Regional differences of superoxide dismutase activity enhance the superoxide–induced electrical heterogeneity in rabbit hearts

Differential effects of superoxide radical on the action potentials in ventricular muscles, Purkinje fibers and atrial muscles in the heart of different aged rats

The purpose of this study was to determine whether the effects of the superoxide donor menadione on myocardial electrical activity were regional and/or age-dependent. Action potentials were recorded in cardiac muscles that were isolated from 18- and 49-day old rat hearts, using a conventional microelectrode technique. Superoxide dismutase (SOD) activity was determined spectrophotometrically in regional cardiac muscles. Menadione (30 microM) significantly increased the action potential duration at 90% repolarization (APD90) in Purkinje fibers and ventricular muscles from 18-day old rats; and it decreased the APD in ventricular muscles from 49-day old rats. ATP-sensitive K+ (KATP) channel opener pinacidil blocked menadione-induced effects on the APD90 of Purkinje fibers and ventricular muscles from 18-day old rats, but did not block menadione-induced effects on the APD90 of Purkinje fibers and ventricular muscles from 49-day old rats. The cGMP-dependent protein kinase (PKG) activator or inhibitor did not inhibit the menadione-induced effects on APD90 of ventricular muscles and Purkinje fibers from 18- and 49-day old rats. The superoxide dismutase (SOD) activities in ventricular muscles from 4-, 18- and 49-day old rats, were: 1185 +/- 135, 1434 +/- 96, and 1760 +/- 144 U/mg protein, respectively; whereas, the SOD activities in atrial muscles, were: 1135.4 +/- 156.4, 1145.4 +/- 148.8, and 1243.5 +/- 175.2 U/mg protein, respectively. However, the ventricular SOD activities were significantly inhibited under hypoxic conditions. These results indicate that menadione can produce regionally differential effects on cardiac APD, which may be due to differences in the SOD activity, depending on the region and age of the cardiac tissue.

Dynamic changes in nitric oxide and mitochondrial oxidative stress with site-dependent differential tissue response during anoxic preconditioning in rat heart

In this study, dynamic changes in nitric oxide (NO) and mitochondrial superoxide (O2•−) were examined during anoxic preconditioning (AP) in rat heart model. AP and anoxia-reoxygenation (A/R) were performed on isolated hearts and single cardiomyocytes. The cellular insult in the form of infarct size and DNA damage were localized and correlated with NO synthases (endothelial and inducible) expression levels. The results showed that endocardium was the most affected region in AP groups, whereas the larger area of infarct was confined to mid- and epicardium in the A/R group. Interestingly, a high-level expression of immunofluorescent NO synthases was restricted to viable areas in the AP. In contrast to the gradual increase in O2•− level that occurred in the AP group, a sudden massive increase in its level was demonstrated at the onset of reoxygenation in the A/R group. The observed increase in NO production during reoxygenation in the AP group was attenuated by inducible NO synthase inhibitor. The study revealed, on a real-time basis, the role played by preconditioning for modulating NO and O2•− levels on behalf of cell survival. The results afford a better understanding of cardiac-adapting mechanism during AP and the role of inducible NO synthase in this important phenomenon.

Sodium Channel Gating Modes During Redox Reaction

BACKGROUND/AIMS

Many studies have confirmed that persistent sodium current (I(NaP)) is altered during a redox reaction, but little attention has been paid to transient sodium current (I(NaT)) and its correlation with I(NaP) during the redox reaction. The aim of the study was to investigate the effect of the redox states on the correlation between I(NaT) and I(NaP) in cardiomyocytes.

METHODS

I(NaT) and I(NaP) were recorded using whole-cell and cell-attached patch-clamp techniques in guinea pig ventricular myocytes.

RESULTS

In whole-cell recordings, dithiothreitol (DTT, 1 mM) simultaneously increased I(NaT) and decreased I(NaP). Hydrogen peroxide (H(2)O(2), 0.3 mM) increased I(NaP) and decreased I(NaT) in a time-dependent manner, which were reversed by DTT (1 mM). In cell-attached recordings, the increasing of I(NaP) and decreasing of I(NaT) induced by H(2)O(2) (0.3 mM) were similarly recovered by DTT (1 mM). H(2)O(2) (0.3 mM) prolonged the action potential (AP) duration of ventricular papillary cells whereas decreased the AP amplitude and maximum rate of depolarization (V(max)) in a time-dependent manner, which were reversed by DTT (1 mM).

CONCLUSION

These results indicate that the redox states could modulate the sodium channel gating modes in guinea pig ventricular myocytes.

Lovastatin specifically prevents focal ischemic ventricular tachycardia due to triggered activity

BACKGROUND

Use of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor has been associated with reduced implantable defibrillator shocks in several multicenter trials, suggesting an antiarrhythmic effect.

OBJECTIVE

The purpose of this study was to determine if lovastatin had an antiarrhythmic effect in a canine model of ischemic and inducible ventricular tachycardia (VT).

METHODS

Forty-seven alpha-chloralose anesthetized dogs underwent left anterior descending coronary occlusion. Three-dimensional activation mapping identified the mechanism of reinducible VT and the response to lovastatin (0.5 mg/kg IV). The endocardium was excised from foci and studied using standard microelectrode techniques with Tyrode's solution.

RESULTS

Lovastatin blocked focal VT in 8 of 13 dogs (P <.01) compared with only 1 of 12 saline-treated dogs with focal VT. Lovastatin had no effect on reentrant VT. Lovastatin did not alter the effective refractory period, arterial pressure, or percentage of ischemic electrograms. Effective plasma concentration of lovastatin hydroxy acid ranged from 21-157 ng/mL (0.8-3.7 x 10(-7) M). In vitro rapid pacing, mostly with isoproterenol (5 x 10(-7) M) superfusion, produced delayed afterdepolarizations and triggered activity (9 +/- 2 action potentials). Lovastatin (10(-7) M) produced no change in action potentials or delayed afterdepolarizations. However, triggered activity was attenuated to 2 +/- 1 action potentials with lovastatin (P <.05, n = 13) but not with vehicle alone. Triggered activity returned to control after lovastatin washout (20 minutes) as well as with co-superfusion with mevalonic acid (10(-6) M, n = 5). 2,2,6,6-Tetramethylpiperidine-N-oxyl, an antioxidant that enters tissues (10(-3) M, n = 8), prevented triggered activity in a fashion similar to lovastatin.

CONCLUSION

Lovastatin, in concentrations achievable in human plasma, specifically suppresses triggered activity and focal VT due to ischemia. A prenylated protein downstream from mevalonic acid may act as an antioxidant, producing the antiarrhythmic effect.