Acute ischemia of canine interventricular septum produces asymmetric suppression of conduction

Ankyrin-G mediates targeting of both Na+ and KATP channels to the rat cardiac intercalated disc

We investigated targeting mechanisms of Na⁺ and K_ATP channels to the intercalated disk (ICD) of cardiomyocytes. Patch clamp and surface biotinylation data show reciprocal downregulation of each other’s surface density. Mutagenesis of the Kir6.2 ankyrin binding site disrupts this functional coupling. Duplex patch clamping and Angle SICM recordings show that I_Na and I_KATP functionally co-localize at the rat ICD, but not at the lateral membrane. Quantitative STORM imaging show that Na⁺ and K_ATP channels are localized close to each other and to AnkG, but not to AnkB, at the ICD. Peptides corresponding to Nav1.5 and Kir6.2 ankyrin binding sites dysregulate targeting of both Na⁺ and K_ATP channels to the ICD, but not to lateral membranes. Finally, a clinically relevant gene variant that disrupts K_ATP channel trafficking also regulates Na⁺ channel surface expression. The functional coupling between these two channels need to be considered when assessing clinical variants and therapeutics.

KATP Channels in the Cardiovascular System

KATP channels are integral to the functions of many cells and tissues. The use of electrophysiological methods has allowed for a detailed characterization of KATP channels in terms of their biophysical properties, nucleotide sensitivities, and modification by pharmacological compounds. However, even though they were first described almost 25 years ago (Noma 1983, Trube and Hescheler 1984), the physiological and pathophysiological roles of these channels, and their regulation by complex biological systems, are only now emerging for many tissues. Even in tissues where their roles have been best defined, there are still many unanswered questions. This review aims to summarize the properties, molecular composition, and pharmacology of KATP channels in various cardiovascular components (atria, specialized conduction system, ventricles, smooth muscle, endothelium, and mitochondria). We will summarize the lessons learned from available genetic mouse models and address the known roles of KATP channels in cardiovascular pathologies and how genetic variation in KATP channel genes contribute to human disease.

ST-segment elevation of right precordial lead (V4 R) is associated with multivessel disease and increased in-hospital mortality in acute anterior myocardial infarction patients

BACKGROUND

ST segment elevation of chest lead V4 R is associated with worse prognosis in acute inferior ST-elevation myocardial infarction (STEMI). This study tried to determine the relationship between ST elevation in the right precordial lead V4 R and acute anterior STEMI.

METHODS

Prospective study of 144 consecutive anterior STEMI patients: all had 15-lead ECG recordings (12 conventional leads and V3 R-V5 R) obtained. Patients were classified into two groups on the basis of presence (Group I, 50 patients) or absence (Group II, 94 patients) of ST-segment elevation ≥0.5 mm in lead V4 R.

RESULTS

Multivessel involvement was significantly higher in Group I compared with Group II (54% and 23% respectively, P < 0.001). Major adverse cardiac events and in-hospital mortality was also significantly higher for those in Group I (P < 0.02 for both). A significant correlation was found between in-hospital mortality and those in Group I (P = 0.03, OR: 6.27, CI: 1.22-32.3). There was an independent relationship between in-hospital mortality and V4 R-ST elevation (P = 0.03, OR: 11.64, CI: 1.3-27.4).

CONCLUSION

ST segment elevation in chest lead V4 R is associated with multivessel disease and increased in-hospital mortality in patients with anterior STEMI that had undergone primary percutaneous coronary intervention to the left anterior descending artery.

Interventricular heterogeneity as a substrate for arrhythmogenesis of decoupled mitochondria during ischemia in the whole heart

Myocardial ischemia results in metabolic changes, which collapse the mitochondrial network, that increase the vulnerability of the heart to ventricular fibrillation (VF). It has been demonstrated at the single cell level that uncoupling the mitochondria using carbonyl cyanide p-(tri-fluoromethoxy)phenyl-hydrazone (FCCP) at low concentrations can be cardioprotective. The aim of our study was to investigate the effect of FCCP on arrhythmogenesis during ischemia in the whole rabbit heart. We performed optical mapping of isolated rabbit hearts ( n = 33) during control and 20 min of global ischemia and reperfusion, both with and without pretreatment with the mitochondrial uncoupler FCCP at 100, 50, or 30 nM. No hearts went into VF during ischemia under the control condition, with or without the electromechanical uncoupler blebbistatin. We found that pretreatment with 100 ( n = 4) and 50 ( n = 6) nM FCCP, with or without blebbistatin, leads to VF during ischemia in all hearts, whereas pretreatment with 30 nM of FCCP led to three out of eight hearts going into VF during ischemia. We demonstrated that 30 nM of FCCP significantly increased interventricular (but not intraventricular) action potential duration and conduction velocity heterogeneity in the heart during ischemia, thus providing the substrate for VF. We showed that wavebreaks during VF occurred between the right and left ventricular junction. We also demonstrated that no VF occurred in the heart pretreated with 10 μM glibenclamide, which is known to abolish interventricular heterogeneity. Our results indicate that low concentrations of FCCP, although cardioprotective at the single cell level, are arrhythmogenic at the whole heart level. This is due to the fact that FCCP induces different electrophysiological changes to the right and left ventricle, thus increasing interventricular heterogeneity and providing the substrate for VF.

