The main difference between the already existing and improved methods was the use of PVC tubes in the ligation process. In the existing surgery method, the myocardial tissue was ligated using the 6-0 silk suture only, which induced damage to the myocardium during ligation resulting in intraoperative death. Moreover, the pulsation of the heart would loosen the slipknot. In contrast, in the improved method with the PVC tube, the slipknot placed in the groove of the tube could be tightened, and the area of the myocardium affected by ligation increased. These benefits were observed during the experimental procedure and confirmed by the TTC staining and percent survival findings.
The critical step of the improved surgery method was placing the soft tube on the proximal LAD coronary artery, accompanied by nerves, lymphatic vessels, and myocardial tissue during ligation in the ischemic period. This pre-prepared soft tube can act as a cushion that protects the peripheral tissues (nerves, myocardia, and lymphatic vessels) and decreases mortality during coronary artery ligation. The surgery performed by the already existing method was similar to the surgery for myocardial infarction. The percent survival findings indicated that rats in the existing model group mainly died during the ischemic period (two rats died at 2 min post-ligation, and two rats died at 45 min post-ligation). Otherwise, the underlying causes of death are still unclear, and there are a series of hypotheses, including additional damage to the nervous structures23, lymphatic vessels, and myocardia.
Regarding nervous damage, previous studies have indicated that during the ischemic period in the animal model, besides the direct local effects of ischemia on the nervous structures, there is also probably a significant decrease in neuropeptide Y (NPY) levels that contribute to disturbances in axoplasmic transport in the sympathetic innervation24. This finding agrees with results reported by Han et al.25, who revealed that a gradual disappearance of NPY occurred within the infarcted myocardium after ligation of the LAD coronary artery in rats. However, the role of NPY in this context remains unclear. Its deletion attenuates cardiac dysfunction and apoptosis during acute myocardial infarction26, and is associated with arrhythmia27, high blood pressure, and coronary microvascular function28.
Furthermore, adverse obstruction of cardiac lymph flow occurred during the ischemic period, leading to severe cardiac edema, left dysfunction, and hemorrhages29, which might be another cause of death in rats. During this pathological process, the ligature of the LAD coronary artery might be attributed to the obstruction of coronary arteries or cardiac lymphatic transport within the infarct area, which can cause additional complications, such as adverse remodeling of the epicardial collector lymphatics, reduced lymphatic flow, and persistent edema30.
Therefore, circulation in lymphatic vessels plays a functional role in cardiac homeostasis31 and wound healing32, and the percent survival findings in this study suggest that the improved MIRI surgical procedure might avoid lymphatic damage and promote lymphatic reperfusion by placing the soft tube on the LAD coronary artery during ligature. In comparison, the existing surgery method is more likely to tear the heart muscle and cause a massive hemorrhage during ligation of the LAD coronary artery, without the cushioning effect of the soft tube. Additionally, the pre-prepared soft tube diameter was much larger than the 6-0 silk suture, and the tube may have contracted and induced a larger infarct size when the slipknot was tied to the tube during the ischemic period.
This study had a few limitations. The infarct size of the heart was analyzed in the preliminary experiment. The substitution formula (N = 7.75) was calculated using a previously reported equation33. Considering the possible death of rats during the operation, N was raised by 25%; hence, n = 10 (ten rats for each group) was decided. Otherwise, the already existing method to generate the MIRI model had a high mortality rate. Therefore, few cases (low sample size) in the experimental model group influenced the statistical findings. Several assessments, including echocardiography30, Evans blue staining34, and the myocardial enzyme measurement35, were essential for cardiac function evaluation and analysis. Owing to the low sample size of this work, these assessments were not performed and will be described in a future study of pharmacodynamic research in MIRI. However, considering that the existing surgical procedure to generate the MIRI model is associated with extensive myocardial damage, it is worthwhile to report this present method to improve the modeling of MIRI in rats and bring light to this preclinical model that correctly simulates ischemic heart disease.
In conclusion, the improved surgery method to generate the MIRI model had a higher survival rate, an elevated ST-T segment, and a larger infarct size than the existing MIRI model generation method, suggesting that the improved model better simulates MIRI pathology.