Currents In Ventricular Myocytes Research Articles

Chrysosplenol-C Increases Contraction by Augmentation of Sarcoplasmic Reticulum Ca2+ Loading and Release via Protein Kinase C in Rat Ventricular Myocytes

Naturally found chrysosplenol-C (4',5,6-trihydroxy-3,3',7-trimethoxyflavone) increases the contractility of cardiac myocytes independent of β-adrenergic signaling. We investigated the cellular mechanism for chrysosplenol-C-induced positive inotropy. Global and local Ca2+ signals, L-type Ca2+ current (ICa), and contraction were measured from adult rat ventricular myocytes using two-dimensional confocal Ca2+ imaging, the whole-cell patch-clamp technique, and video-edge detection, respectively. Application of chrysosplenol-C reversibly increased Ca2+ transient magnitude with a maximal increase of ∼55% within 2- to 3-minute exposures (EC50 ≅ 21 μM). This chemical did not alter ICa and slightly increased diastolic Ca2+ level. The frequency and size of resting Ca2+ sparks were increased by chrysosplenol-C. Chrysosplenol-C significantly increased sarcoplasmic reticulum (SR) Ca2+ content but not fractional release. Pretreatment of protein kinase C (PKC) inhibitor but not Ca2+/calmodulin-dependent protein kinase II (CaMKII) inhibitor abolished the stimulatory effects of chrysosplenol-C on Ca2+ transients and Ca2+ sparks. Chrysosplenol-C-induced positive inotropy was removed by the inhibition of PKC but not CaMKII or phospholipase C. Western blotting assessment revealed that PKC-δ protein level in the membrane fractions significantly increase within 2 minutes after chrysosplenol-C exposure with a delayed (5-minute) increase in PKC-α levels in insoluble membrane. These results suggest that chrysosplenol-C enhances contractility via PKC (most likely PKC-δ)-dependent enhancement of SR Ca2+ releases in ventricular myocytes. SIGNIFICANCE STATEMENT: Study shows that chrysosplenol-C, a natural flavone showing a positive inotropic effect, increases SR Ca2+ releases on depolarizations and Ca2+ sparks with an increase of SR Ca2+ loading but not L-type Ca2+ current in ventricular myocytes. Chrysosplenol-C-induced enhancement in contraction is eliminated by PKC inhibition, and it is associated with redistributions of PKC to the membrane. These indicate that chrysosplenol-C enhances contraction via PKC-dependent augmentations of SR Ca2+ release and Ca2+ loading during action potentials.

Hemodynamic and electromechanical effects of paraquat in rat heart.

Paraquat (PQ) is a highly lethal herbicide. Ingestion of large quantities of PQ usually results in cardiovascular collapse and eventual mortality. Recent pieces of evidence indicate possible involvement of oxidative stress- and inflammation-related factors in PQ-induced cardiac toxicity. However, little information exists on the relationship between hemodynamic and cardiac electromechanical effects involved in acute PQ poisoning. The present study investigated the effects of acute PQ exposure on hemodynamics and electrocardiogram (ECG) in vivo, left ventricular (LV) pressure in isolated hearts, as well as contractile and intracellular Ca2+ properties and ionic currents in ventricular myocytes in a rat model. In anesthetized rats, intravenous PQ administration (100 or 180 mg/kg) induced dose-dependent decreases in heart rate, blood pressure, and cardiac contractility (LV +dP/dtmax). Furthermore, PQ administration prolonged the PR, QRS, QT, and rate-corrected QT (QTc) intervals. In Langendorff-perfused isolated hearts, PQ (33 or 60 μM) decreased LV pressure and contractility (LV +dP/dtmax). PQ (10-60 μM) reduced the amplitudes of Ca2+ transients and fractional cell shortening in a concentration-dependent manner in isolated ventricular myocytes. Moreover, whole-cell patch-clamp experiments demonstrated that PQ decreased the current amplitude and availability of the transient outward K+ channel (Ito) and altered its gating kinetics. These results suggest that PQ-induced cardiotoxicity results mainly from diminished Ca2+ transients and inhibited K+ channels in cardiomyocytes, which lead to LV contractile force suppression and QTc interval prolongation. These findings should provide novel cues to understand PQ-induced cardiac suppression and electrical disturbances and may aid in the development of new treatment modalities.

