Background and Purpose: Exercise in hot environments increases core body temperature and increases the risk of heat-related illnesses, which can lead to negative physiological and functional effects, including damage to the heart muscle. Therefore, identifying and investigating effective strategies to reduce these consequences is of great importance. The present study aimed to investigate the effect of selenium supplementation on indices of heart muscle damage after a period of exercise in a hot environment in rats.

Materials and Methods: This experimental study was conducted on 36 adults male Wistar rats (8 weeks old, weighing 225±25 g). The animals were randomly divided into four groups of 8, including control, exercise in a normal environment, exercise in a warm environment, and exercise in a warm environment with selenium supplementation. The exercise protocol included endurance exercise on a treadmill for 8 weeks (6 sessions per week) at 36±1°C. Supplementation with selenium at a rate of 45.0 mg/kg was administered via pellet. After the end of the period, the animals were killed and samples were taken. The expression levels of CTnI and CK-MB genes were examined by Real-time RT-PCR and hematoxylin-eosin staining. Data analysis was performed with the nonparametric Kruskal-Wallis test and Dunn's post hoc test at a significance level of p≤0.05.

Results: The findings of the present study showed that heat stress caused by exercise in a hot environment significantly decreased cTnI gene expression (p < 0.01) and significantly increased CK-MB gene expression (p < 0.01). The level of cTnI gene expression in the HS group remained at a level close to the control group (p > 0.05), indicating that selenium supplementation effectively neutralized this decrease and maintained gene expression at a level close to baseline (control) conditions. However, the level of CK-MB gene expression in the HS group did not provide a complete protective effect. Based on the histological examination of the myocardium by H&E staining, it was determined that the control group had a healthy and regular tissue structure. Exercise in a normal environment caused mild cellular changes, while exercise in a hot environment caused severe myocardial damage, including extensive necrosis, edema, and fiber misalignment. The use of percooling or selenium supplementation alone had a partial protective effect, but the greatest protection was observed when percooling and selenium supplementation were applied simultaneously; in this group, necrosis was localized, mild hemorrhage, and the overall tissue structure was preserved, indicating a significant reduction in damage caused by thermal stress and an approximation of the tissue state to normal conditions.

Conclusion: Training in a hot environment can lead to heat stress and heart damage, but selenium supplementation plays an effective role in reducing these damages and improving physiological responses.