Relative importance of SR load and cytoplasmic calcium concentration in the genesis of aftercontractions in cardiac myocytes

Mitochondrial mechanosensor in cardiovascular diseases

The role of mitochondria in cardiac tissue is of utmost importance due to the dynamic nature of the heart and its energetic demands, necessary to assure its proper beating function. Recently, other important mitochondrial roles have been discovered, namely its contribution to intracellular calcium handling in normal and pathological myocardium. Novel investigations support the fact that during the progression toward heart failure, mitochondrial calcium machinery is compromised due to its morphological, structural and biochemical modifications resulting in facilitated arrhythmogenesis and heart failure development. The interaction between mitochondria and sarcomere directly affect cardiomyocyte excitation-contraction and is also involved in mechano-transduction through the cytoskeletal proteins that tether together the mitochondria and the sarcoplasmic reticulum. The focus of this review is to briefly elucidate the role of mitochondria as (mechano) sensors in the heart.

Exploring Impaired SERCA Pump-Caused Alternation Occurrence in Ischemia

Impaired sarcoplasmic reticulum (SR) calcium transport ATPase (SERCA) gives rise to Ca²⁺ alternans and changes of the Ca2+release amount. These changes in Ca²⁺ release amount can reveal the mechanism underlying how the interaction between Ca²⁺ release and Ca²⁺ uptake induces Ca²⁺ alternans. This study of alternans by calculating the values of Ca²⁺ release properties with impaired SERCA has not been explored before. Here, we induced Ca²⁺ alternans by using an impaired SERCA pump under ischemic conditions. The results showed that the recruitment and refractoriness of the Ca²⁺ release increased as Ca²⁺ alternans occurred. This indicates triggering Ca waves. As the propagation of Ca waves is linked to the occurrence of Ca²⁺ alternans, the “threshold” for Ca waves reflects the key factor in Ca²⁺ alternans development, and it is still controversial nowadays. We proposed the ratio between the diastolic network SR (NSR) Ca content (Ca_nsr) and the cytoplasmic Ca content (Ca_i) (Ca_nsr/Ca_i) as the “threshold” of Ca waves and Ca²⁺ alternans. Diastolic Ca_nsr, Ca_i, and their ratio were recorded at the onset of Ca²⁺ alternans. Compared with certain Ca_nsr and Ca_i, the “threshold” of the ratio can better explain the comprehensive effects of the Ca²⁺ release and the Ca²⁺ uptake on Ca²⁺ alternans onset. In addition, these ratios are related with the function of SERCA pumps, which vary with different ischemic conditions. Thus, values of these ratios could be used to differentiate Ca²⁺ alternans from different ischemic cases. This agrees with some experimental results. Therefore, the certain value of diastolic Ca_nsr/Ca_i can be the better “threshold” for Ca waves and Ca²⁺ alternans.

Action Potential Prolongation, β-Adrenergic Stimulation, and Angiotensin II as Co-factors in Sarcoplasmic Reticulum Instability

Introduction: Increases in action potential duration (APD), genetic or acquired, and arrhythmias are often associated; nonetheless, the relationship between the two phenomena is inconstant, suggesting coexisting factors. β-adrenergic activation increases sarcoplasmic reticulum (SR) Ca²⁺-content; angiotensin II (ATII) may increase cytosolic Ca²⁺ and ROS production, all actions stimulating RyRs opening. Here we test how APD interacts with β-adrenergic and AT-receptor stimulation in facilitating spontaneous Ca²⁺ release events (SCR).

Methods: Under “action potential (AP) clamp”, guinea-pig cardiomyocytes (CMs) were driven with long (200 ms), normal (150 ms), and short (100 ms) AP waveforms at a CL of 500 ms; in a subset of CMs, all the 3 waveforms could be tested within the same cell. SCR were detected as inward current transients (I_TI) following repolarization; I_TI incidence and repetition within the same cycle were measured under increasing isoprenaline concentration ([ISO]) alone, or plus 100 nM ATII (30 min incubation+superfusion).

Results: I_TI incidence and repetition increased with [ISO]; at longer APs the [ISO]-response curve was shifted upward and I_TI coupling interval was reduced. ATII increased I_TI incidence more at low [ISO] and under normal (as compared to long) APs. Efficacy of AP shortening in suppressing I_TI decreased in ATII-treated myocytes and at higher [ISO].