Monensin-Induced Increase in Intracellular Na+ Induces Changes in Na+ and Ca2+ Currents and Regulates Na+-K+ and Na+-Ca2+ Transport in Cardiomyocytes

Background/Aims: Monensin, an Na ionophore, increases intracellular Na ([Na]i). Alteration of [Na]i influences ion transport through the sarcolemmal membrane. So far, the effects of monensin on ventricular myocytes have not been examined in detail. The main objective of this study was to elucidate the mechanism via which monensin-evoked increases in [Na]i affect the membrane potential and currents in ventricular myocytes of guinea pigs. Methods: Membrane potentials and currents were measured using the whole-cell patch-clamp technique in single myocytes. The concentration of intracellular Ca ([Ca]i) was evaluated by measuring fluorescence intensity of Fluo-4. Results: Monensin (10⁻⁵M) shortened the action potential duration (APD) and reduced the amplitude of the plateau phase. In addition, monensin decreased the sodium current (I_Na) and shifted the inactivation curve to the hyperpolarized direction. Moreover, it decreased the L-type calcium current (I_Ca). However, this effect was attenuated by increasing the buffering capacity of [Ca]i. The Na-Ca exchange current (I_Na-Ca) was activated particularly in the reverse mode. Na-K pump current (I_Na-K) was also activated. Notably, the inward rectifying K current (I_K1) was not affected, and the change in the delayed outward K current (I_K) was not evident. Conclusion: These results suggest that the monensin-induced shortened APD and reduced amplitude of the plateau phase are primarily due to the decrease in the I_Ca, the activation of the reverse mode of I_Na-Ca, and the increased I_Na-K, and second due to the decreased I_Na. The I_K and the I_K1 may not be associated with the abovementioned changes induced by monensin. The elevation of [Na]i can exert multiple influences on electrophysiological phenomena in cardiac myocytes.

Pseudo-ginsengenin DQ ameliorated aconitine-induced arrhythmias by influencing Ca2+ and K+ currents in ventricular myocytes.

Pseudo-ginsengenin DQ (PDQ) is the product of the oxidative cyclization of protopanaxadiol. PDQ exhibits various bioactivities, including reversal of multidrug resistance in cancer, renal protective effects against acute nephrotoxicity and attenuating myocardial ischemia injury induced by isoproterenol or ligation of coronary arterials, but its effect on arrhythmias has not been clear until now. Because of the complicated effects of ginseng on the cardiovascular system, it is necessary to investigate whether PDQ affects arrhythmias, which are always concomitant with other cardiac diseases. Aconitine was used to induce arrhythmia in vivo. To understand its electrophysiological fundamental, whole-cell patch-clamp was used to record the L-type calcium current (ICa,L) and potassium currents (IK and IK1) in the ventricular myocytes in rats. Oral administration of PDQ exerted obvious antiarrhythmic effects, as indicated by the decreased incidence rate, lower number of occurrences, and shorter duration time of ventricular tachycardia and ventricular tachycardia, decreased mortality rate and increased survival time. ICa,L and IK were inhibited by PDQ treatment while IK1 was not affected. To conclude, PDQ may have an anti-arrhythmia effect through inhibiting ICa,L and IK.

Luteolin improves heart preservation through inhibiting hypoxia-dependent L-type calcium channels in cardiomyocytes.

