Antioxidant Defenses in the Kidneys and Heart of the Freshwater Fish <i>Astyanax lacustris</i> Subjected to High (31°C) and Low (15°C) Temperatures

Aquatic ecosystems have their abiotic and biotic factors constantly altered by various factors. Among them, water temperature is an abiotic factor that can significantly affect fish physiology, increasing energy demand, which can impact homeostasis and survival. Endocrine and metabolic changes and enzymatic modulation are referred to as stress responses, which can lead to oxidative stress, generating negative physiological effects when temperature limits are exceeded. Oxidative stress biomarkers used in combination can highlight the effects of a stressful condition. Here, we seek to understand how the species Astyanax lacustris, which is native to Brazil and has ecological and economic importance, as well as remarkable research potential, responds to changes in water temperature. Thus, we evaluated the effects of high (31°C ± 1°C) and low (15°C ± 1°C) thermal stress on the antioxidant defense system in the heart and kidneys of A. lacustris. Specimens were collected from artificial lakes in União da Vitória (PR) and exposed to different temperatures for periods of 2, 6, 12, 24, 48, 72, or 96 h, with a control group mantained at 23°C ± 1°C. The results indicated that in the heart exposed to 31°C, there was modulation in the biomarkers superoxide dismutase (SOD), glutathione peroxidase (GPx), and glutathione (GSH), while at 15°C only GPx activity was altered. In the kidneys of fish exposed to 31°C, there was a change in the activity of the biomarkers catalase (CAT), glutathione-S-transferase (GST), and lipid peroxidation (LPO), while at 15°C there was modulation of the glutathione reductase (GR) biomarker and changes in the levels of reactive oxygen species (ROS). Responses to heat stress were organ-specific, influenced by temperature and exposure time. Principal component analysis (PCA) indicated an association of glutathione-dependent biomarkers at high temperatures in the kidneys, while responses in the heart were similar across temperatures. Overall, A. lacustris exhibited distinct antioxidant responses in different tissues under thermal stress, with kidney response being more sensitive to heat, while cardiac responses were less variable across treatments.