activation and reactive oxygen species (ROS) production and induced apoptosis. These effects of NE on cardiomyocytes were diminished by taurine (0.5 mg/kg) but not beta-alanine. Inhibition of gp91(phox)-NADPH oxidase or ROS production protected cardiomyocytes from apoptosis. NE also induced calpain-1 activation in cardiomyocytes. This effect of NE on calpain was abrogated by gp91(phox)-NADPH oxidase inhibition or ROS scavengers and was mimicked by H(2)O(2) (25 microM) in cardiomyocytes. Pharmacological inhibitors of calpain or overexpression of calpastatin, a specific calpain inhibitor, blocked calpain activation and prevented cardiomyocyte apoptosis during NE stimulation. Furthermore, taurine treatment inhibited NE- or H(2)O(2)-induced calpain activation in cardiomyocytes. In conclusion, NADPH oxidase induces calpain activation, leading to apoptosis in NE-induced cardiomyocytes. Taurine inhibits NADPH oxidase and calpain activation. Thus, inhibition of NADPH oxidase-mediated calpain activation may be an important mechanism for taurine's antiapoptotic action in cardiomyocytes.     

Voltage noise influences action potential duration in cardiac myocytes

Stochastic gating of ion channels introduces noise to membrane currents in cardiac muscle cells (myocytes). Since membrane currents drive membrane potential, noise thereby influences action potential duration (APD) in myocytes. To assess the influence of noise on APD, membrane potential is in this study formulated as a stochastic process known as a diffusion process, which describes both the current-voltage relationship and voltage noise. In this framework, the response of APD voltage noise and the dependence of response on the shape of the current-voltage relationship can be characterized analytically. We find that in response to an increase in noise level, action potential in a canine ventricular myocytes is typically prolonged and that distribution of APDs becomes more skewed towards long APDs, which may lead to an increased frequency of early after-depolarization formation. This is a novel mechanism by which voltage noise may influence APD. The results are in good agreement with those obtained from more biophysically-detailed mathematical models, and increased voltage noise (due to gating noise) may partially underlie an increased incidence of early after-depolarizations in heart failure.     

Cardiac action potential duration and calcium regulation in males and females

Adult women have longer QT intervals compared with men of a similar age, indicating differences in the speed of repolarisation of the ventricles. We investigate the influences of gender on ventricular electrophysiology and intracellular Ca²⁺ regulation of the guinea pig heart. Comparing sexually mature animals, females exhibited a significantly longer APD. Peak L-type Ca²⁺ current (I_CaL) was larger in females and when this current was inhibited with nifedipine the gender differences in APD were removed. APD differences also disappeared when the SR was depleted of Ca²⁺. Inactivation of I_CaL during a clamp step is faster in females but slower during an action potential and SR Ca²⁺ content is larger. We suggest that gender differences in APD result from variation in the kinetics of I_CaL stemming from alterations to Ca²⁺ release.     

Mechanisms underlying adaptation of action potential duration by pacing rate in rat myocytes

Heart rate is an essential determinant of cardiac performance. In rat ventricular myocytes, a sudden increase in rate yields to a prolongation of the action potential duration (APD). The mechanism underlying this prolongation is controversial: it has been proposed that the longer APD is due to either: (1) a decrease in K⁺ currents only or (2) an increase in Ca²⁺ current only. The aim of this study was to quantitatively investigate the contribution of Ca²⁺ and K⁺ currents in the adaptation of APD to pacing rate. Simulation using a mathematical model of ventricular rat cardiac cell model [Pandit, S.V., Clark, R.B., Giles, W.R., Demir, S.S., 2001. A mathematical model of action potential heterogeneity in adult rat left ventricular myocytes. Biophys. J. 81, 3029–3051] predicted a role in the prolongation of APD for K⁺ currents only. In patch clamp experiments, increasing the pacing rate leads to a significant increase in APD in both control and detubulated myocytes, although it was more marked in control than detubulated myocytes. Supporting the model prediction, we observed that increasing stimulation frequency leads to a decrease in K⁺ currents in voltage clamped rat ventricular myocytes (square and action potential waveforms), and to a similar extent in both cell types. We have also observed that frequency-dependent facilitation of Ca²⁺ current occurred in control cells but not in detubulated cells (square and action potential waveforms). From these experiments, we calculated that the relative contribution of Ca²⁺ and K⁺ currents to the longer APD following an increase in pacing rate is 65% and 35%, respectively. Therefore, in contrast to the model prediction, Ca²⁺ current has a significant role in the adaptation of APD to pacing rate. Finally, we have introduced a simplistic modification to the Pandit's model to account for the frequency-dependent facilitation of Ca²⁺ current.     