The current study aimed to evaluate whether luteolin could improve long-term heart preservation; this was achieved by evaluating the heart following long-term storage in University of Wisconsin solution (the control group) and in solutions containing three luteolin concentrations. The effects of different preservation methods were evaluated with respect to cardiac function while hearts were in custom-made ex vivo Langendorff perfusion systems. Different preservation methods were evaluated with respect to the histology, ultrastructure and apoptosis rate of the hearts, and the function of cardiomyocytes. In the presence of luteolin, the rate pressure product of the left ventricle was increased within 60 min of reperfusion following a 12-h preservation, coronary flow was higher within 30 min of reperfusion, cardiac contractile function was higher throughout reperfusion following 12- and 18-h preservations, and the left ventricle peak systolic pressure was significantly higher compared with the control group (all P<0.05). The expression levels of apoptosis regulator Bax and apoptosis regulator Bcl-2 in the luteolin groups were significantly decreased and increased, respectively. Lactate dehydrogenase, creatine kinase and malondialdehyde enzymatic activity was increased following long-term storage, while the activity of superoxide dismutase was significantly decreased. Furthermore, luteolin inhibited L-type calcium currents in ventricular myocytes under hypoxia conditions. Thus, luteolin demonstrated protective effects during long-term heart preservation in what appeared to be a dose-dependent manner, which may be accomplished through inhibiting hypoxia-dependent L-type calcium channels.

Complex electrophysiological remodeling in postinfarction ischemic heart failure

Heart failure (HF) following myocardial infarction (MI) is associated with high incidence of cardiac arrhythmias. Development of therapeutic strategy requires detailed understanding of electrophysiological remodeling. However, changes of ionic currents in ischemic HF remain incompletely understood, especially in translational large-animal models. Here, we systematically measure the major ionic currents in ventricular myocytes from the infarct border and remote zones in a porcine model of post-MI HF. We recorded eight ionic currents during the cell's action potential (AP) under physiologically relevant conditions using selfAP-clamp sequential dissection. Compared with healthy controls, HF-remote zone myocytes exhibited increased late Na+ current, Ca2+-activated K+ current, Ca2+-activated Cl- current, decreased rapid delayed rectifier K+ current, and altered Na+/Ca2+ exchange current profile. In HF-border zone myocytes, the above changes also occurred but with additional decrease of L-type Ca2+ current, decrease of inward rectifier K+ current, and Ca2+ release-dependent delayed after-depolarizations. Our data reveal that the changes in any individual current are relatively small, but the integrated impacts shift the balance between the inward and outward currents to shorten AP in the border zone but prolong AP in the remote zone. This differential remodeling in post-MI HF increases the inhomogeneity of AP repolarization, which may enhance the arrhythmogenic substrate. Our comprehensive findings provide a mechanistic framework for understanding why single-channel blockers may fail to suppress arrhythmias, and highlight the need to consider the rich tableau and integration of many ionic currents in designing therapeutic strategies for treating arrhythmias in HF.

AKAP150 participates in calcineurin/NFAT activation during the down-regulation of voltage-gated K+ currents in ventricular myocytes following myocardial infarction

The Ca2+-responsive phosphatase calcineurin/protein phosphatase 2B dephosphorylates the transcription factor NFATc3. In the myocardium activation of NFATc3 down-regulates the expression of voltage-gated K+ (Kv) channels after myocardial infarction (MI). This prolongs action potential duration and increases the probability of arrhythmias. Although recent studies infer that calcineurin is activated by local and transient Ca2+ signals the molecular mechanism that underlies the process is unclear in ventricular myocytes. Here we test the hypothesis that sequestering of calcineurin to the sarcolemma of ventricular myocytes by the anchoring protein AKAP150 is required for acute activation of NFATc3 and the concomitant down-regulation of Kv channels following MI. Biochemical and cell based measurements resolve that approximately 0.2% of the total calcineurin activity in cardiomyocytes is associated with AKAP150. Electrophysiological analyses establish that formation of this AKAP150–calcineurin signaling dyad is essential for the activation of the phosphatase and the subsequent down-regulation of Kv channel currents following MI. Thus AKAP150-mediated targeting of calcineurin to sarcolemmal micro-domains in ventricular myocytes contributes to the local and acute gene remodeling events that lead to the down-regulation of Kv currents.

Epinephrine reversed high-concentration bupivacaine-induced inhibition of calcium channels and transient outward potassium current channels, but not on sodium channel in ventricular myocytes of rats.

