Thermal tolerance of turbot based on cardiac performance and analysis of molecular regulatory mechanisms.

The heart is an important factor affecting the upper limit of thermal tolerance in fish, and plays a major role in their physiological plasticity and acclimation to different thermal conditions. Turbot (Scophthalmus maximus (L.)) is a cold-water marine fish, and in our previous studies, selecting a high-temperature resistant strain (HR) which increased the upper limit of high-temperature tolerance by more than 2 °C. The study evaluated the difference of the thermal tolerance between the high-temperature resistant strain (HR) and general group (GG) by using Arrhenius break temperatures (ABT). The ABT between them showed a significant difference that suggested different levels of thermal resistance. The result was consistent with assessments of upper thermal tolerance, as well as the activity of creatine kinase and lactate dehydrogenase. Meanwhile, the key genes regulating cardiac performance were analyzed in molecular level. Quantitative real-time PCR results demonstrated that the RNA expression of β-ARs in the heart at increasing temperature followed an upward trend. However, the expression of mmp2, timp2, fosa, and pcna genes in the heart decreased. Additionally, western blot results demonstrated that the levels of β1-AR and β3-AR protein expression steadily increased. Overall, in response to acute thermal stress, the primary effector genes in the β-AR signalling pathway were β-ARs. Our study identified an effective and accurate measurement for evaluating the turbot's thermal tolerance and initially explored the molecular mechanisms that regulate cardiac performance under thermal stress. These would be greatly beneficial in aquaculture and breeding projects of turbot.