Exploring the role of dimethylarginine dimethylaminohydrolase-mediated reduction in tissue asymmetrical dimethylarginine levels in cardio-protective mechanism of ischaemic postconditioning in rats

Objectives: Reperfusion of ischaemic myocardium results in reduced nitric oxide (NO) biosynthesis by endothelial nitric oxide synthase (eNOS) leading to endothelial dysfunction and subsequent tissue damage. Impaired NO biosynthesis may be partly due to increased levels of asymmetrical dimethylarginine (ADMA), an endogenous inhibitor of eNOS. As dimethylarginine dimethylaminohydrolase (DDAH) is a key enzyme responsible for degradation of ADMA, the present study was designed to explore the role of DDAH/ADMA/NO pathway in cardio-protective mechanism of ischaemic postconditioning. Materials and Methods: Isolated rat hearts were subjected to myocardial ischaemia for 30 min followed by reperfusion for 2 hours in control group. Myocardial injury was assessed by measurement of infarct size, left ventricular developed pressure (LVDP), lactate dehydrogenase (LDH) and creatine kinase (CK) enzymes in coronary effluents. The reperfused hearts were homogenised and tissue concentration of nitrite, ADMA level and DDAH enzyme activity was determined. Results: A significant increase in infarct size, LDH, CK release in coronary effluents and ADMA level in myocardial tissue was observed in control group. The increase in tissue ADMA coincided with reductions of NO tissue concentrations and DDAH activity. Ischaemic postconditioning significantly attenuated ischaemia-reperfusion induced myocardial injury manifested in the terms of decreased infarct size, LDH, CK, tissue ADMA along with increase in NO levels and DDAH enzyme activity. Pretreatment with L-Homocysteine (300 microM), a competitive inhibitor of DDAH, and L-NG-nitroarginine methyl ester (L-NAME; 100 microM), an inhibitor of eNOS, completely abolished ischaemic postconditioning-induced myocardial protection. Conclusion: Enhancing DDAH activity by postconditioning may be a novel target to reduce ADMA level and increase NO bioavailability to prevent myocardial ischaemia-reperfusion injury.