الفهرس 
IntroductionOnly 14 pages are availabe for public view
 |  | from | 163 |  |  |
| | | from | 163 | | |
Abstract
Cardiovascular diseases (CVDs) are one of the leading causes of death in the world (Parlakpinar et al., 2013). Organic nitrates, particularly NG, has long been one of the key therapies for CVDs including coronary artery disease, acute myocardial infarction and congestive heart failure (Munzel et al., 2013). The ability of mammalian cells to convert NG to vaso-relaxant NO played a significant part in the discovery of the unique role of both NO and NG in acute angina and congestive heart disease (Fung, 2004).
Despite of the clear benefits of NG treatment, yet it rapidly loses its hemodynamic and anti-ischemic clinical effectiveness when used as long-term therapy and this phenomenon is referred as nitrate tolerance (Parker, 2004). Moreover, acute high-dose of organic nitrates is likely to cause a similar loss of potency in the form of tachyphylaxis (Daiber and Munzel, 2015).
This multifactorial phenomenon is likely related to impairment in NG biotransformation by inhibiting ALDH2 and generation of reactive oxygen species (ROS) as superoxide (O2.-) and reactive nitrogen species (RNS) as peroxynitrite (ONOO−) anions diminishing NO bioavailability (Gongadze et al., 2016).
A cross talk between ROS, mitochondrial and endothelial dysfunction was documented in nitrate tolerance process. In this scenario, substantial interplay occurs between sources generating ROS including uncoupled mitochondrial respiratory chain (Fridovich, 2004), activated nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (Griendling et al., 2000) and uncoupled NO synthase (Landmesser et al., 2003).
Metformin, an oral hypoglycemic drug, showed cardioprotective effect against I/R injury independent of its hypoglycemic actions (Whittington et al., 2013). It displayed antioxidant potentials through activating 5’ adenosine monophosphate-activated protein kinase (AMPK) essential in maintaining intracellular redox status (Wang et al., 2012), inhibiting NADPH oxidases and increasing SOD expression (Wang et al., 2012).
The present work was sought to study firstly, the effect of continuous pretreatment with 3 different doses of NG (25, 50, 100mg/kg) on cardiac function, oxidative stress, mitochondrial function, endothelial function and myocardial infarction severity following induction of I/R model in male albino rats. Additionally, based on the cardioprotective and antioxidant effects of metformin, our second aim was to explore the potential beneficial effect of concurrent metformin administration to NG.
The present study was conducted on male Albino rats that were randomly allocated into 2 main groups: Sham group (10 rats) and I/R group: which was further subdivided into 8 groups (10 rats each) that underwent ischemia for 35 minutes through ligation of left anterior descending (LAD) coronary artery, and then reperfusion for 60 minutes by releasing the ligation after end of treatment and before scarification (Chen et al., 2008): Untreated group; NG 25, 50, 100 mg groups (receiving NG three times/day for three successive days); metformin group (receiving metformin 300mg/kg/day for two weeks); NG 25, 50, 100mg+metformin groups (receiving metformin for 2 weeks and NG in the last 3 days).
The experimental work was divided into: cardiac hemodynamics measurement: (Left ventricular end diastolic pressure (LVEDP) & left ventricular delta pressure/delta time (LV dP/dtmax)), Biochemical studies: (Measurement of cardiac ALDH2, Measurement of cardiac 4HNE and SOD to evaluate oxidative stress, Mitochondrial dysfunction evaluated by measuring cardiac mitochondrial complex I and cytochrome C, Endothelial dysfunction evaluated by measuring aortic ET1 and NO). Histopathological studies of heart and aorta: (Aortic tunica intima length and tunica intima derangement was assessed, measurement of infarct size grossly and myocardial infarction (MI) severity score microscopically).
The results of the present work could be summarized as follow: rats exposed to I/R exhibited a significant deterioration of all parameters (cardiac hemodynamics, infarction size, markers of oxidative stress, mitochondrial and endothelial functions). Regarding the three doses of NG, NG 25 mg improved most of the parameters except markers of mitochondrial dysfunction (complex I and cytochrome C) and NO. NG 50 mg produced partial deterioration of measured parameters as it insignificantly affected LVEDP, ALDH2, SOD, cytochrome C and myocardial infarction score but significantly decreased cardiac contractility, increase 4HNE, decrease mitochondrial complex I, increased ET1, decreased NO and increased both infarction size and tunica intima length. NG 100 mg produced complete deterioration of all parameters. Metformin per se improved all parameters except SOD. Metformin+ NG 25mg showed additive improvement in all parameters when compared to I/R untreated group and NG 25 mg group, while metformin plus NG 50 or 100 mg improved all parameters when compared to I/R untreated group and their corresponding NG group except for SOD level when metformin combined with NG 100mg/kg showed insignificant effect in comparison to I/R untreated group.
Conclusion:
NG pretreatment for 3 consecutive days/ 3 times daily showed dose dependent effect on cardiac hemodynamic, oxidative stress parameters, mitochondrial function, endothelial function and myocardial infarction severity following induction of myocardial ischemia reperfusion in male albino rats compared to I/R untreated group. The least deleterious effects were reported with NG dose 25 mg/kg.
Metformin co-administration to NG succeeded to ameliorate the deleterious effects reported with NG. The effect of metformin could be explained by its ability to increase ALDH2 which is important for NG bioactivation, decrease 4HNE, preserve mitochondrial and endothelial functions and decreased myocardial infarction severity when compared to I/R untreated group.
Clinical implications: results of the present study highlighted the issue of using nitrate for treatment of ischemia associated with development of myocardial infarction and if used, the exact dose and duration require further investigation. Another important point raised by the results of the current study emphasizing the beneficial effect of adding metformin to NG in this I/R animal model. Therefore, the beneficial effect of co-administration of metformin to NG need further clinical assessment to access these points.