Is there a role of inducible nitric oxide synthase activation in the delayed antiarrhythmic effect of sodium nitrite?

Long-term inorganic nitrate administration protects against myocardial ischemia-reperfusion injury in female rats

BACKGROUND

The favorable effects of nitrate against myocardial ischemia-reperfusion injury (MIRI) have primarily focused on male rats and in short term. Here we determine the impact of long-term nitrate intervention on baseline cardiac function and the resistance to MIRI in female rats.

METHODS

Female Wistar rats were randomly divided into untreated and nitrate-treated (100 mg/L sodium nitrate in drinking water for 9 months) groups (n = 14/group). At intervention end, levels of serum progesterone, nitric oxide metabolites (NOx), heart NOx concentration, and mRNA expressions of NO synthase isoforms (NOS), i.e., endothelial (eNOS), neuronal (nNOS), and inducible (iNOS), were measured. Isolated hearts were exposed to ischemia, and cardiac function indices (CFI) recorded. When the ischemia-reperfusion (IR) period ended, infarct size, NO metabolites, eNOS, nNOS, and iNOS expression were measured.

RESULTS

Nitrate-treated rats had higher serum progesterone (29.8%, P = 0.013), NOx (31.6%, P = 0.035), and higher heart NOx (60.2%, P = 0.067), nitrite (131%, P = 0.018), and eNOS expression (200%, P = 0.005). Nitrate had no significant effects on baseline CFI but it increased recovery of left ventricular developed pressure (LVDP, 19%, P = 0.020), peak rate of positive (+ dp/dt, 16%, P = 0.006) and negative (-dp/dt, 14%, P = 0.014) changes in left ventricular pressure and decreased left ventricular end-diastolic pressure (LVEDP, 17%, P < 0.001) and infarct size (34%, P < 0.001). After the IR, the two groups had significantly different heart nitrite, nitrate, NOx, and eNOS and iNOS mRNA expressions.

CONCLUSIONS

Long-term nitrate intervention increased the resistance to MIRI in female rats; this was associated with increased heart eNOS expression and circulating progesterone before ischemia and blunting ischemia-induced increased iNOS and decreased eNOS after MIRI.

Examination of the Changes in Calcium Homeostasis in the Delayed Antiarrhythmic Effect of Sodium Nitrite

We have evidence that the intravenous infusion of sodium nitrite (NaNO₂) results in an antiarrhythmic effect when given 24 h prior to an ischemia and reperfusion (I/R) insult in anaesthetized dogs. This protection was associated with the reduction of reactive oxygen species resulting from I/R through the attenuation of mitochondrial respiration. Here, we examined whether the changes in calcium, which also contributes to arrhythmia generation, play a role in the NaNO₂-induced effect. On the first day, 30 anaesthetized dogs were treated either with saline or NaNO₂ (0.2 µmol/kg/min) for 20 min. Some animals were subjected to a 25 min LAD (anterior descending branch of the left coronary artery) occlusion and 2 min reperfusion (I/R = 4; NaNO₂-I/R = 6), or the heart was removed 24 h later. We have shown that nitrite prevented the I/R-induced increase in cellular and mitochondrial calcium deposits. During simulated I/R, the amplitude of the calcium transient and the diastolic calcium level were significantly lower in the nitrite-treated hearts and the ERP (effective refractory period) fraction of the action potential was significantly increased. Furthermore, nitrite also enhanced the mitochondrial respiratory response and prevented the MPTPT opening during calcium overload. These results suggest that nitrite can reduce the harmful consequences of calcium overload, perhaps directly by modulating ion channels or indirectly by reducing the mitochondrial ROS (reactive oxygen species) production.

Examination of the Role of Mitochondrial Morphology and Function in the Cardioprotective Effect of Sodium Nitrite Administered 24 h Before Ischemia/Reperfusion Injury

Background: We have previous evidence that in anesthetized dogs the inorganic sodium nitrite protects against the severe ventricular arrhythmias, resulting from coronary artery occlusion and reperfusion, when administered 24 h before. The present study aimed to examine, whether in this effect changes in mitochondrial morphology and function would play a role. Methods: Thirty dogs were infused intravenously either with saline (n = 15) or sodium nitrite (0.2 μmol/kg/min; n = 15) for 20 min, and 24 h later, 10 dogs from each group were subjected to a 25 min period of occlusion and then reperfusion of the left anterior descending coronary artery. The severity of ischaemia and ventricular arrhythmias were examined in situ. Left ventricular tissue samples were collected either before the occlusion (5 saline and 5 nitrite treated dogs) or, in dogs subjected to occlusion, 2 min after reperfusion. Changes in mitochondrial morphology, in complex I and complex II-dependent oxidative phosphorylation (OXPHOS), in ATP, superoxide, and peroxynitrite productions were determined. Results: The administration of sodium nitrite 24 h before ischemia/reperfusion significantly attenuated the severity of ischaemia, and markedly reduced the number and incidence of ventricular arrhythmias. Nitrite also attenuated the ischaemia and reperfusion (I/R)-induced structural alterations, such as reductions in mitochondrial area, perimeter, and Feret diameter, as well as the increase in mitochondrial roundness. The administration of nitrite, however, enhanced the I/R-induced reduction in the mitochondrial respiratory parameters; compared to the controls, 24 h after the infusion of nitrite, there were further significant decreases, e.g., in the complex I-dependent OXPHOS (by -20 vs. -53%), respiratory control ratio (by -14 vs. -61%) and in the P/E control coupling ratio (by 2 vs. -36%). Nitrite also significantly reduced the I/R-induced generation of superoxide, without substantially influencing the ATP production. Conclusions: The results suggest that sodium nitrite may have an effect on the mitochondria; it preserves the mitochondrial structure and modifies the mitochondrial function, when administered 24 h prior to I/R. We propose that nitrite affects primary the phosphorylation system (indicated by the decreased P/E ratio), and the reduction in superoxide production would result from the subsequent suppression of the ROS producing complexes; an effect which may certainly contribute to the antiarrhythmic effect of nitrite.