Objective To explore the protective effects of L-arginine(L-Arg) on the myocardial tissue of diabetic rats.Methods The rat models of diabetes mellitus(DM)(n=28) were made by intraperitoneal injection of streptozotocin(60 mg/kg),and allocated into DM group(n=14) and L-Arg treatment group(300 mg/(kg·d))(L-Arg group)(n=14) at random.Another 12 normal rats served as normal control group(NC group).After 12 weeks,the rats were sacrificed.Morphological changes in myocardium in 3 groups were observed under transmission electromicroscope(TEM).The mRNA expression levels of endothelial nitric oxide synthase(eNOS),inducible NOS(iNOS) and endothelin(ET-1) in myocardial tissue were determined by reverse transcription polymerase chain reaction(RT-PCR).The activity of eNOS,iNOS,Na+-K+-ATPase,Ca2+-ATPase and the content of NO,ET-1 in myocardial homogenate were also determined.Results In DM rats,TEM examination showed the lysis of myofibril,dilation of sarcoplasmic reticulum and swelling of mitochondria of myocardial tissue.Compared with NC rats,the mRNA expression level of eNOS and the activity of Na+-K+-ATPase,Ca2+-ATPase,as well as the NO content decreased,but the mRNA expression and content of ET-1 increased in DM rats.The pathological changes in myocardium induced by intraperitoneal injection of streptozotocin were reversed in L-Arg group.Compared with DM rats,the mRNA expression level of eNOS,the activity of Na+-K+-ATPase,Ca2+-ATPase and the content of NO increased,but the mRNA expression and content of ET-1 decreased in L-Arg group.Conclusion L-Arg exerts protective effects on diabetic myocardium,which may be through increasing the NO content and decreasing the ET-1 content,so as to improve the blood supply of myocardial tissue.