Left-to-right ventricular differences in I(KATP) underlie epicardial repolarization gradient during global ischemia

BACKGROUND

The ionic mechanisms of electrical heterogeneity in the ischemic ventricular epicardium remain poorly understood.

OBJECTIVE

This study sought to test the hypothesis that the adenosine triphosphate (ATP)-activated K+ current (I(KATP)) plays an important role in mediating repolarization differences between the right ventricle (RV) and left ventricle (LV) during global ischemia.

METHODS

Electrical activity in Langendorff-perfused guinea pig hearts was recorded optically during control, ischemia, and reperfusion. Patch-clamp experiments were used to quantify I(KATP) density in isolated myocytes. Molecular correlates of I(KATP) (Kir6/SUR) were probed via reverse transcriptase-polymerase chain reaction. The role of I(KATP) in modulating repolarization was studied using computer simulations.

RESULTS

Action potential duration (APD) was similar between LV and RV in control hearts, but significantly different in global ischemia. Pretreatment of hearts with 10 μM glibenclamide (I(KATP) blocker) abolished the APD gradient during ischemia. In the absence of ischemia, pinacidil (I(KATP) opener) tended to shorten the APD more in the LV, and caused a small but significant increase in APD dispersion. In voltage clamp experiments, the density of the whole-cell current activated by pinacidil at depolarized potentials was significantly larger in LV, compared with RV epicardial myocytes. The mRNA levels of Kir6.1/Kir6.2 were significantly higher in LV compared with RV. Simulations showed that I(KATP) is the main determinant of LV-RV APD gradient, whereas cell-to-cell coupling modified the spatial distribution of this APD gradient.

CONCLUSION

I(KATP) is an important determinant of the epicardial LV-RV APD gradient during global ischemia, in part due to a higher current density and molecular expression in the LV.

Transseptal dispersion of repolarization and its role in the development of Torsade de Pointes arrhythmias

OBJECTIVE

This study was designed to quantitate transseptal dispersion of repolarization (DR) and delineate its role in arrhythmogenesis using the calcium agonist BayK 8644 to mimic the gain of function of calcium channel current responsible for Timothy syndrome.

BACKGROUND

Amplification of transmural dispersion of repolarization (TDR) has been shown to contribute to development of Torsade de Pointes (TdP) arrhythmias under long-QT conditions.

METHODS

An arterially perfused septal wedge preparation was developed via cannulation of the septal artery. Action potentials (APs) were recorded using floating microelectrodes together with a transseptal electrocardiogram (ECG). These data were compared to those recorded from arterially perfused canine left ventricular (LV) wedge preparations.

RESULTS

Under control conditions, the shortest AP duration measured at 90% repolarization (APD(90)) was observed in right ventricular (RV) endocardium (181.8 +/- 15 ms), APD(90) peaked close to midseptum (278.0 +/- 32 ms), and abbreviated again as LV endocardium was approached (207.3 +/- 9 ms). Transseptal DR averaged 106 +/- 24 ms and T(peak)-T(end) 84 +/- 7 ms (n = 6). TDR and T(peak)-T(end) recorded from LV wedge were 36 +/- 9 ms and 34 +/- 19 ms, respectively (n = 30). BayK 8644 increased transseptal DR to 123.2 +/- 35 ms (n = 5) and induced early and delayed afterdepolarizations (3/5), rate-dependent ST-T-wave alternans (5/5), and TdP arrhythmias (3/5).

CONCLUSIONS

Our data indicate that dispersion of repolarization across the interventricular septum is twice that of the LV free wall, predisposing to development of TdP under long-QT conditions. Our findings suggest that the coronary-perfused ventricular septal preparation may be a sensitive model in which to assess the potential arrhythmogenic effects of drugs and pathophysiological conditions.