Abstract：Rainbow trout (Oncorhynchus mykiss), which is a cold-water species of salmonid, is cultivated as the main variety in China. The high temperature is an important threat to aquaculture of it in summer. In this study, the experimental fish belonged to full-sibling inbreeding families and the average weight of (400±10.5) g. In order to define heat stress response (HSR) and the changes of sensitive parameters, the study took slow heating process which was considered to simulate natural heating process in cage culture water, and 10 fish were sampled to investigate the changes of some main serum non-specific immunity parameters at 18, 21, 23, 24, 25 and 26 ℃, respectively. The results showed that in the process of heat stress, alanine transaminase (ALT) and aspartase aminotransferase (AST) activity at 25 and 26 ℃ were significantly higher than that at other temperature points (P＜0.05), respectively. The difference of total bilirubin (T-Bill) content was not significant in the heating process of 18~23 ℃ (P＞0.05), increased significantly at 24 ℃ (P＜0.05), and peaked at 26 ℃. Alkaline phosphatase (AKP) activity maintained the highest level at 18 ℃, decreased gradually with the water temperature rising and at 21 ℃ was significantly lower than that at 18 ℃ (P＜0.05).The difference of creatine kinase (CK) activity was not significant in the heating process of 21~25 ℃, but increased significantly at 26 ℃ (P＜0.05). Lysozyme (LZM) activity increased gradually in the heating process of 21~24 ℃, peaked at 24 ℃, and at 25 and 26 ℃ were significantly lower than that at other temperature points (P＜0.05), respectively. Complement 3 (C3) content increased gradually in the heating process. The differences of serum proteins content which included total protein (TP), albumin (ALB), globulin (GLO) were all not significant in the heating process (P＞0.05). From the above results, a conclusion can be drawn by analyzing the changes of sensitive parameters. Significant decrease of AKP activity at 21 ℃ indicated that heat stress response occurred and the body changed from normal state into adaptive regulation state at 21 ℃. The fact of significant decrease of LZM activity, significant increase of ALT and AST activity after 24 ℃, and the significant increase of T-Bill content at 24 ℃ indicated that the rainbow trout body's immune function decreased significantly and the body appeared severe tissues damage after 24 ℃. Therefore, 24 ℃ was a "key high-temperature point", the point was a critical point that the body changed from adaptive regulation state to "injury". The fact that AST, ALT, CK activity and T-Bill content peaked at 26 ℃, AKP and LZM activity reached the nadir at 26 ℃ indicated that tissues were damaged seriously and the body might show upper limit of thermal tolerance to temperatures at 26 ℃. The above results will contribute to the development of heat resistant breeding programs. Meanwhile, the key high-temperature point of 24 ℃ has important application value in the future study of heat stress, for example, when screening differentially expressed genes under heat stress, this study can offer a theoretical basis.