The shortening and action potential of the cortex in the main pulvinus ofMimosa pudica

The shortening and action potential of the upper and lower cortices of the main pulvinus ofMimosa pudica were recorded simultaneously. The shortening and action potential were observed only in the lower cortex. The extensibility and the excitability of the cortex are discussed.     

Diazepam reduces action potential duration in guinea-pig papillary muscle by a cAMP-dependent mechanism.

In this study, we have analyzed the role of cyclic AMP (cAMP) as the mediator of the decrease in action potential duration induced by diazepam. Diazepam (1-100 microM) reduced, in a dose-dependent manner, the duration of intracellular action potential recorded in the papillary muscle obtained from the right ventricle of the guinea pig heart. This effect was mimicked by the analog of cyclic AMP, 8-Br-cAMP (100 microM), but not by gamma-amino-butyric acid (GABA). Also, the selective antagonist of the benzodiazepine receptors, flumazenil did not modify the effect of diazepam. The diazepam-induced shortening of action potential duration was partially antagonized by the inhibitor of cAMP synthesis carbachol (1 microM) or the blocker of the cAMP-dependent protein kinase A, Rp-cAMP[S] (1 microM). These results indicate that cyclic AMP is involved in the diazepam-induced shortening of the action potential duration of the guinea pig papillary muscle.     

Tyrosine kinase signaling in action potential shortening and expression of HSP72 in late preconditioning

We investigated the role of tyrosine kinase (TK) signaling in the opening of the ATP-sensitive K(+) (K(ATP)) channel and 72-kDa heat shock protein (HSP72) expression during late preconditioning. Rabbits were subjected to surgical operation (sham) or were preconditioned (PC) with four cycles of 5 min of ischemia and 10 min of reperfusion. Twenty-four hours later, animals were subjected to 30 min of ischemia and 180 min of reperfusion. Genistein (1 mg/kg ip) was used to block the receptor TK. Six groups were studied: control, sham, genistein-sham, PC, genistein-PC, and vehicle-PC group (1% dimethyl sulfoxide). Genistein or vehicle was given 30 min before the surgical procedure. Genistein pretreatment decreased the expression of HSP72 in PC hearts and suppressed action potential duration shortening during ischemia in sham and PC groups. Infarct size (%risk area) was reduced in the PC (11.6 +/- 1.0%) and vehicle-PC (19.3 +/- 2.0%) compared with the control (40.0 +/- 3.8%) or sham (46.0 +/- 2.0%) groups (P < 0.05). Genistein pretreatment increased infarct size to 46.4 +/- 4.1% in the PC hearts. We conclude that TK signaling is involved in K(ATP) channel opening and HSP72 expression during late PC.     

Acid sensing ion channels regulate neuronal excitability by inhibiting BK potassium channels

Acid sensing ion channels (ASICs), Ca²⁺ and voltage-activated potassium channels (BK) are widely present throughout the central nervous system. Previous studies have shown that when expressed together in heterologous cells, ASICs inhibit BK channels, and this inhibition is relieved by acidic extracellular pH. We hypothesized that ASIC and BK channels might interact in neurons, and that ASICs may regulate BK channel activity. We found that ASICs inhibited BK currents in cultured wild-type cortical neurons, but not in ASIC1a/2/3 triple knockout neurons. The inhibition in the wild-type was partially relieved by a drop in extracellular pH to 6. To test the consequences of ASIC-BK interaction for neuronal excitability, we compared action potential firing in cultured cortical neurons from wild-type and ASIC1a/2/3 null mice. We found that in the knockout, action potentials were narrow and exhibited increased after-hyperpolarization. Moreover, the excitability of these neurons was significantly increased. These findings are consistent with increased BK channel activity in the neurons from ASIC1a/2/3 null mice. Our data suggest that ASICs can act as endogenous pH-dependent inhibitors of BK channels, and thereby can reduce neuronal excitability.     