Conclusions: AP prolongation sensitized the SR to the destabilizing actions of ISO and ATII. Summation of ISO, ATII and AP duration effects had a “saturating” effect on SCR incidence, thus suggesting convergence on a common factor (RyRs stability) “reset” by the occurrence of spontaneous Ca²⁺ release events.

Effect of ovariectomy on intracellular Ca2+ regulation in guinea pig cardiomyocytes

This study addressed the hypothesis that long-term deficiency of ovarian hormones after ovariectomy (OVx) alters cellular Ca²⁺-handling mechanisms in the heart, resulting in the formation of a proarrhythmic substrate. It also tested whether estrogen supplementation to OVx animals reverses any alterations to cardiac Ca²⁺ handling and rescues proarrhythmic behavior. OVx or sham operations were performed on female guinea pigs using appropriate anesthetic and analgesic regimes. Pellets containing 17β-estradiol (1 mg, 60-day release) were placed subcutaneously in selected OVx animals (OVx + E). Cardiac myocytes were enzymatically isolated, and electrophysiological measurements were conducted with a switch-clamp system. In fluo-4-loaded cells, Ca²⁺ transients were 20% larger, and fractional sarcoplasmic reticulum (SR) Ca²⁺ release was 7% greater in the OVx group compared with the sham group. Peak L-type Ca²⁺ current was 16% larger in OVx myocytes with channel inactivation shifting to more positive membrane potentials, creating a larger "window" current. SR Ca²⁺ stores were 22% greater in the OVx group, and these cells showed a higher frequency of Ca²⁺ sparks and waves and shorter wave-free intervals. OVx myocytes showed higher frequencies of early afterdepolarizations, and a greater percentage of these cells showed delayed afterdepolarizations after exposure to isoprenaline compared with sham myocytes. The altered Ca²⁺ regulation occurring in the OVx group was not observed in the OVx + E group. These findings suggest that long-term deprivation of ovarian hormones in guinea pigs lead to changes in myocyte Ca²⁺-handling mechanisms that are considered proarrhythmogenic. 17β-Estradiol replacement prevented these adverse effects.NEW & NOTEWORTHY Ovariectomized guinea pig cardiomyocytes have higher frequencies of Ca²⁺ waves, and isoprenaline-challenged cells display more early afterdepolarizations, delayed afterdepolarizations, and extra beats compared with sham myocytes. These alterations to Ca²⁺ regulation were not observed in myocytes from ovariectomized guinea pigs supplemented with 17β-estradiol, suggesting that ovarian hormone deficiency modifies cardiac Ca²⁺ regulation, potentially creating proarrhythmic substrates.

NCX is an important determinant for premature ventricular activity in a drug-induced model of Andersen-Tawil syndrome

AIMS

Andersen-Tawil syndrome (ATS1)-associated ventricular arrhythmias are initiated by premature ventricular activity (PVA) resulting from diastolic Ca(2+) (Ca(D)) accumulation. We hypothesized that relatively high Na(+)-Ca(2+) exchanger (NCX) expression coupled with slower Ca(2+) uptake may constitute an arrhythmogenic substrate during drug-induced ATS1 (DI-ATS1).

METHODS AND RESULTS

DI-ATS1 was induced with 10 µmol/L BaCl(2) and 2 mmol/L [K(+)](o). Ca(2+) transients and action potentials were optically mapped from Langendorff-perfused guinea pig ventricles. Intracellular Ca(2+) handling was modulated by either direct NCX inhibition with 5 µmol/L KB-R7943 or by sarcoplasmic reticulum Ca(2+)-ATPase (SERCA2a) inhibition with cyclopiazonic acid (CPA). During DI-ATS1, PVA was more frequent in left ventricular (LV)-base (LVB) vs. LV-apex (LVA) (2.2 ± 0.8 vs. 0.6 ± 0.3 PVA/10 min), consistent with greater Ca(D) (1.65 ± 0.13 vs. 1.42 ± 0.09 normalized-Ca(D) units) and western blot-assessed NCX protein expression (81.2 ± 30.9%) in LVB relative to LVA. Further, regions of high NCX (LVB) evidenced a shorter PVA coupling interval relative to regions of low NCX expression (LVA, 67.7 ± 3.5 vs. 78.5 ± 3.6%). Inhibiting NCX during DI-ATS1 lowered the incidence of ventricular tachycardias (VTs, 0 vs. 25%) and PVA (1.5 ± 0.4 vs. 4.3 ± 1.4 PVA/10 min), but it did not affect PVA coupling intervals in LVB nor LVA (70.8 ± 4.3 vs. 73.8 ± 2.5%). Conversely, inhibition of SERCA2a with CPA, thereby increasing the role of NCX in Ca(2+) handling, significantly increased the incidence of VTs and PVA relative to DI-ATS1 alone, while decreasing the PVA coupling interval in all regions.

