Introduction: Diabetic cardiomyopathy is an important factor in increasing the risk of heart failure and mortality in people with diabetes. Cathepsin D has been shown to play a crucial role in maintaining multiple physiological functions, including the degradation of intracellular proteins and enzyme precursors. This research aimed to investigate the combined effects of vitamin D3 supplementation and aerobic exercise on cardiac cathepsins and insulin resistance in diabetic rats induced by the combination of high-fat diet and streptozotocin (HFD-STZ). Materials and Methods: In 40 male Wistar rats, type 2 diabetes was induced by a high-fat diet fed+ streptozotocin, and they were divided into 5 groups: 1) Normal control (NC), 2) Diabetic control (DC), 3) Diabetes+Aerobic training (DAT), 4) Diabetes+Vitamin D3 (DVD), and 5) Diabetes+Aerobic training and Vitamin D3 (DVDAT). After eight weeks of aerobic training and vitamin D3 supplementation, cathepsin D protein content of heart tissue and serum levels of insulin, glucose, vitamin D3, and insulin resistance index were measured. Results: One-way ANOVA showed that cathepsin D content increased by 335% in the DC group compared to the NC group (P<0.0001). In contrast, there were significant decreases in the DAT, DVD, and DVDAT groups (44.37%, 50.80%, and 62.76%, respectively) compared to the DC group (P<0.0001). The HOMA-IR index also decreased in the DAT, DVD, and DVDAT groups by 52.95%, 39.27%, and 63.68%, respectively (P<0.0001). Conclusion: Eight weeks of aerobic training with vitamin D3 supplementation improves glucose metabolism in diabetic rats induced by HFD-STZ, probably via reducing cardiac tissue level of cathepsin D and insulin resistance index.
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