Effects of low salinity stress on Na⁺-K⁺-ATPase activities, expression of Na⁺-K⁺-ATPase β-subunit mRNA and microscopical structure in gill filaments of juvenile Mugil cephalus

In natural aquatic system, salinity can largely influence growth, breeding, and even fishes survival. In general, euryhaline fishes often possess strong osmotic adjustment capacity. The gill filaments play an important role in the maintenance of blood ion and acid-base balance in seawater-acclimated fishes, especially if there is a sudden change in water salinity. Seawater-acclimated fishes drink seawater to counter osmotic loss of water, and secrete excess Na⁺ and Cl^- across the gill filaments. When there is a change between freshwater and seawater, euryhaline fishes must transform their gill filaments from an ion-absorbing epithelium to an ion-secreting epithelium, or vice versa. Associated with this transformation is change of Na+/K+-ATPase activity, Na⁺/K⁺-ATPase gene expression and structural change of the gill. Euryhaline teleost fishes, such as killifish Fundulus heteroclitus and Mozambique tilapia Oreochromis mossambicus, have been used as model animals for the study of osmo- and ion-regulation in teleost fish. However, there are little accumulating researches and available literature on juvenile Mugil cephalus. Thus, the objective of the present study is to explore the response and adjustment mechanisms of juvenile M. cephalus to salinity stress.The present study examined the effects of low salinity stress (salinity 15, 10, 5, 0, and salinity 20 as control) on Na⁺-K⁺-ATPase activities, expression of Na⁺-K⁺-ATPase β-subunit mRNA and structural changes in the gill filaments of juvenile M. cephalus during the experimental period of 20 days.The results showed that no fish mortality occurred for all experimental groups during the study period. The Na⁺-K⁺-ATPase activities in the gill filament tissue were significantly lower in the control group than in the treatment groups. At the low salinity groups (0 and 5), the Na⁺-K⁺-ATPase activities showed rising trends by high and low amplitude during the early and later experimental period, respectively, and the activities at salinity 10 and 15 groups attained the maximum value in 15 days, and then slightly decreased. The expression of Na⁺-K⁺-ATPase β-subunit mRNA changed according with the Na⁺-K⁺-ATPase activity. It showed rising trend with the decrease of salinity. There were significant differences in Na⁺-K⁺-ATPases and their β-subunit mRNA expressions between the control group and salinity 0 group from day 5 to day 20, as well as between the control group and salinity 5 group from day 10 to day 20 (P<0.05). The highest expression of Na⁺-K⁺-ATPase β- subunit mRNA at the salinity 0, 5 groups was 4.10 and 2.51 times higher than that at the control group respectively, suggesting that the Na⁺-K⁺-ATPase β-subunit mRNA played an important role in the process of regulation. With the decrease of salinity, the gill lamellae gradually became wide, and the distance between them as well as the number and size of chloride cells showed decreasing trend, while the size of pavement cells and the number of mucous cells consistently increased.In conclusion, the structure and physiological functions of the gill of juvenile M. cephalus adapt themselves to the salinity change.