BackgroundEpinephrine is a first-line drug for cardiopulmonary resuscitation, but its efficacy in the treatment of bupivacaine-induced cardiac toxicity is still in question. We hypothesized that epinephrine can reverse cardiac inhibition of bupivacaine by modulating ion flows through the ventricular myocyte membrane channels of rats. The aim of this study was to observe and report the effects of epinephrine on high-concentration bupivacaine-induced inhibition of sodium (INa), L-type calcium (ICa-L), and transient outward potassium (Ito) currents in the ventricular myocytes of rats.MethodsThe ventricular myocytes were isolated from Sprague-Dawley rats (250-300 g) by acute enzymatic dissociation. The whole-cell patch clamp technique was used to record the ion channel currents in single ventricular myocytes both before and after administration of medications.ResultAdministration of bupivacaine 100 μmol/L significantly reduced INa, (P < 0.05). However, administration of bupivacaine 100 μmol/L in conjunction with epinephrine 0.15 μg/ml had no effect in restoring INa to its previous state. Similarly, a sharp decline of ICa-L and Ito was observed after administration of bupivacaine 100 μmol/L (P < 0.05). In contrast to INa, ICa-L and Ito were significantly improved after the administration of the aforementioned combination of bupivacaine and epinephrine (P < 0.05).ConclusionEpinephrine can reverse high-concentration bupivacaine induced inhibition of ICa-L and Ito, but not INa. Thus, epinephrine’s effectiveness in reversal of bupivacaine-induced cardiac toxicity secondary to sodium channel inhibition may be limited.

Loss of PI3K-Gamma Scaffold Function causes Severe Electrical Remodeling in Mice Ventricular Myocytes

In the heart the kinase/scaffold protein phosphoinositide-3-kinase-γ (PI3Kγ) has been associated with maladaptive remodeling. However little is known about the relative contribution of kinase-scaffold roles on the normal electrical activity in cardiac cells.Here, we compared the action potentials and ionic currents in ventricular myocytes isolated from conventional KO mice or mice expressing a catalytically inactive PI3K (“kinase-dead”, KD). Action potentials were evoked at 1Hz (I-clamp). K+ and L-type Ca2+ (ICaL) currents were measured by step depolarizations (V-clamp).Membrane capacitances (Cm) were not different between genotypes. KO myocytes showed marked prolongation of the action potential duration (APD) at 90% of repolarization (APD90: WT 64.36±4.1 ms, KO 149.6±18 ms). APD90 beat-to-beat variability (BVR) was larger in KO and, consistently, the incidence of early afterdepolarizations was higher in KO than WT (respectively, 66% and 18%). In contrast, KD cells showed a slight prolongation of APD90 (96±7.9 ms), but BVR and EADs incidence were comparable to WT. While voltage-gated K+ currents (Ito, IKur and Iss) were strongly downregulated in KO, KD myocytes showed only a small reduction of Ito. ICaL density was significantly lower in both KO and KD mice at negative potentials only. Consistently, both KD and KO mice showed a positive shift of both steady-state activation (ΔV1/2 vs WT: KO 4.2 mV, KD 5.5 mV) and inactivation curves (ΔV1/2 vs WT: KO 3.2 mV, KD 3.4 mV). Addition of isoproterenol (100 nM) resulted in ICaL enhancement, whose magnitude was larger in KO compared to KD and WT myocytes (% increase,WT: 58±6%,KD:45±7%,KO:95.5±12%). These data suggest distinct action related to either kinase and scaffold functions: while the former has an impact on ICaL biophysical properties, the latter prevents arrhythmias in murine ventricular myocytes by preserving K+ currents.

Effects of sleep deprivation on action potential and transient outward potassium current in ventricular myocytes in rats.