CONCLUSION

PVA preferentially occurs in regions of enhanced NCX expression with relatively slower Ca(2+) uptake and during perfusion of CPA which further reduces sarcoplasmic reticular Ca(2+) uptake.

Role and mechanism of subcellular Ca2+ distribution in the action of two inotropic agents with different toxicity

Pro-arrhythmic risk strongly limits the therapeutic value of current inotropic interventions. Istaroxime (previously PST2744) is a novel inotropic agent, significantly less pro-arrhythmic than digoxin that, in addition to block Na(+)/K(+) pump, stimulates sarcoplasmic reticulum (SR) Ca(2+) ATPase (SERCA2). Here we compare istaroxime and digoxin effects to further address the role of SR modulation in reducing the toxicity associated with Na(+)/K(+) pump blockade. In murine ventricular myocytes both compounds increased cell twitch (inotropy) in a concentration-dependent fashion. At high concentrations digoxin, but not istaroxime, induced unstimulated contractions, a sign of pro-arrhythmic toxicity. To evaluate the mechanism of this difference, we compared the two drugs at concentrations exerting equal inotropy but different toxicity. At these concentrations: (1) the two drugs equally inhibited the Na(+)/K(+) pump; (2) digoxin induced larger increases in resting Ca(2+) and in diastolic Ca(2+) during pacing; (3) neither drug affected the relationship between RyR-mediated SR Ca(2+) leak and Ca(2+) content; (4) istaroxime, but not digoxin, enhanced SR Ca(2+) reuptake rate. In conclusion, digoxin toxicity was associated to larger accumulation of cytosolic Ca(2+), which did not result from RyR facilitation, but which might ultimately induce it to promote unstimulated Ca(2+) release. The lower toxicity of Na(+)/K(+) pump blockade by istaroxime may thus reflect improved Ca(2+) confinement within the SR, likely to result from concomitant SERCA2 stimulation.

Cardiac ankyrins in health and disease

Ankyrins are critical components of ion channel and transporter signaling complexes in the cardiovascular system. Over the past 5 years, ankyrin dysfunction has been linked with abnormal ion channel and transporter membrane organization and fatal human arrhythmias. Loss-of-function variants in the ankyrin-B gene (ANK2) cause "ankyrin-B syndrome" (previously called type 4 long QT syndrome), manifested by a complex cardiac phenotype including ventricular arrhythmias and sudden cardiac death. More recently, dysfunction in the ankyrin-B-based targeting pathway has been linked with a highly penetrant and severe form of human sinus node disease. Ankyrin-G (a second ankyrin gene product) is required for normal expression, membrane localization, and biophysical function of the primary cardiac voltage-gated sodium channel, Na(v)1.5. Loss of the ankyrin-G/Na(v)1.5 interaction is associated with human cardiac arrhythmia (Brugada syndrome). Finally, in the past year ankyrin dysfunction has been associated with more common arrhythmia and cardiovascular disease phenotypes. Specifically, large animal studies reveal striking remodeling of ankyrin-B and associated proteins following myocardial infarction. Additionally, the ANK2 locus has been linked with QT(c) interval variability in the general human population. Together, these findings identify a host of unanticipated and exciting roles for ankyrin polypeptides in cardiac function. More broadly, these findings illustrate the importance of local membrane organization for normal cardiac physiology.

