This animal model simulates the type I diabetic model of human disease, the advantages of this model not only avoid the effect of various confounding factors in patients on diabetic cardiomyopathy, such as age, the course of the disease, and coexisting diseases, but also obtain the imaging indicators of myocardial fibrosis by non-invasive imaging method and compare them with myocardial histological alterations to insight into the pathogenesis of DCM. The main findings of this study were that 1) When 3 months of diabetic model, a longer duration of diabetes resulted in a higher myocardial T1 value, ECV and LVM, while left ventricular GLS decreased with the prolongation of modeling time, and 2) at 6 months, abnormalities were observed in EAT compared with before modeling.
T1 mapping can early monitor myocardial focal and diffuse fibrosis by quantifying myocardial T1 and extracellular volume values, and have been confirmed in previous histological studies[13]. At present, they have important value in the diagnosis, treatment and prognosis evaluation of heart disease. Our results showed that T1 and ECV values of type 1 DM animal models increased gradually with the prolongation of DM course after 3 months of modeling, which was consistent with the previous experimental results on rabbits[14]. There are many factors causing this myocardial damage, including many molecular mechanisms, animal studies in vivo have shown that hyperglycemia leads to cardiomyocyte apoptosis through the activation of reactive oxygen species (ROS)[15], which in turn can induce the formation of end glycation end products and eventually lead to myocardial fibrosis[16]. Modeling for 3 months, T1 value and ECV value gradually increased with the extension of diabetes duration, which was consistent with the results of Zeng[14], indicating that the longer the course of diabetes, the higher the degree of myocardial fibrosis. One possible explanation is that long term chronic hyperglycemia may aggravate myocardial interstitial matrix expansion, and promote myocardial apoptosis and necrosis, and eventually lead to continuous myocardial injury.
In this study, it was found that the LVM was increased after modeling 1.5 month, and similar results were obtained by Nakai et al[17]. We also found that LV remodeling index (LV mass/volume ratio) was increased after modeling 4.5 month. This may be related to the concentric LV remodeling due to myocardial hypertrophy caused by diabetes[18, 19]. According to relevant reports, the pathogenesis of diabetic left ventricular hypertrophy was related to diabetic myocardial interstitial fibrosis and myocardial triglyceride accumulation[20, 21]. In our this experiment, diffuse fibrosis of myocardial interstitium has been confirmed in the histological analysis, so next we will expand our sample size, and conduct anti-myocardial fibrosis therapy in diabetic pigs to observe the histological changes of isolated heart to verify whether the above treatment can protect or reverse myocardial damage.
EAT comprises adipocytes, stromo-vascular cells, neurons, and immune cells[22, 23],it’s is metabolically very active, and the secretome of EAT disrupts cardiomyocyte metabolism[24], depresses cardiomyocyte contractile function[5] and alters expression of adhesion markers of primary cardiac endothelial cells[25]. In our study, we also find the EAT in diabetic model was significantly increased when 6 months of diabetic model. It's been reported previously that the amount of EAT is increased in patients with T2DM[26] and the high level of EAT is associated with cardiac systolic dysfunction in patients with T2DM[27]. Thus, in our study, at 6 months, whether dysfunction of cardiomyocyte contractile function(LVGLS) can be ascribed to the true effect of DM or to a confounding effect of DM and EAT still requires further study.
At 3 months, CMR-FT was used to find that the value of LVGLS began to decreased, while other strain parameters remained unchanged. The myocardium consists of 3 layers[28]. The orientation of left ventricle subendocardial muscle fibers is longitudinal, corresponds to longitudinal strain, and is more susceptible to ischemia and to other mechanical, toxic, or metabolic factors[29, 30]. Thus, the vaule of LVGLS decreased may document the subendocardial dysfunction although the global cardiac function remains normal, as assessed by standard EF. This shows that CMR-FT is a very sensitive tool to detect subtle endocardial impairment in the early stages of DM. In addition, this study also found that the longer the course of diabetes, the lower the value of LVGLS, indicating the more serious the degree of endocardial damage. Therefore, effective reduction of blood glucose level is essential to protect the myocardium. In the following experiments, we will also conduct hypoglycemic treatment on diabetic pigs to observe whether myocardial mechanical injury can be reversed or ameliorated, so as to provide an important theoretical basis for clinical improvement of cardiac function in patients with DCM.
In this study, two limitations should be considered. First, our animal sample size was small, which greatly reduced the power of the study and did not allow us to draw generalized conclusions. We will continue to expand our sample size in our future studies on this topic. Second, Our study was unable to carry out serial histological validation evaluation at various timepoints in type 1 DM animal models because pigs are large animals and limited by many experimental conditions.