BackgroundSleep deprivation contributes to the development and recurrence of ventricular arrhythmias. However, the electrophysiological changes in ventricular myocytes in sleep deprivation are still unknown.Material/MethodsSleep deprivation was induced by modified multiple platform technique. Fifty rats were assigned to control and sleep deprivation 1, 3, 5, and 7 days groups, and single ventricular myocytes were enzymatically dissociated from rat hearts. Action potential duration (APD) and transient outward current (Ito) were recorded using whole-cell patch clamp technique.ResultsCompared with the control group, the phases of APD of ventricular myocytes in 3, 5, and 7 days groups were prolonged and APD at 20% and 50% level of repolarization (APD20 and APD50) was significantly elongated (The APD20 values of control, 1, 3, 5, and 7 days groups: 5.66±0.16 ms, 5.77±0.20 ms, 8.28±0.30 ms, 11.56±0.32 ms, 13.24±0.56 ms. The APD50 values: 50.66±2.16 ms, 52.77±3.20 ms, 65.28±5.30 ms, 83.56±7.32 ms, 89.24±5.56 ms. P<0.01, n=18). The current densities of Ito significantly decreased. The current density-voltage (I–V) curve of Ito was vitally suppressed downward. The steady-state inactivation curve and steady-state activation curve of Ito were shifted to left and right, respectively, in sleep deprivation rats. The inactivation recovery time of Ito was markedly retarded and the time of closed-state inactivation was markedly accelerated in 3, 5, and 7 days groups.ConclusionsAPD of ventricular myocytes in sleep deprivation rats was significantly prolonged, which could be attributed to decreased activation and accelerated inactivation of Ito.

Contribution of sodium channel neuronal isoform Nav1.1 to late sodium current in ventricular myocytes from failing hearts.

Late Na(+) current (INaL) contributes to action potential remodelling and Ca(2+)/Na(+) changes in heart failure. The molecular identity of INaL remains unclear. The contributions of different Na(+) channel isoforms, apart from the cardiac isoform, remain unknown. We discovered and characterized a substantial contribution of neuronal isoform Nav1.1 to INaL. This new component is physiologically relevant to the control of action potential shape and duration, as well as to cell Ca(2+) dynamics, especially in heart failure. Late Na(+) current (INaL) contributes to action potential (AP) duration and Ca(2+) handling in cardiac cells. Augmented INaL was implicated in delayed repolarization and impaired Ca(2+) handling in heart failure (HF). We tested if Na(+) channel (Nav) neuronal isoforms contribute to INaL and Ca(2+) cycling defects in HF in 17 dogs in which HF was achieved via sequential coronary artery embolizations. Six normal dogs served as control. Transient Na(+) current (INaT ) and INaL in left ventricular cardiomyocytes (VCMs) were recorded by patch clamp while Ca(2+) dynamics was monitored using Fluo-4. Virally delivered short interfering RNA (siRNA) ensured Nav1.1 and Nav1.5 post-transcriptional silencing. The expression of six Navs was observed in failing VCMs as follows: Nav1.5 (57.3%) > Nav1.2 (15.3%) > Nav1.1 (11.6%) > Nav2.1 (10.7%) > Nav1.3 (4.6%) > Nav1.6 (0.5%). Failing VCMs showed up-regulation of Nav1.1 expression, but reduction of Nav1.6 mRNA. A similar Nav expression pattern was found in samples from human hearts with ischaemic HF. VCMs with silenced Nav1.5 exhibited residual INaT and INaL (∼30% of control) with rightwardly shifted steady-state activation and inactivation. These currents were tetrodotoxin sensitive but resistant to MTSEA, a specific Nav1.5 blocker. The amplitude of the tetrodotoxin-sensitive INaL was 0.1709 ± 0.0299 pA pF(-1) (n = 7 cells) and the decay time constant was τ = 790 ± 76 ms (n = 5). This INaL component was lacking in VCMs with a silenced Nav1.1 gene, indicating that, among neuronal isoforms, Nav1.1 provides the largest contribution to INaL. At -10 mV this contribution is ∼60% of total INaL. Our further experimental and in silico examinations showed that this new Nav1.1 INaL component contributes to Ca(2+) accumulation in failing VCMs and modulates AP shape and duration. In conclusion, we have discovered an Nav1.1-originated INaL component in dog heart ventricular cells. This component is physiologically relevant to controlling AP shape and duration, as well as to cell Ca(2+) dynamics.