Mechanisms and potential therapeutic targets for ventricular arrhythmias associated with impaired cardiac calcium cycling

The close relationship between life-threatening ventricular arrhythmias and contractile dysfunction in the heart implicates intracellular calcium cycling as an important underlying mechanism of arrhythmogenesis. Despite this close association, however, the mechanisms of arrhythmogenesis attributable to impaired calcium cycling are not fully appreciated or understood. In this report we review some of the current thinking regarding arrhythmia mechanisms associated with either abnormal impulse initiation (i.e. arrhythmia triggers) or impulse propagation (i.e. arrhythmia substrates). In all cases, the mechanisms are primarily related to dysfunction of calcium regulatory proteins associated with the sarcomere. These findings highlight the broad scope of arrhythmias associated with abnormal calcium cycling, and provide a basis for a causal relationship between cardiac electrical instability and contractile dysfunction. Moreover, calcium cycling proteins may provide much needed targets for novel antiarrhythmic therapies.

Calcium-mediated triggered activity is an underlying cellular mechanism of ectopy originating from the pulmonary vein in dogs

Paroxysmal atrial fibrillation associated with focal ectopy originating from the pulmonary vein (PV) can be preceded by variations in autonomic tone; however, the underlying cellular mechanisms are not clear. To determine the mechanisms of autonomically mediated PV ectopy, high-resolution optical mapping techniques were used to measure action potentials and Ca2+ transients from the PV and the ligament of Marshall area in the arterially perfused canine left atrium. Rapid pacing was used to initiate ectopic activity during pituitary adenylate cyclase-activating polypeptide (PACAP) injection (1 nmol), as a surrogate for autonomic imbalance, before ( n = 9) and after ( n = 6) verapamil (10 nmol) administration. In all preparations, spontaneous activity was absent before rapid pacing. During PACAP injection, rapid pacing induced ectopic activity in eight of nine preparations. In contrast, before PACAP injection, rapid pacing did not induce ectopic activity. Activation maps of each episode of ectopic activity indicated that the site of origin occurred more frequently in the PV (70%) than in the ligament of Marshall (30%) area. As rapid pacing cycle length increased, so did the ectopic beat coupling interval. In addition, PACAP-induced ectopic activity was associated with large Ca2+ transient amplitudes and was always suppressed by verapamil, a Ca2+ channel blocker ( P < 0.05). Finally, during PACAP injection in the absence of an ectopic beat, spontaneous Ca2+ release and delayed afterdepolarizations were observed simultaneously after termination of rapid pacing. In conclusion, these data suggest that autonomically mediated PV ectopy may be due to Ca2+-mediated triggered activity arising from delayed afterdepolarizations.

The effect of isoflurane during reoxygenation on the sarcoplasmic reticulum and cellular injury in isolated ventricular myocytes

In contrast to pretreatment with isoflurane its benefit when applied during reperfusion in rat hearts was only modest. As cellular injury during reoxygenation is greatly determined by sarcoplasmic reticulum (SR) calcium [Ca2+] handling we investigated the effect of isoflurane after simulated ischemia in rat ventricular myocytes. Hypoxic metabolic inhibition was induced by exposure to an acidic medium (pH: 6.3) containing deoxyglucose. Ambient pO2 was reduced to <15 mm Hg. After 30 min, cells were reoxygenated for 30 min with a glucose containing medium (pH: 7.4) in air (Air) or in the presence of isoflurane (Iso), or two SR blockers, i.e. either 3 microM ryanodine (Rya) or 10 microM of cyclopiazonic acid (CPA). During inhibition, diastolic cytosolic calcium ([Ca2+]i) increased and systolic cell shortening decreased. [Ca2+]i further increased in all groups towards the end of reoxygenation. However, [Ca2+]i in the Iso and the Rya group climbed twice as high as in the Air and the CPA group (P < 0.05). Hypercontracture occurred in 23% and 18% in the Iso and the Rya and in 10% and 9% in the Air and the CPA group, respectively (P < 0.05). Cell relengthening and shortening was impaired in Iso, Rya, and CPA treated cells (P < 0.05 vs. Air). Isoflurane given solely during reoxygenation appears to augment cellular injury. Its action seems to be blockade of SR Ca2+ release and Ca2+ efflux. SR Ca2+ overload induces spontaneous Ca2+ oscillations that cause hypercontracture. However, [Ca2+]i does not independently govern cellular systolic and diastolic dysfunction.

