Effect of addition of salt on oxidant activity and apoptosis of Coilia nasus juveniles under air exposure stress

Air exposure, as a stressor, can cause death of most fish species in aquaculture. However, little has been known about the responses to the stress caused by air exposure. In the present study, the influence of addition of salt on mitigating the stress caused by air exposure was investigated in Coilia nasus juveniles for the first time. Five experimental groups (control (C), air exposure group without addition of salt (FAF), addition of salt after air exposure (FAS), addition of salt before air exposure (SAF), and addition of salt both before and after air exposure (SAS)) were analyzed in an acute air exposure stress model. The results showed that air exposure triggered high mortality; antioxidase activities and total antioxidant capacity were decreased; malondialdehyde (MDA) and lipid peroxide (LPO) were increased; and Caspase 3 expression was activated. The expression of genes associated with mitochondrial apoptosis (Caspase 9, Apaf1, Bax, Bcl-2), endoplasmic reticulum apoptosis (CHOP, PERK, ATF4), and death receptor apoptosis (CFLAR, TRADD) was activated after air exposure. Compared to FAF, mortality was decreased by 5-fold in the SAS group. Additionally, antioxidant activities were increased, and MDA and LPO levels were decreased. SAS could also inhibit expressions of the genes related to apoptosis. These results indicated that high mortality, oxidant stress, and apoptosis were caused by air exposure, but the addition of salt both before and after air exposure could significantly reduce mortality, inhibit oxidant stress and apoptosis. Our data demonstrated that the addition of salt both before and after air exposure would be an effective method to mitigate air exposure stress in C. nasus juveniles.