Bone marrow mesenchymal stem cells protected post-infarcted myocardium against arrhythmias via reversing potassium channels remodelling.

Bone marrow mesenchymal stem cells (BMSCs) emerge as a promising approach for treating heart diseases. However, the effects of BMSCs-based therapy on cardiac electrophysiology disorders after myocardial infarction were largely unclear. This study was aimed to investigate whether BMSCs transplantation prevents cardiac arrhythmias and reverses potassium channels remodelling in post-infarcted hearts. Myocardial infarction was established in male SD rats, and BMSCs were then intramyocardially transplanted into the infarcted hearts after 3 days. Cardiac electrophysiological properties in the border zone were evaluated by western blotting and whole-cell patch clamp technique after 2 weeks. We found that BMSCs transplantation ameliorated the increased heart weight index and the impaired LV function. The survival of infarcted rats was also improved after BMSCs transplantation. Importantly, electrical stimulation-induced arrhythmias were less observed in BMSCs-transplanted infarcted rats compared with rats without BMSCs treatment. Furthermore, BMSCs transplantation effectively inhibited the prolongation of action potential duration and the reduction of transient and sustained outward potassium currents in ventricular myocytes in post-infarcted rats. Consistently, BMSCs-transplanted infarcted hearts exhibited the increased expression of KV4.2, KV4.3, KV1.5 and KV2.1 proteins when compared to infarcted hearts. Moreover, intracellular free calcium level, calcineurin and nuclear NFATc3 protein expression were shown to be increased in infarcted hearts, which was inhibited by BMSCs transplantation. Collectively, BMSCs transplantation prevented ventricular arrhythmias by reversing cardiac potassium channels remodelling in post-infarcted hearts.

GW24-e1851 An increase of CO2 levels, enhancing late sodium current, is an independent factor of proarrhythmia in the heart

ObjectivesIncreased CO2 levels in the myocardium may be global due to inadequate ventilation/circulation or local due to myocardial ischaemia/infarction (by 4 times higher than the normal), and are associated with...

Abstract 008: Bone Marrow Mesenchymal Stem Cells Prevented Cardiac Arrhythmias Of Infarcted Heart Via Regulating Calcineurin And Potassium Channels Expression

Bone marrow mesenchymal stem cells (BMSCs) emerge as a new promising approach for treating heart diseases. However, the effects of BMSCs-based therapy on cardiac electrophysiological remodeling following myocardial infarction were largely unclear. This study was aimed to investigate whether BMSCs transplantation prevented cardiac arrhythmias in post-infarcted hearts. Myocardial infarction was established in male SD rats, and BMSCs were then intramyocardially transplanted into the infarcted hearts after 3 days. Cardiac electrophysiological remodeling in the border zone was evaluated by western blotting and whole-cell path clamp technique after 2 weeks. We found that BMSCs transplantation significantly ameliorated the increased heart weight index, the impaired left ventricular functions, and the enhanced cardiac fibrosis in infarcted hearts. The survival ratio of infarcted rats was also improved after BMSCs transplantation. Importantly, electrical stimulation-induced arrhythmias were seldom observed in BMSCs-transplanted infarcted rats compared with rats without BMSCs treatment. Furthermore, BMSCs transplantation effectively inhibited the prolongation of action potential duration and the reduction of transient and sustained outward potassium currents in ventricular myocytes from post-infarcted rats. Consistently, BMSCs-transplanted infarcted hearts exhibited the more expression of K V 4.2, K V 4.3, K V 1.5 and K V 2.1 proteins than infarcted hearts. Moreover, intracellular free calcium level, calcineurin and nuclear NFATc3 protein expression was shown activated in infarcted hearts, which was inhibited after BMSCs transplantation. Collectively, BMSCs transplantation prevented ventricular arrhythmias by reversing the reduction of cardiac potassium channels in post-infarcted heats, which involves attenuated intracellular calcium overload and calcineurin/NFATc3 signal pathway.