Sodium calcium exchange as a target for antiarrhythmic therapy

In search of better antiarrhythmic therapy, targeting the Na/Ca exchanger is an option to be explored. The rationale is that increased activity of the Na/Ca exchanger has been implicated in arrhythmogenesis in a number of conditions. The evidence is strong for triggered arrhythmias related to Ca2+ overload, due to increased Na+ load or during adrenergic stimulation; the Na/Ca exchanger may be important in triggered arrhythmias in heart failure and in atrial fibrillation. There is also evidence for a less direct role of the Na/Ca exchanger in contributing to remodelling processes. In this chapter, we review this evidence and discuss the consequences of inhibition of Na/Ca exchange in the perspective of its physiological role in Ca2+ homeostasis. We summarize the current data on the use of available blockers of Na/Ca exchange and propose a framework for further study and development of such drugs. Very selective agents have great potential as tools for further study of the role the Na/Ca exchanger plays in arrhythmogenesis. For therapy, they may have their specific indications, but they carry the risk of increasing Ca2+ load of the cell. Agents with a broader action that includes Ca2+ channel block may have advantages in other conditions, e.g. with Ca2+ overload. Additional actions such as block of K+ channels, which may be unwanted in e.g. heart failure, may be used to advantage as well.

Delayed after depolarization-mediated triggered activity associated with slow calcium sequestration near the endocardium

INTRODUCTION

Previously, we have shown that cells near the endocardium are more prone to elevated diastolic intracellular calcium levels than cells near the epicardium. The arrhythmogenic consequence of such regional differences in calcium handling is not clear.

METHODS AND RESULTS

Using optical mapping techniques, calcium transients and action potentials were recorded simultaneously from ventricular sites across the transmural wall of the arterially perfused canine left ventricular wedge preparation during control conditions, and under conditions of increased calcium entry (I(K) blockade and beta-adrenergic stimulation). Under conditions of enhanced calcium entry, the decay of the calcium transient and diastolic calcium levels during rapid pacing were slower (38%, P < 0.01) and higher (215%, P < 0.02), respectively, near (within approximately 3 mm) the endocardium compared to the epicardium (n = 9). Immediately after termination of rapid pacing under conditions of increased calcium entry, ectopic activity and simultaneous delayed after depolarizations and spontaneous calcium release events were observed. Over all experiments, ectopic activity occurred more frequently closer to the endocardium compared to the epicardium.

CONCLUSIONS

Under conditions of enhanced calcium entry, myocytes closer to the endocardium exhibit a higher level of diastolic calcium and greater ectopic activity compared to the epicardium. We show for the first time simultaneous delayed after depolarization and spontaneous calcium release events from myocytes in a normally coupled multicellular preparation. These data combined suggest that myocytes near the endocardium are more susceptible to calcium-mediated triggered activity.

Cellular mechanism of calcium-mediated triggered activity in the heart

Calcium overload due to enhanced calcium entry is a mechanism for spontaneous calcium release (SCR) from the sarcoplasmic reticulum, delayed-afterdepolarizations (DAD), and triggered activity. However, the exact mechanistic relationship between elevated intracellular calcium levels and triggered activity originating from a specific location remains unclear. We hypothesize that under conditions of enhanced calcium entry, elevation of intracellular calcium will result in multiple calcium release events of which only one is more likely to initiate a triggered beat. We used optical mapping of action potentials and ratiometric calcium transients in an electromechanically-uncoupled canine wedge model of enhanced calcium entry, using I(Ks) blockade with beta-adrenergic stimulation. Under conditions of enhanced calcium entry, the rate of calcium uptake was faster compared with control conditions; however, during rapid pacing, cytoplasmic calcium elevation at the endocardium was significantly increased (15+/-4%) compared with control (10+/-3, P<0.04). Rapid pacing induced multiple simultaneous SCR events with largest amplitude and earliest onset near the endocardium compared with the epicardium. Furthermore, SCR events with largest amplitude and earliest onset served as a focus for DAD-mediated triggered activity. Interestingly, polymorphic VT occurred in some experiments when multiple SCR events occurred. In conclusion, multiple, simultaneous SCR events occur over a broad region of relatively slower calcium uptake and elevated diastolic calcium levels. However, SCR events closer to the endocardium have the largest amplitude and earliest onset and are, thereby, more likely to initiate DAD-mediated triggered activity. Finally, multiple SCR events may be a mechanism of polymorphic VT under calcium overload conditions.