The regenerative capacity of zebrafish reverses cardiac failure caused by genetic cardiomyocyte depletion

Natural models of heart regeneration in lower vertebrates such as zebrafish are based on invasive surgeries causing mechanical injuries that are limited in size. Here, we created a genetic cell ablation model in zebrafish that facilitates inducible destruction of a high percentage of cardiomyocytes. Cell-specific depletion of over 60% of the ventricular myocardium triggered signs of cardiac failure that were not observed after partial ventricular resection, including reduced animal exercise tolerance and sudden death in the setting of stressors. Massive myocardial loss activated robust cellular and molecular responses by endocardial, immune, epicardial and vascular cells. Destroyed cardiomyocytes fully regenerated within several days, restoring cardiac anatomy, physiology and performance. Regenerated muscle originated from spared cardiomyocytes that acquired ultrastructural and electrophysiological characteristics of de-differentiation and underwent vigorous proliferation. Our study indicates that genetic depletion of cardiomyocytes, even at levels so extreme as to elicit signs of cardiac failure, can be reversed by natural regenerative capacity in lower vertebrates such as zebrafish.     

Transmembrane action potential heterogeneity in the canine isolated arterially perfused right atrium: effect of IKr and IKur/Ito block

The role of electrical heterogeneity in development of cardiac arrhythmias is well recognized. The extent to which transmembrane action potential (TAP) heterogeneity contributes to the normal electrophysiology of well-oxygenated atria is not well defined. The principal objective of the present study was to define regional and transmural differences in characteristics of the TAP in isolated superfused and arterially perfused canine right atrial (RA) preparations under baseline, rapidly activating delayed rectifier K(+) current (I(Kr)) block, and combined block of ultrarapid delayed rectifier and transient outward K(+) current (I(Kur)/I(to) block). Superfused preparations that survived generally displayed a triangle-shaped TAP. Exceptions included cells from the crista terminalis, where TAPs with a normal plateau could be recorded. In contrast, most TAPs recorded from throughout the perfused RA displayed a spike-and-dome and/or plateau morphology. The perfused RA displayed a heterogeneous distribution of repolarization, V(max), and spike-and-dome morphology along the epicardial and endocardial surfaces as well as transmurally, in the region of the upper crista terminalis. I(Kr) block with E-4031 prolonged repolarization homogeneously in the perfused RA, whereas I(Kur)/I(to) block using low concentrations of 4-aminopyridine abbreviated action potential duration at 90% repolarization heterogeneously, leading to a reduction in dispersion of repolarization. Our data indicate that the electrical heterogeneities, previously described for the canine ventricle, also exist within the atria and that I(Kr) block does not accentuate and I(Kur)/I(to) block reduces RA dispersion of repolarization. Our study also points to major differences in the transmembrane activity recorded using superfused vs. arterially perfused atrial preparations.     

IL-22 ameliorated cardiomyocyte apoptosis in cardiac ischemia/reperfusion injury by blocking mitochondrial membrane potential decrease,inhibiting ROS and cytochrome C

Irreversible cardiomyocyte death is one of the main reasons of heart failure following cardiac injury. Therefore, controlling cardiomyocyte death is an effective method to delay the progression of cardiac disease after injury. IL-22 plays critical roles in tissue homeostasis and repair, and has become an important bridge between the immune system and specific tissues or organs. However, whether IL-22 can prevent of cardiomyocyte apoptosis from cardiac injury remains unclear. Therefore, the present work would address the above question. Our results showed that, in vitro, IL-22 prevented cardiomyocyte apoptosis induced by Angiotensin II via enhancing the activity of SOD, blocking the decrease of mitochondrial membrane potential, inhibiting ROS production and release of cytochrome C. The similar results were also found in vivo and patients. Our results shed a light on the therapy of cardiac injury.