The cardiomyocyte molecular clock, regulation of Scn5a, and arrhythmia susceptibility

The molecular clock mechanism underlies circadian rhythms and is defined by a transcription-translation feedback loop. Bmal1 encodes a core molecular clock transcription factor. Germline Bmal1 knockout mice show a loss of circadian variation in heart rate and blood pressure, and they develop dilated cardiomyopathy. We tested the role of the molecular clock in adult cardiomyocytes by generating mice that allow for the inducible cardiomyocyte-specific deletion of Bmal1 (iCSΔBmal1). ECG telemetry showed that cardiomyocyte-specific deletion of Bmal1 (iCSΔBmal1(-/-)) in adult mice slowed heart rate, prolonged RR and QRS intervals, and increased episodes of arrhythmia. Moreover, isolated iCSΔBmal1(-/-) hearts were more susceptible to arrhythmia during electromechanical stimulation. Examination of candidate cardiac ion channel genes showed that Scn5a, which encodes the principle cardiac voltage-gated Na(+) channel (Na(V)1.5), was circadianly expressed in control mouse and rat hearts but not in iCSΔBmal1(-/-) hearts. In vitro studies confirmed circadian expression of a human Scn5a promoter-luciferase reporter construct and determined that overexpression of clock factors transactivated the Scn5a promoter. Loss of Scn5a circadian expression in iCSΔBmal1(-/-) hearts was associated with decreased levels of Na(V)1.5 and Na(+) current in ventricular myocytes. We conclude that disruption of the molecular clock in the adult heart slows heart rate, increases arrhythmias, and decreases the functional expression of Scn5a. These findings suggest a potential link between environmental factors that alter the cardiomyocyte molecular clock and factors that influence arrhythmia susceptibility in humans.

The Electrophysiological Effects of Qiliqiangxin on Cardiac Ventricular Myocytes of Rats

Qiliqiangxin, a Chinese herb, represents the affection in Ca channel function of cardiac myocytes. It is unknown whether Qiliqiangxin has an effect on Na current and K current because the pharmacological actions of this herb's compound are very complex. We investigated the rational usage of Qiliqiangxin on cardiac ventricular myocytes of rats. Ventricular myocytes were exposed acutely to 1, 10, and 50 mg/L Qiliqiangxin, and whole cell patch-clamp technique was used to study the acute effects of Qiliqiangxin on Sodium current (I Na), outward currents delayed rectifier outward K+ current (I K), slowly activating delayed rectifier outward K+ current (I Ks), transient outward K+ current (I to), and inward rectifier K+ current (I K1). Qiliqiangxin can decrease I Na by 28.53% ± 5.98%, and its IC50 was 9.2 mg/L. 10 and 50 mg/L Qiliqiangxin decreased by 37.2% ± 6.4% and 55.9% ± 5.5% summit current density of I to. 10 and 50 mg/L Qiliqiangxin decreased I Ks by 15.51% ± 4.03% and 21.6% ± 5.6%. Qiliqiangxin represented a multifaceted pharmacological profile. The effects of Qiliqiangxin on Na and K currents of ventricular myocytes were more profitable in antiarrhythmic therapy in the clinic. We concluded that the relative efficacy of Qiliqiangxin was another choice for the existing antiarrhythmic therapy.

Eschewing ischemia or responding to it

This month’s installment of Generally Physiological considers the regulation of nitric oxide (NO) delivery to vascular smooth muscle cells, the effects of an increase in free intracellular magnesium (Mg2+i) on calcium current in ventricular myocytes during transient cardiac ischemia, and the

Absence of Rgs5 prolongs cardiac repolarization and predisposes to ventricular tachyarrhythmia in mice

The aim of this study was to elucidate the effects of regulator of G-protein signaling 5 (Rgs5), a negative regulator of G-protein-mediated signaling, on cardiac repolarization and arrhythmia in mice. Wild-type and Rgs5−/− mice were subjected to in vivo, in vitro, and cellular electrophysiological experiments. Rgs5−/− mouse hearts showed significantly prolonged cardiac repolarization, including prolonged QT interval and action potential duration (APD). Consistent with these findings, measurement of K+ currents in ventricular myocytes of Rgs5−/− mice revealed significant reduction of the outward voltage-dependent K+ currents, including Ipeak, Ito,IKur, and Iss, compared to that in wild-type mice. Transcript and protein expression levels of Kv4.2, Kv4.3, Kv1.5, and Kv2.1 were downregulated in Rgs5−/− mouse ventricles compared with those in wild-type mice (P<0.05). In addition, electrically induced ventricular tachyarrhythmias were facilitated by Rgs5−/− in isolated hearts. Importantly, the increased incidence and duration of electrically induced ventricular tachyarrhythmias were associated with enhanced dispersion of APD and spatial heterogeneity of Ipeak, Ito and IKur between the epicardium and endocardium in the Rgs5−/− heart. This study showed the relationship between the absence of Rgs5 and cardiac electrophysiological abnormality. The results strongly indicate that Rgs5−/− induced prolonged repolarization and ventricular tachyarrhythmia, which were closely related to the remodeling of voltage-dependent K+ currents.

Effects of Salusin-β on action potential and ionic currents in ventricular myocytes of rats

Salusin-β is a regulatory peptide that exerts negative inotropic effect on ventricular muscle, but its electrophysiological effects on ventricular myocytes are still unknown. Action potential and channel currents such as sodium current (I(N) (a) ), transient outward potassium current (I(to) ), steady-state potassium current (I(sus) ), sodium-calcium exchange current (I(N) (aCa) ) and inward rectifier potassium current (I(K) (1) ) were measured in ventricular myocytes isolated from 12 to 16weeks rats by whole-cell voltage-clamp techniques. Salusin-β dose-dependently shortened the duration of action potential in rat ventricular myocytes. Furthermore, salusin-β significantly inhibited I(N) (aCa) and increased I(to) , but did not affect I(N) (a) , I(sus) and I(K) (1) . These results suggest that the effect of salusin-β on action potential may be partly attributed to a decrease in inward currents and an increase in outward currents.

Rosiglitazone induces arrhythmogenesis in diabetic hypertensive rats with calcium handling alteration

BackgroundDiabetes and hypertension have significant effects on cardiac calcium (Ca2+) regulation, which plays an essential role in determining cardiac function. The effect of peroxisome proliferator-activated receptor (PPAR)-γ agonists on Ca2+ regulation in the cardiomyocytes is unclear. ObjectiveThe purpose of this study was to investigate the effects of hypertension, diabetes, and PPAR-γ agonist-rosiglitazone on the regulation of Ca2+ and the electrophysiological characteristics of isolated ventricular myocytes. MethodsThe indo-1 fluorometric ratio technique and whole-cell patch clamp were used to investigate intracellular Ca2+ (Ca2+i), action potentials, and ionic currents in ventricular myocytes from rats of Wistar–Kyoto (WKY), diabetic WKY (induced by streptozotocin), diabetic WKY treated with rosiglitazone (5mg/kg), spontaneously hypertensive rats (SHR), diabetic SHR, and diabetic SHR treated with rosiglitazone. Western blot was used to evaluate protein expressions of sarcoplasmic reticulum ATPase (SERCA2a), Na+–Ca2+ exchanger (NCX), and ryanodine receptor (RyR). ResultsDiabetic WKY and diabetic SHR had smaller sarcoplasmic reticulum Ca2+ contents, and Ca2+i transients with a prolonged decay portion, down-regulated SERCA2a, NCX, and RyR protein expressions and smaller L-type Ca2+ currents than non-diabetic WKY and SHR, respectively. The Ca2+ dysregulations in diabetes were attenuated in rats treated with rosiglitazone. Diabetes and hypertension both prolonged the action potential duration which were enhanced by the use of rosiglitazone, and induced the genesis of triggered activity. ConclusionsDiabetes and hypertension modulate Ca2+ handling. Rosiglitazone significantly changed the Ca2+ regulation and electrophysiological characteristics, and may contain an arrhythmogenic potential in diabetes with hypertension.